MRI monitoring of the thermal ablation of tissue: effects of long exposure times

An augmented hybrid multibaseline and referenceless MR thermometry motion compensation algorithm for MRgHIFU hyperthermia

PURPOSE

A hybrid principal component analysis and projection onto dipole fields (PCA-PDF) MR thermometry motion compensation algorithm was optimized with atlas image augmentation and validated.

METHODS

Experiments were conducted on a 3T Philips MRI and Profound V1 Sonalleve high intensity focused ultrasound (high intensity focused ultrasound system. An MR-compatible robot was configured to induce motion on custom gelatin phantoms. Trials with periodic and sporadic motion were introduced on phantoms while hyperthermia was administered. The PCA-PDF algorithm was augmented with a predictive atlas to better compensate for larger sporadic motion.

RESULTS

During periodic motion, the temperature SD in the thermometry was improved from1 . 1 ± 0 . 1 $$ 1.1\pm 0.1 $$ to0 . 5 ± 0 . 1 ∘ $$ 0.5\pm 0.{1}^{\circ } $$ C with both the original and augmented PCA-PDF application. For large sporadic motion, the augmented atlas improved the motion compensation from the original PCA-PDF correction from8 . 8 ± 0 . 5 $$ 8.8\pm 0.5 $$ to0 . 7 ± 0 . 1 ∘ $$ 0.7\pm 0.{1}^{\circ } $$ C.

CONCLUSION

The PCA-PDF algorithm improved temperature accuracy to <1°C during periodic motion, but was not able to adequately address sporadic motion. By augmenting the PCA-PDF algorithm, temperature SD during large sporadic motion was also reduced to <1°C, greatly improving the original PCA-PDF algorithm.

What happens to brain outside the thermal ablation zones? An assessment of needle-based therapeutic ultrasound in survival swine

BACKGROUND

In stereotactic radiosurgery, isodose lines must be considered to determine how surrounding tissue is affected. In thermal ablative therapy, such as laser interstitial thermal therapy (LITT), transcranial MR-guided focused ultrasound (tcMRgFUS), and needle-based therapeutic ultrasound (NBTU), how the surrounding area is affected has not been well studied.

OBJECTIVE

We aimed to quantify the transition zone surrounding the ablation core created by magnetic resonance-guided robotically-assisted (MRgRA) delivery of NBTU using multi-slice volumetric 2-D magnetic resonance thermal imaging (MRTI) and subsequent characterization of the resultant tissue damage using histopathologic analysis.

METHODS

Four swine underwent MRgRA NBTU using varying duration and wattage for treatment delivery. Serial MRI images were obtained, and the most representative were overlaid with isodose lines and compared to brain tissue acquired postmortem which underwent histopathologic analysis. These results were also compared to predicted volumes using a finite element analysis model. Contralateral brain tissue was used for control data.

RESULTS

Intraoperative MRTI thermal isodose contours were characterized and comprehensively mapped to post-operative MRI images and qualitatively compared with histological tissue sections postmortem. NBTU 360° ablations induced smaller lesion volumes (33.19 mm³; 120 s, 3 W; 30.05 mm³, 180 s, 4 W) versus 180° ablations (77.20 mm³, 120 s, 3 W; 109.29 mm³; 180 s; 4 W). MRTI/MRI overlay demonstrated the lesion within the proximal isodose lines. The ablation-zone was characterized by dense macrophage infiltration and glial/neuronal loss as demonstrated by glial fibrillary acidic protein (GFAP) and neurofilament (NF) absence and avid CD163 staining. The transition-zone between lesion and normal brain demonstrated decreased macrophage infiltration and measured ∼345 microns (n - 3). We did not detect overt hemorrhages or signs of edema in the adjacent spared tissue.

CONCLUSION

We successfully performed MRgRA NBTU ablation in swine and demonstrated minimal histologic changes extended past the ablation-zone. The lesion was characterized by macrophage infiltration and glial/neuronal loss which decreased through the transition-zone.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

Carbon nanotube-mediated high intensity focused ultrasound

High intensity focused ultrasound (HIFU) is emerging as a novel therapeutic technique for cancer treatment through a hyperthermal mechanism using ultrasound. However, collateral thermal damages to healthy tissue and skin burns due to the use of high levels of ultrasonic energy during HIFU treatment remain major challenges to clinical application. The main objective of the current study is to evaluate the potential of carbon nanotubes (CNTs) as effective absorption-enhancing agents for HIFU to mediate the heating process at low ultrasonic power levels, and consequently upgrade hyperthermal therapeutic effects of HIFU. An experimental study using in vitro tissue phantoms was conducted to assess the effects of CNTs on HIFU’s heating mechanism. Detailed information was extracted from the experiments for thermal analysis, including rate of absorbed energy density and temperature rise profile at the focal region. Parametric studies were carried out, revealing the effects of ultrasound parameters (ultrasonic power and driving frequency) on the performance of CNTs in various concentrations. The results indicated that CNTs significantly enhanced the thermal effect of HIFU by elevating energy absorption rate and consequential temperature rise. Moreover, it was demonstrated that an increase in ultrasonic power and driving frequency could lead to a better performance of CNTs during HIFU ablation procedures; the effects of CNTs could be further enhanced by increasing their volume concentration inside the medium.

Quantitative prediction of the extent of pelvic tumour ablation by magnetic resonance-guided high intensity focused ultrasound

BACKGROUND

Patient suitability for magnetic resonance-guided high intensity focused ultrasound (MRgHIFU) therapy of pelvic tumors is currently assessed by visual estimation of the proportion of tumor that can be reached by the device's focus (coverage). Since it is important to assess whether enough energy reaches the tumor to achieve ablation, a methodology for estimating the proportion of the tumor that can be ablated (treatability) was developed. Predicted treatability was compared against clinically achieved thermal ablation.

METHODS

MR Dixon sequence images of five patients with recurrent gynecological tumors were acquired during their treatment. Acousto-thermal simulations were performed using k-Wave for three exposure points (the deepest and shallowest reachable focal points within the tumor, identified from tumor coverage analysis, and a point halfway in-between) per patient. Interpolation between the resulting simulated ablated tissue volumes was used to estimate the maximum treatable depth and hence, tumor treatability. Predicted treatability was compared both to predicted tumor coverage and to the clinically treated tumor volume. The intended and simulated volumes and positions of ablated tissues were compared.

RESULTS

Predicted treatability was less than coverage by 52% (range: 31-78%) of the tumor volume. Predicted and clinical treatability differed by 9% (range: 1-25%) of tumor volume. Ablated tissue volume and position varied with beam path length through tissue.

CONCLUSION

Tumor coverage overestimated patient suitability for MRgHIFU therapy. Employing patient-specific simulations improved treatability assessment. Patient treatability assessment using simulations is feasible.

Iterative Curve Fitting of the Bioheat Transfer Equation for Thermocouple-Based Temperature Estimation In Vitro and In Vivo

Temperature measurements with thin thermocouples embedded in ultrasound fields are strongly subjected to a viscous heating artifact (VHA). The artifact contribution decays over time; therefore, it can be minimized at late temperature readings. However, previous studies have failed to demonstrate a rigorous method for determining the optimal time point at which the artifact contribution is negligible. In this study, we present an iterative processing method based on successive curve fittings using an artifact-independent model. The fitting starting point moves at each iteration until the maximum R² indicates where the viscous heating is minimum. A solution of the bioheat transfer equation is used to account for blood perfusion, thus enabling in vivo measurements. Three T-type thermocouples with different diameters and sensitivities were assessed in an excised canine liver and in the mouse brain in vivo. We found that the artifact constitutes up to 81% ± 5% of wire thermocouple readings. The best-fit time varied in the liver samples ( n = 3 ) from 0 to 3.335 ± 0.979 s and in the mouse brain ( n = 5 ) from 0 to 0.498 ± 0.457 s at variable experimental conditions, which clearly demonstrates the need of the method for finding the appropriate starting time point of the fit. This study introduces a statistical method to determine the best time to fit a curve that can back-estimate temperature in tissues under ultrasound exposure using thermocouples. This method allows temperature evaluation in vivo and in vitro during a validation and safety assessment of a wide range of therapeutic and diagnostic ultrasound modalities.

Volumetric analysis of magnetic resonance-guided focused ultrasound thalamotomy lesions

OBJECTIVE Magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy was recently approved for use in the treatment of medication-refractory essential tremor (ET). Previous work has described lesion appearance and volume on MRI up to 6 months after treatment. Here, the authors report on the volumetric segmentation of the thalamotomy lesion and associated edema in the immediate postoperative period and 1 year following treatment, and relate these radiographic characteristics with clinical outcome. METHODS Seven patients with medication-refractory ET underwent MRgFUS thalamotomy at Brigham and Women's Hospital and were monitored clinically for 1 year posttreatment. Treatment effect was measured using the Clinical Rating Scale for Tremor (CRST). MRI was performed immediately postoperatively, 24 hours posttreatment, and at 1 year. Lesion location and the volumes of the necrotic core (zone I) and surrounding edema (cytotoxic, zone II; vasogenic, zone III) were measured on thin-slice T2-weighted images using Slicer 3D software. RESULTS Patients had significant improvement in overall CRST scores (baseline 51.4 ± 10.8 to 24.9 ± 11.0 at 1 year, p = 0.001). The most common adverse events (AEs) in the 1-month posttreatment period were transient gait disturbance (6 patients) and paresthesia (3 patients). The center of zone I immediately posttreatment was 5.61 ± 0.9 mm anterior to the posterior commissure, 14.6 ± 0.8 mm lateral to midline, and 11.0 ± 0.5 mm lateral to the border of the third ventricle on the anterior commissure-posterior commissure plane. Zone I, II, and III volumes immediately posttreatment were 0.01 ± 0.01, 0.05 ± 0.02, and 0.33 ± 0.21 cm³, respectively. These volumes increased significantly over the first 24 hours following surgery. The edema did not spread evenly, with more notable expansion in the superoinferior and lateral directions. The spread of edema inferiorly was associated with the incidence of gait disturbance. At 1 year, the remaining lesion location and size were comparable to those of zone I immediately posttreatment. Zone volumes were not associated with clinical efficacy in a statistically significant way. CONCLUSIONS MRgFUS thalamotomy demonstrates sustained clinical efficacy at 1 year for the treatment of medication-refractory ET. This technology can create accurate, predictable, and small-volume lesions that are stable over time. Instances of AEs are transient and are associated with the pattern of perilesional edema expansion. Additional analysis of a larger MRgFUS thalamotomy cohort could provide more information to maximize clinical effect and reduce the rate of long-lasting AEs.

Focused ultrasound in neurosurgery: a historical perspective

Focused ultrasound (FUS) has been under investigation for neurosurgical applications since the 1940s. Early experiments demonstrated ultrasound as an effective tool for the creation of intracranial lesions; however, they were limited by the need for craniotomy to avoid trajectory damage and wave distortion by the skull, and they also lacked effective techniques for monitoring. Since then, the development and hemispheric distribution of phased arrays has resolved the issue of the skull and allowed for a completely transcranial procedure. Similarly, advances in MR technology have allowed for the real-time guidance of FUS procedures using MR thermometry. MR-guided FUS (MRgFUS) has primarily been investigated for its thermal lesioning capabilities and was recently approved for use in essential tremor. In this capacity, the use of MRgFUS is being investigated for other ablative indications in functional neurosurgery and neurooncology. Other applications of MRgFUS that are under active investigation include opening of the blood-brain barrier to facilitate delivery of therapeutic agents, neuromodulation, and thrombolysis. These recent advances suggest a promising future for MRgFUS as a viable and noninvasive neurosurgical tool, with strong potential for yet-unrealized applications.

Magnetic resonance thermometry and its biological applications - Physical principles and practical considerations

Most parameters that influence the magnetic resonance imaging (MRI) signal experience a temperature dependence. The fact that MRI can be used for non-invasive measurements of temperature and temperature change deep inside the human body has been known for over 30 years. Today, MR temperature imaging is widely used to monitor and evaluate thermal therapies such as radio frequency, microwave, laser, and focused ultrasound therapy. In this paper we cover the physical principles underlying the biological applications of MR temperature imaging and discuss practical considerations and remaining challenges. For biological tissue, the MR signal of interest comes mostly from hydrogen protons of water molecules but also from protons in, e.g., adipose tissue and various metabolites. Most of the discussed methods, such as those using the proton resonance frequency (PRF) shift, T₁, T₂, and diffusion only measure temperature change, but measurements of absolute temperatures are also possible using spectroscopic imaging methods (taking advantage of various metabolite signals as internal references) or various types of contrast agents. Currently, the PRF method is the most used clinically due to good sensitivity, excellent linearity with temperature, and because it is largely independent of tissue type. Because the PRF method does not work in adipose tissues, T₁- and T₂-based methods have recently gained interest for monitoring temperature change in areas with high fat content such as the breast and abdomen. Absolute temperature measurement methods using spectroscopic imaging and contrast agents often offer too low spatial and temporal resolution for accurate monitoring of ablative thermal procedures, but have shown great promise in monitoring the slower and usually less spatially localized temperature change observed during hyperthermia procedures. Much of the current research effort for ablative procedures is aimed at providing faster measurements, larger field-of-view coverage, simultaneous monitoring in aqueous and adipose tissues, and more motion-insensitive acquisitions for better precision measurements in organs such as the heart, liver, and kidneys. For hyperthermia applications, larger coverage, motion insensitivity, and simultaneous aqueous and adipose monitoring are also important, but great effort is also aimed at solving the problem of long-term field drift which gets interpreted as temperature change when using the PRF method.

Drift correction for accurate PRF-shift MR thermometry during mild hyperthermia treatments with MR-HIFU

There is growing interest in performing hyperthermia treatments with clinical magnetic resonance imaging-guided high-intensity focused ultrasound (MR-HIFU) therapy systems designed for tissue ablation. During hyperthermia treatment, however, due to the narrow therapeutic window (41-45 °C), careful evaluation of the accuracy of proton resonant frequency (PRF) shift MR thermometry for these types of exposures is required.

PURPOSE

The purpose of this study was to evaluate the accuracy of MR thermometry using a clinical MR-HIFU system equipped with a hyperthermia treatment algorithm.

METHODS

Mild heating was performed in a tissue-mimicking phantom with implanted temperature sensors using the clinical MR-HIFU system. The influence of image-acquisition settings and post-acquisition correction algorithms on the accuracy of temperature measurements was investigated. The ability to achieve uniform heating for up to 40 min was evaluated in rabbit experiments.

RESULTS

Automatic centre-frequency adjustments prior to image-acquisition corrected the image-shifts in the order of 0.1 mm/min. Zero- and first-order phase variations were observed over time, supporting the use of a combined drift correction algorithm. The temperature accuracy achieved using both centre-frequency adjustment and the combined drift correction algorithm was 0.57° ± 0.58 °C in the heated region and 0.54° ± 0.42 °C in the unheated region.

CONCLUSION

Accurate temperature monitoring of hyperthermia exposures using PRF shift MR thermometry is possible through careful implementation of image-acquisition settings and drift correction algorithms. For the evaluated clinical MR-HIFU system, centre-frequency adjustment eliminated image shifts, and a combined drift correction algorithm achieved temperature measurements with an acceptable accuracy for monitoring and controlling hyperthermia exposures.

Noninvasive Dynamic Imaging of Tumor Early Response to Nanoparticle-mediated Photothermal Therapy

In spite of rapidly increasing interest in the use of nanoparticle-mediated photothermal therapy (PTT) for treatment of different types of tumors, very little is known on early treatment-related changes in tumor response. Using graphene oxide (GO) as a model nanoparticle (NP), in this study, we tracked the changes in tumors after GO NP-mediated PTT by magnetic resonance imaging (MRI) and quantitatively identified MRI multiple parameters to assess the dynamic changes of MRI signal in tumor at different heating levels and duration. We found a time- and temperature-dependent dynamic change of the MRI signal intensity in intratumor microenvironment prior to any morphological change of tumor, mainly due to quick and effective eradication of tumor blood vessels. Based on the distribution of GO particles, we also demonstrated that NP-medited PTT caused heterogeneous thermal injury of tumor. Overall, these new findings provide not only a clinical-related method for non-invasive early tracking, identifying, and monitoring treatment response of NP-mediated PTT but also show a new vision for better understanding mechanisms of NP-mediated PTT.

T2-based temperature monitoring in abdominal fat during MR-guided focused ultrasound treatment of patients with uterine fibroids

Background

Near-field heating is a potential problem in focused ultrasound treatments, as it can result in thermal injury to skin, subcutaneous fat, and other tissues. Our goals were to determine if T2-based temperature mapping could be used reliably to measure near-field heating in adipose tissue and whether it is practical to perform such mapping during focused ultrasound treatments.

Methods

We investigated the dependence of T2 on temperature in ex vivo adipose tissue at 3T using a double-echo fast spin echo (FSE) sequence. We implemented and evaluated the T2-based temperature mapping technique in the adipose tissue of two healthy volunteers. Finally, we applied the technique during magnetic resonance-guided focused ultrasound (MRgFUS) treatments to measure near-field heating in eight patients with uterine fibroids.

Results

Calibration experiments in porcine adipose tissue determined a temperature coefficient of 6.16 ms/°C during heating and 5.37 ms/°C during cooling. The volunteer experiments demonstrated a strong correlation between the skin temperature and T2-based temperature measurements in the fat layer. During the treatments of patients with uterine fibroids, we observed a measurable change in the T2 of fat tissue within the path of the ultrasound beam and a temperature increase of up to 15 °C with sustained heating of more than 10 °C.

Conclusions

Our results demonstrate the feasibility and importance of monitoring near-field heating in fatty tissues. The implementation of near-field monitoring between sonications can shorten treatments by reducing the cooling time. It can help improve safety by avoiding excessive heating in the near field.

Modulation of the interstitial fluid pressure by high intensity focused ultrasound as a way to alter local fluid and solute movement: insights from a mathematical model

High intensity focused ultrasound (HIFU) operated in thermal mode has been reported to reduce interstitial fluid pressure and improve the penetration of large macromolecules and nanoparticles in tumor and normal tissue. Little is understood about how the interstitial fluid pressure and velocity as well as the interstitial macromolecule transport are affected by HIFU exposure. A mathematical model is presented here which sheds light on the initial biophysical changes brought about HIFU. Our continuum model treats tissue as an effective poro-elastic material that reacts to elevated temperatures with a rapid drop in interstitial elastic modulus. Using parameters from the literature, the model is extrapolated to derive information on the effect in tumors, and to predict its impact on the convective and diffusive transport of macromolecular drugs. The model is first solved using an analytical approximation with step-wise changes at each boundary, and then solved numerically starting from a Gaussian beam approximation of the ultrasound treatment. Our results indicate that HIFU causes a rapid drop in interstitial fluid pressure that may be exploited to facilitate convection of macromolecules from vasculature to the exposed region. However, following a short recovery period in which the interstitial fluid pressure is normalized, transport returns to normal and the advantages disappear over time. The results indicate that this effect is strongest for the delivery of large molecules and nanoparticles that are in the circulation at the time of treatment. The model may be easily applied to more complex situations involving effects on vascular permeability and diffusion.

Multiple applicator hepatic ablation with interstitial ultrasound devices: theoretical and experimental investigation

PURPOSE

To evaluate multiple applicator implant configurations of interstitial ultrasound devices for large volume ablation of liver tumors.

METHODS

A 3D bioacoustic-thermal model using the finite element method was implemented to assess multiple applicator implant configurations for thermal ablation with interstitial ultrasound energy. Interstitial applicators consist of linear arrays of up to four 10 mm-long tubular ultrasound transducers, each under separate and dynamic power control, enclosed within a water-cooled delivery catheter (2.4 mm OD). The authors considered parallel implants with two and three applicators (clustered configuration), spaced 2-3 cm apart, to simulate open surgical placement. In addition, the authors considered two applicator implants with applicators converging and diverging at angles of ∼20°, 30°, and 45° to simulate percutaneous placement. Heating experiments (10-15 min) were performed and compared against simulations employing the same experimental parameters. To estimate the performance of parallel, multiple applicator configurations in an in vivo setting, simulations were performed taking into account a range of blood perfusion levels (0, 5, 12, and 15 kg m(-3) s(-1)) that may occur in tumors of varying vascularity. The impact of tailoring the power supplied to individual transducer elements along the length of applicators is explored for applicators inserted in non-parallel (converging and diverging) configurations. Thermal dose (t(43) > 240 min) and temperature thresholds (T > 52 °C) were used to define the ablation zones, with dynamic changes to tissue acoustic and thermal properties incorporated within the model.

RESULTS

Experiments in ex vivo bovine liver yielded ablation zones ranging between 4.0-5.6 cm × 3.2-4.9 cm, in cross section. Ablation zone dimensions predicted by simulations with similar parameters to the experiments were in close agreement (within 5 mm). Simulations of in vivo heating showed that 15 min heating and interapplicator spacing less than 3 cm are required to obtain contiguous, complete ablation zones. The ability to create complete ablation zone profiles for nonparallel implants was illustrated by tailoring applied power levels along the length of applicators.

CONCLUSIONS

Parallel implants consisting of three interstitial ultrasound applicators in a triangular configuration yield complete ablation zones measuring up to 6.2 cm × 5.7 cm after 15 min heating. At larger interapplicator spacing, the level of blood perfusion in the tumor may yield indentations along the periphery of the ablation zone. Tailoring applied power along the length of the applicator can accommodate for nonparallel implants, without compromising safety.

Magnetic resonance image-guided versus ultrasound-guided high-intensity focused ultrasound in the treatment of breast cancer

Image-guided high-intensity focused ultrasound (HIFU) has been used for more than ten years, primarily in the treatment of liver and prostate cancers. HIFU has the advantages of precise cancer ablation and excellent protection of healthy tissue. Breast cancer is a common cancer in women. HIFU therapy, in combination with other therapies, has the potential to improve both oncologic and cosmetic outcomes for breast cancer patients by providing a curative therapy that conserves mammary shape. Currently, HIFU therapy is not commonly used in breast cancer treatment, and efforts to promote the application of HIFU is expected. In this article, we compare different image-guided models for HIFU and reviewed the status, drawbacks, and potential of HIFU therapy for breast cancer.

A unified approach to combine temperature estimation and elastography for thermal lesion determination in focused ultrasound thermal therapy

Sonogram-based temperature estimation and elastography have both shown promise as methods of monitoring focused ultrasound (FUS) treatments to induce thermal ablation in tissue. However, each method has important limitations. Temperature estimates based on echo delays become invalid when the relationship between sound speed and temperature is nonlinear, and are further complicated by thermal expansion and other changes in tissue. Elastography can track thermal lesion formation over a wider range of elasticity, but with low specificity and high noise. Furthermore, this method is poor at small lesion detection. This study proposes integrating the two estimates to improve the quality of monitoring FUS-induced thermal lesions. Our unified computational kernel is tested on three types of phantoms. Experiments with type I and type II phantoms were conducted to calibrate the thermal mapping and elastography methods, respectively. The optimal settings were then used in experiments with the type III phantom, which contains ex vivo swine liver tissue. Three different spatial-peak temporal-average intensities (I(spta); 35, 133 and 240 W cm(-2)) were delivered with a sonication time of 60 s. The new procedure can closely monitor heating while identifying the dimensions of the thermal lesion, and is significantly better at the latter task than either approach alone. This work may help improve the current clinical practice, which employs sonograms to guide the FUS-induced thermal ablation procedure.

Focused ultrasound as a local therapy for liver cancer

Conventional surgical treatments of liver cancer are invasive (including minimally invasive) with a high incidence of new metastasis and poor success, even after multiple resections or ablations. These limitations motivated research into new, less invasive solutions for liver cancer treatment.Focused ultrasound surgery (FUS), or high-intensity focused ultrasound, has been recognized as a noninvasive technology for benign and malignant tumor treatment. Previously, FUS was guided with ultrasound that has limited target definition and monitoring capability of the ablation process. Combining magnetic resonance imaging (MRI) with multiple-element phased-array transducers to create MRI-guided focused ultrasound thermal therapy provides more accurate targeting and real-time temperature monitoring. This treatment is hindered by the ribcage that limits the acoustic windows to the liver and the respiratory motion of the liver. New advances in MRI and transducer design will likely resolve these limitations and make MRI-guided FUS a powerful tool in local liver cancer therapy. This article reviews this technology and advances that can expand its use for cancer treatment in general and liver cancer in particular.

Noninvasive assessment of tissue heating during cardiac radiofrequency ablation using MRI thermography

BACKGROUND

Failure to achieve properly localized, permanent tissue destruction is a common cause of arrhythmia recurrence after cardiac ablation. Current methods of assessing lesion size and location during cardiac radiofrequency ablation are unreliable or not suited for repeated assessment during the procedure. MRI thermography could be used to delineate permanent ablation lesions because tissue heating above 50°C is the cause of permanent tissue destruction during radiofrequency ablation. However, image artifacts caused by cardiac motion, the ablation electrode, and radiofrequency ablation currently pose a challenge to MRI thermography in the heart. In the current study, we sought to demonstrate the feasibility of MRI thermography during cardiac ablation.

METHODS AND RESULTS

An MRI-compatible electrophysiology catheter and filtered radiofrequency ablation system was used to perform ablation in the left ventricle of 6 mongrel dogs in a 1.5-T MRI system. Fast gradient-echo imaging was performed before and during radiofrequency ablation, and thermography images were derived from the preheating and postheating images. Lesion extent by thermography was within 20% of the gross pathology lesion.

CONCLUSIONS

MR thermography appears to be a promising technique for monitoring lesion formation and may allow for more accurate placement and titration of ablation, possibly reducing arrhythmia recurrences.

MRI-guided focused ultrasound surgery

MRI-guided focused ultrasound (MRgFUS) surgery is a noninvasive thermal ablation method that uses magnetic resonance imaging (MRI) for target definition, treatment planning, and closed-loop control of energy deposition. Integrating FUS and MRI as a therapy delivery system allows us to localize, target, and monitor in real time, and thus to ablate targeted tissue without damaging normal structures. This precision makes MRgFUS an attractive alternative to surgical resection or radiation therapy of benign and malignant tumors. Already approved for the treatment of uterine fibroids, MRgFUS is in ongoing clinical trials for the treatment of breast, liver, prostate, and brain cancer and for the palliation of pain in bone metastasis. In addition to thermal ablation, FUS, with or without the use of microbubbles, can temporarily change vascular or cell membrane permeability and release or activate various compounds for targeted drug delivery or gene therapy. A disruptive technology, MRgFUS provides new therapeutic approaches and may cause major changes in patient management and several medical disciplines.

Conductive interstitial thermal therapy device for surgical margin ablation:In vivoverification of a theoretical model

PURPOSE

To demonstrate the efficacy and predictability of a new conductive interstitial thermal therapy (CITT) device to ablate surgical margins.

METHOD

The temperature distributions during thermal ablation of CITT were calculated with finite element modelling in a geometrical representation of perfused tissue. The depth of ablation was derived using the Arrhenius and the Sapareto and Dewey (S&D) models for the temperature range of 90 to 150 degrees C. The female pig animal model was used to test the validity of the mathematical model. Breast tissues were ablated to temperatures in the range of 79-170 degrees C, in vivo. Triphenyltetrazolium chloride viability stain was used to delineate viable tissue from ablated regions and the ablation depths were measured using digital imaging.

RESULTS

The calculations suggest that the CITT can be used to ablate perfused tissues to a 10-15 mm width within 20 minutes. The measured and calculated depths of ablation were statistically equivalent (99% confidence intervals) within +/- 1mm at 170 degrees C. At lower temperatures the equivalence between the model and the observations was within +/- 2 mm.

CONCLUSION

The CITT device can reliably and uniformly ablate a 10-15 mm wide region of soft tissue. Thus, it can be used to secure negative margins following the resection of a primary tumor, which could impede local recurrences in the treatment of local diseases such as early staged, non-metastatic, breast cancer.

Analysis of the spatial and temporal accuracy of heating in the prostate gland using transurethral ultrasound therapy and active MR temperature feedback

A new MRI-guided therapy is being developed as a minimally invasive treatment for localized prostate cancer utilizing high-intensity ultrasound energy to generate a precise region of thermal coagulation within the prostate gland. The purpose of this study was to evaluate in vivo the capability to produce a spatial heating pattern in the prostate that accurately matched the shape of a target region using transurethral ultrasound heating and active MR temperature feedback. Experiments were performed in a canine model (n = 9) in a 1.5 T MR imager using a prototype device comprising a single planar transducer operated under rotational control. The spatial temperature distribution, measured every 5 s with MR thermometry, was used to adjust the acoustic power and rotation rate in order to achieve a temperature of 55 degrees C along the outer boundary of the target region. The results demonstrated the capability to produce accurate spatial heating patterns within the prostate gland. An average temperature of 56.2 +/- 0.6 degrees C was measured along the outer boundary of the target region across all experiments in this study. The average spatial error between the target boundary and the 55 degrees C isotherm was 0.8 +/- 0.7 mm (-0.2 to 3.2 mm), and the overall treatment time was < or =20 min for all experiments. Excellent spatial agreement was observed between the temperature information acquired with MRI and the pattern of thermal damage measured on H&E-stained tissue sections. This study demonstrates the benefit of adaptive energy delivery using active MR temperature feedback, and an excellent capability to treat precise regions within the prostate gland with this technology.

MRI-based thermal dosimetry and diffusion-weighted imaging of MRI-guided focused ultrasound thermal ablation of uterine fibroids

PURPOSE

To investigate tissue changes observed in diffusion-weighted imaging (DWI) and its relation to contrast imaging, thermal dosimetry, and changes in the apparent diffusion coefficient (ADC) after MRI-guided focused ultrasound surgery (MRgFUS) of uterine fibroids.

MATERIALS AND METHODS

Imaging data were analyzed from 45 fibroids in 42 women treated with MRgFUS. The areas of the hyperintense regions in DWI and of nonperfused regions in T1-weighted contrast enhanced imaging (both acquired immediately after treatment) were compared with each other and to thermal dosimetry based estimates. Changes in ADC were also calculated.

RESULTS

Hyperintense regions were observed in 35/45 fibroids in DWI. When present, the areas of these regions were comparable on average to the thermal dose estimates and to the nonperfused regions, except for in several large treatments in which the nonperfused region extended beyond the treated area. ADC increased in 19 fibroids and decreased in the others.

CONCLUSION

DWI changes, which includes changes in both in T2 and ADC, may be useful in many cases to delineate the treated region resulting from MRgFUS. However, clear DWI changes were not always observed, and in some large treatments, the extent of the nonperfused region was under estimated. ADC changes immediately after MRgFUS were unpredictable.

Prevention of post-focal thermal damage by formation of bubbles at the focus during high intensity focused ultrasound therapy

Safety concerns exist for potential thermal damage at tissue-air or tissue-bone interfaces located in the post-focal region during high intensity focused ultrasound (HIFU) treatments. We tested the feasibility of reducing thermal energy deposited at the post-focal tissue-air interfaces by producing bubbles (due to acoustic cavitation and/or boiling) at the HIFU focus. HIFU (in-situ intensities of 460-3500 W/cm2, frequencies of 3.2-5.5 MHz) was applied for 30 s to produce lesions (in turkey breast in-vitro (n = 37), and rabbit liver (n = 4) and thigh muscle in-vivo (n = 11)). Tissue temperature was measured at the tissue-air interface using a thermal (infrared) camera. Ultrasound imaging was used to detect bubbles at the HIFU focus, appearing as a hyperechoic region. In-vitro results showed that when no bubbles were present at the focus (at lower intensities of 460-850 W/cm2), the temperature at the interface increased continuously, up to 7.3 +/- 4.0 degrees C above the baseline by the end of treatment. When bubbles formed immediately after the start of HIFU treatment (at the high intensity of 3360 W/cm2), the temperature increased briefly for 3.5 s to 7.4 +/- 3.6 degrees C above the baseline temperature and then decreased to 4.0 +/- 1.4 degrees C above the baseline by the end of treatment. Similar results were obtained in in-vivo experiments with the temperature increases (above the baseline temperature) at the muscle-air and liver-air interfaces at the end of the high intensity treatment lower by 7.1 degrees C and 6.0 degrees C, respectively, as compared to the low intensity treatment. Thermal effects of HIFU at post-focal tissue-air interfaces, such as in bowels, could result in clinically significant increases in temperature. Bubble formation at the HIFU focus may provide a method for shielding the post-focal region from potential thermal damage.

Investigation of the viscous heating artefact arising from the use of thermocouples in a focused ultrasound field

Accurate temperature measurements in therapeutic ultrasound fields are necessary for understanding damage mechanisms, verification of thermal modelling and calibration of non-invasive clinical thermometry. However, artefactual heating, primarily due to viscous forces which result from motion relative to the surrounding tissue, occurs when metal thermocouples are used in an ultrasound field. The magnitude and time dependence of this artefact has been characterized by comparison with novel thin-film thermocouples (TFTs) at 1-2 cm focal depths in fresh degassed ex vivo bovine liver. High-intensity focused ultrasound exposures (1.7 MHz; free-field spatial-peak temporal-average intensities 40-600 W cm(-2)) were used. Subtraction of the TFT data from that obtained for other thermocouples yielded the time dependence of the viscous heating artefact. This was found to be intensity independent up to 600 W cm(-2) (below the threshold for cavitation and lesion formation) and remained significant at radial distances out to the first side lobe in the focal plane. The contribution of viscous heating to cooling was also found to be significant for at least 5 s after the end of insonation. The ratio of viscous artefact to absorptive heating after 5 s was: 1.76 +/- 0.07 for a fine-wire, 0.45 +/- 0.07 and 1.93 +/- 0.07 for two different sheathed-wires and 0.24 +/- 0.07 for a needle thermocouple.

Magnetic resonance guidance for radiofrequency ablation of liver tumors

Image-guided thermal ablation of liver tumors is a minimally invasive treatment option. Techniques used for thermal ablation are radiofrequency (RF) ablation, laser interstitial thermotherapy (LITT), microwave (MW) ablation, high-intensity focused ultrasound (HIFU), and cryoablation. Among these techniques RF ablation attained widespread consideration. Image guidance should ensure a precise ablation therapy leading to a complete coagulation of tumor tissue without injury to critical structures. Therefore, the modality of image guidance has an important impact on the safety and efficacy of percutaneous RF ablation. The current literature regarding percutaneous RF ablation mainly describes the use of computed tomography (CT) and ultrasonography (US) guidance. In addition, interventional MR systems offer the possibility to utilize the advantages of MR imaging such as excellent soft-tissue contrast, multiplanar and interactive capabilities, and sensitivity to thermal effects during the entire RF ablation procedure. Monitoring of thermally induced coagulation by MR imaging is supportive to control the ablation procedure. MR imaging can be advantageously used to guide overlapping ablation if necessary as well as to define the endpoint of RF ablation after complete coverage of the target tissue is verified. Furthermore, monitoring of thermal effects is essential in order to prevent unintended thermal damage from critical structures surrounding the target region. Therefore, MR-guided RF ablation offers the possibility for a safe and effective therapy option in the treatment of primary and secondary hepatic malignancies. The article summarizes the role of MR guidance for RF ablation of liver tumors.

MR thermometry

Minimally invasive thermal therapy as local treatment of benign and malignant diseases has received increasing interest in recent years. Safety and efficacy of the treatment require accurate temperature measurement throughout the thermal procedure. Noninvasive temperature monitoring is feasible with magnetic resonance (MR) imaging based on temperature-sensitive MR parameters such as the proton resonance frequency (PRF), the diffusion coefficient (D), T1 and T2 relaxation times, magnetization transfer, the proton density, as well as temperature-sensitive contrast agents. In this article the principles of temperature measurements with these methods are reviewed and their usefulness for monitoring in vivo procedures is discussed. Whereas most measurements give a temperature change relative to a baseline condition, temperature-sensitive contrast agents and spectroscopic imaging can provide absolute temperature measurements. The excellent linearity and temperature dependence of the PRF and its near independence of tissue type have made PRF-based phase mapping methods the preferred choice for many in vivo applications. Accelerated MRI imaging techniques for real-time monitoring with the PRF method are discussed. Special attention is paid to acquisition and reconstruction methods for reducing temperature measurement artifacts introduced by tissue motion, which is often unavoidable during in vivo applications.

Evaluating tissue changes with ultrasound during radiofrequency ablation

The purpose of this study was to estimate tissue changes during radiofrequency (RF) ablation by correlating echo frequency shifts and temperature elevations. Experiments were performed on phantoms (tissue mimicking gel) and in-vitro turkey breast. Heating was performed with a modified RF-ablation system. Intermittent RF was applied and the temperature at the electrode tip was continually measured by an embedded thermocouple. Various voltages (10-30V) were applied to achieve a wide range of temperature elevations between 10 and 80 degrees C and ablation sizes between 5 and 27 mm in width. B-mode images and raw data were acquired every 5 s by a modified ultrasound imaging system. The raw data from each line and frame was processed using an algorithm to measure spectral shifts of the echo signals in the power spectrum. The phantom experiments showed positive frequency shifts as the temperature rose, with dependency on the heating rate. A linear relationship (R(2) > 0.96) was found between the RF-applied voltage and the width of the heated area, defined by frequency changes larger than 0.05 MHz. In-vitro experiments showed a correlation (R(2) = 0.84) between the width of the coagulated area and the maximal width of the region with more than 0.12 MHz frequency shifts, but a lower correlation (R(2) = 0.4) between the width of the coagulated area and the temperature elevation. In conclusion, correlation was found between echo frequency shifts and temperature elevations and between echo frequency shifts and the width of the ablated area during intermittent RF ablation. Our results suggest that, with further refinement and validation, ultrasound could be used to measure RF heating and its induced coagulation.

Treatment monitoring and thermometry for therapeutic focused ultrasound

Therapeutic ultrasound is currently enjoying increasingly widespread clinical use especially for the treatment of cancer of the prostate, liver, kidney, breast, pancreas and bone, as well as for the treatment of uterine fibroids. The optimum method of treatment delivery varies between anatomical sites, but in all cases monitoring of the treatment is crucial if extensive clinical acceptance is to be achieved. Monitoring not only provides the operating clinician with information relating to the effectiveness of treatment, but can also provide an early alert to the onset of adverse effects in normal tissue. This paper reviews invasive and non-invasive monitoring methods that have been applied to assess the extent of treatment during the delivery of therapeutic ultrasound in the laboratory and clinic (follow-up after treatment is not reviewed in detail). The monitoring of temperature and, importantly, the way in which this measurement can be used to estimate the delivered thermal dose, is dealt with as a separate special case. Already therapeutic ultrasound has reached a stage of development where it is possible to attempt real-time feedback during exposure in order to optimize each and every delivery of ultrasound energy. To date, data from MR imaging have shown better agreement with the size of regions of damage than those from diagnostic ultrasound, but novel ultrasonic techniques may redress this balance. Whilst MR currently offers the best method for non-invasive temperature measurement, the ultrasound techniques under development, which could potentially offer more rapid visualisation of results, are discussed.

A real-time measure of cavitation induced tissue disruption by ultrasound imaging backscatter reduction

A feedback method for obtaining real-time information on the mechanical disruption of tissue through ultrasound cavitation is presented. This method is based on a substantial reduction in ultrasound imaging backscatter from the target volume as the tissue structure is broken down. Ex-vivo samples of porcine liver were exposed to successive high-intensity ultrasound pulses at a low duty cycle to induce mechanical disruption of tissue parenchyma through cavitation (referred to as histotripsy). At the conclusion of treatment, B-scan imaging backscatter was observed to have decreased by 22.4 +/- 2.3 dB in the target location. Treated samples of tissue were found to contain disrupted tissue corresponding to the imaged hypoechoic volume with no remaining discernable structure and a sharp boundary. The observed, substantial backscatter reduction may be an effective feedback mechanism for assessing treatment efficacy in ultrasound surgery using pulsed ultrasound to create cavitation.

MR-guided Radiofrequency Ablation: Do Magnetic Fields Influence Extent of Coagulation in ex Vivo Bovine Livers?

PURPOSE

To prospectively determine if static magnetic fields of magnetic resonance (MR) imagers affect radiofrequency (RF) ablation coagulation volume and shape.

MATERIALS AND METHODS

Ex vivo RF ablations of bovine livers were performed with magnetic field strengths of 0.2, 1.5, and 3.0 T and were compared with ablations performed outside the magnetic field in a control group. Two MR-compatible monopolar RF devices (internally cooled single and cluster electrodes) were systematically tested. Length of long axis (y-axis), length of two short axes (x- and z-axes), and coagulation volume and shape measured outside and inside different magnetic fields were compared with the Dunnett test. Significance level was set to .05.

RESULTS

For the single electrode, no significant difference was observed between length of short axes and coagulation volume and shape measured inside and outside the magnetic field. Mean x- and z-axis lengths were 2.3 and 2.6 cm, respectively, outside the magnetic field; 2.4 and 2.4 cm, respectively, at 0.2 T; 2.5 and 2.6 cm, respectively, at 1.5 T; and 2.2 and 2.5 cm, respectively, at 3.0 T. Differences between length of long axis, length of short axis perpendicular to static magnetic field, and coagulation volume and shape achieved with the cluster electrode inside and outside the magnetic field were not significant. Mean x- and z-axis lengths were 3.9 and 3.9 cm, respectively, outside the magnetic field; 3.7 and 3.8 cm, respectively, at 0.2 T; 4.0 and 4.3 cm, respectively, at 1.5 T; and 3.8 and 3.8 cm, respectively, at 3.0 T. Differences between ablations performed at 1.5 T and those performed in the control group with the cluster electrode were significant (P = .026). In this case, a difference of 4 mm in the length of the short axis parallel to the magnetic field was detected, but there was no significant difference in coagulation volume.

CONCLUSION

No significant differences in coagulation volume and shape could be recorded between RF ablations performed outside and those performed inside the static magnetic field.

Uterine leiomyomas: MR imaging-based thermometry and thermal dosimetry during focused ultrasound thermal ablation

PURPOSE

To retrospectively evaluate magnetic resonance (MR) imaging-based thermometry and thermal dosimetry during focused ultrasound treatments of uterine leiomyomas (ie, fibroids).

MATERIALS AND METHODS

All patients gave written informed consent for the focused ultrasound treatments and the current HIPAA-compliant retrospective study, both of which were institutional review board approved. Thermometry performed during the treatments of 64 fibroids in 50 women (mean age, 46.6 years +/- 4.5 [standard deviation]) was used to create thermal dose maps. The areas that reached dose values of 240 and 18 equivalent minutes at 43 degrees C were compared with the nonperfused regions measured on contrast material-enhanced MR images by using the Bland-Altman method. Volume changes in treated fibroids after 6 months were compared with volume changes in nontreated fibroids and with MR-based thermal dose estimates.

RESULTS

While the thermal dose estimates were shown to have a clear relationship with resulting nonperfused regions, the nonperfused areas were, on average, larger than the dose estimates (means of 1.9 +/- 0.7 and 1.2 +/- 0.4 times as large for areas that reached 240- and 18-minute threshold dose values, respectively). Good correlation was observed for smaller treatment volumes at the lower dose threshold (mean ratio, 1.0 +/- 0.3), but for larger treatment volumes, the nonperfused region extended to locations within the fibroid that clearly were not heated. Variations in peak temperature increase were as large as a factor of two, both between patients and within individual treatments. On average, the fibroid volume reduction at 6 months increased as the ablated volume estimated by using the thermal dose increased.

CONCLUSION

Study results showed good correlation between thermal dose estimates and resulting nonperfused areas for smaller ablated volumes. For larger treatment volumes, nonperfused areas could extend within the fibroid to unheated areas.

Analysis of factors important for transurethral ultrasound prostate heating using MR temperature feedback

The feasibility of using MR thermometry for temperature feedback to control a transurethral ultrasound heating applicator with planar transducers was investigated. The sensitivity of a temperature-based feedback algorithm to spatial (control point area, slice thickness, angular alignment) and non-spatial (imaging time, temperature uncertainty) parameters was evaluated through numerical simulations. The angular alignment of the control point with the ultrasound beam was an important parameter affecting the average spatial error in heat delivery. The other spatial parameters were less influential, thus providing an opportunity to reduce spatial resolution for increased SNR in the MR imaging. The update time was the most important non-spatial parameter determining the performance of the control algorithm. Combined non-spatial and spatial parameters achieved acceptable performance with a voxel size of 3 mm x 3 mm, a 10 mm slice thickness and a 5 s update time. Temperature uncertainty of up to 2 degrees C had little effect on the performance of the control algorithm but did reduce the average error slightly due to a systematic, noise-induced overestimation of the boundary temperature. These simulations imply that MR thermometry performed on clinical 1.5 T imaging systems is of sufficient quality for use as thermal feedback for conformal prostate thermal therapy with transurethral ultrasound heating applicators incorporating planar transducers.

Pulsed cavitational ultrasound therapy for controlled tissue homogenization

Methods were investigated to acoustically control the extent to which cavitation-mediated tissue homogenization is responsible for lesion formation in vitro. These results may guide potential therapeutic procedures that induce damage predominantly via mechanical disruption and, thereby, avoid limitations associated with thermal ablative modalities. Porcine myocardium was insonified at 750 kHz using pulse sequences consisting of high-amplitude pulses (22 MPa Pr) interleaved with variable-amplitude "sustaining" pulses (e.g., 6.9 MPa Pr), which were intended to provide sufficient acoustic input to maintain cavitation activity between primary pulses, but to increase the spatial peak temporal average intensity (I(SPTA)) only marginally. Using modest temporal-average intensities (e.g., I(SPTA) approximately 200 W/cm2), approximately 0.5 cm3 lesions were produced consisting of homogenate that could be irrigated away to reveal smooth cavities. The prevalence of homogenate in a given lesion was sensitive to both pulse-repetition frequency and sustaining pulse amplitude, suggesting the existence of optimum acoustic parameters for producing homogenized lesions largely via mechanical perturbation.

Noninvasive Magnetic Resonance Thermography of Recurrent Rectal Carcinoma in a 1.5 Tesla Hybrid System

To implement noninvasive thermometry, we installed a hybrid system consisting of a radiofrequency multiantenna applicator (SIGMA-Eye) for deep hyperthermia (BSD-2000/3D) integrated into the gantry of a 1.5 Tesla magnetic resonance (MR) tomograph Symphony. This system can record MR data during radiofrequency heating and is suitable for application and evaluation of methods for MR thermography. In 15 patients with preirradiated pelvic rectal recurrences, we acquired phase data sets (25 slices) every 10 to 15 minutes over the treatment time (60-90 minutes) using gradient echo sequences (echo time = 20 ms), transformed the phase differences to MR temperatures, and fused the color-coded MR-temperature distributions with anatomic T1-weighted MR data sets. We could generate one complete series of MR data sets per patient with satisfactory quality for further analysis. In fat, muscle, water bolus, prostate, bladder, and tumor, we delineated regions of interest (ROI), used the fat ROI for drift correction by transforming these regions to a phase shift zero, and evaluated the MR-temperature frequency distributions. Mean MR temperatures (T(MR)), maximum T(MR), full width half maximum (FWHM), and other descriptors of tumors and normal tissues were noninvasively derived and their dependencies outlined. In 8 of 15 patients, direct temperature measurements in reference points were available. We correlated the tumor MR temperatures with direct measurements, clinical response, and tumor features (volume and location), and found reasonable trends and correlations. Therefore, the mean T(MR) of the tumor might be useful as a variable to evaluate the quality and effectivity of heat treatments, and consequently as optimization variable. Feasibility of noninvasive MR thermography for regional hyperthermia has been shown and should be further investigated.

Interleaved echo-planar imaging for fast multiplanar magnetic resonance temperature imaging of ultrasound thermal ablation therapy

PURPOSE

To develop a multiplanar magnetic resonance temperature imaging (MRTI) technique based on interleaved gradient-echo echo-planar imaging (EPI), verify in phantom, develop software tools to process and display data on a clinical scanner in near real-time, and demonstrate feasibility to monitor ultrasound thermal ablation therapy in vivo.

MATERIALS AND METHODS

Temperature estimation used complex phase-difference subtraction of the EPI MRTI data to indirectly measure the temperature-dependent water proton-resonance-frequency shift. Software tools were developed to run on a clinical 1.5-T MR scanner that processed and displayed relevant temperature and thermal dosimetry data during the course of thermal ablation treatments in canine brain and prostate in vivo.

RESULTS

EPI MRTI provided multi-planar acquisitions and increased temperature sensitivity and lipid suppression. Relative to a single-plane fast gradient-echo MRTI sequence at comparable spatial and temporal resolutions in phantom, EPI MRTI demonstrated a three-fold increase in sensitivity and slice coverage per TR. In vivo monitoring of ultrasound thermal ablation therapy in canine brain and prostate demonstrated the usefulness of the temperature and thermal dose information.

CONCLUSION

Multi-planar MRTI allowed progression of thermal damage to be monitored and treatment parameters adjusted in near real-time (less than five second delay). EPI MRTI is an effective multi-planar monitoring method during ultrasound thermal ablation procedures.

Monitoring the formation of thermal lesions with heat-induced echo-strain imaging: a feasibility study

We investigated the feasibility of using echo-strain images to visualize the extent of high-intensity ultrasound (US)-induced thermal lesions during their formation. Echo-strain, defined as the relative deformation of the backscattered ultrasonic signal, is due to tissue expansion and to changes in the speed of sound during heating. First, a theoretical framework was developed to predict the influence of these effects on the echo signal. Then, a simulation tool was developed to create simulated echo-strain images in thermal lesions. Finally, experimental echo-strain images were acquired in 10 porcine liver samples in vitro for various exposure durations and ultrasonic intensities (resulting in lesions that extended 3 to 8 mm deep from the surface). For this purpose, radiofrequency (RF) frames were acquired at 8 frames per s while heating. For each consecutive pair of RF frames, an echo-strain image was calculated using standard elastographic processing. The echo-strain images were cumulated and displayed. The experimental echo-strain images were compared with gross pathology. The (isoechoic) lesions were visible both in simulated and in experimental cumulated echo-strain images as apparent expansion areas (tensile echo-strain), whereas surrounding tissues exhibited apparent compression. The tensile echo-strain area underestimated the lesion in simulations, but was representative of the lesion in experiments. High correspondence was found between the lesion depth measured from experimental cumulative echo-strain images (y) and from gross pathology (x) (Pearson's correlation = 0.90, linear regression y = x-0.1 mm, residual error = 0 +/- 0.9 mm). We hypothesized that significant tissue expansion made the thermal lesions highly visible in the experimental echo-strain images. In two cases, the ultrasonic intensity was too low to induce a lesion, and the corresponding experimental echo-strain images showed no visible lesion. We conclude that cumulative echo-strain images have the potential to monitor the formation of high-intensity US-induced thermal lesions.

MR imaging–guided breast ablative therapy

The integration of imaging and thermal therapy can provide a minimally invasive or even noninvasive alternative to breast surgery for small tumors. Ongoing trials seek to show safety and efficacy for laser, radiofrequency, microwave, cryoablation, and focused ultrasound surgery. To be successful, these therapies must achieve equivalent or even greater efficacy as surgical outcomes and must demonstrate total ablation of the dominant lesion with negative margins, while sparing normal tissue beyond the target tissue. Procedures have been validated by histopathology subsequent to resection.

Termoablação a laser de tumores hepáticos: atualização

A termoablação por raio laser de tumores hepáticos tem despontado como alternativa válida de tratamento em pacientes que não são candidatos a ressecção cirúrgica. O procedimento pode ser realizado por via percutânea, laparoscópica ou por laparotomia, e orientado por métodos de imagem. O objetivo deste trabalho é apresentar o mecanismo de ação deste método, bem como as suas indicações, contra-indicações, complicações e resultados clínicos, baseados em revisão bibliográfica.

Magnetic Resonance Imaging Characteristics of Six Radiofrequency Electrodes in a Phantom Study

PURPOSE

To evaluate and compare visibility and artifacts in magnetic resonance (MR) compatible radiofrequency (RF) electrodes for MR-guided RF ablation.

MATERIAL AND METHODS

Six different MR compatible electrodes for RF ablation including two internally cooled single needles, one internally cooled cluster needle, two expandable needles and one perfused needle were tested in a phantom study at 0.2 Tesla and at 1.5 Tesla field strength. Fluoroscopic, T1- and T2-weighted fast spin echo (FSE) and gradient echo (GE) sequences, which are usually used for MR-guided interventions, were evaluated. Qualitative and quantitative evaluations were performed. Length, width, noise, tip artifacts, global artifacts and global visualization of the RF electrodes that showed all sequences at different angles.

RESULTS

Qualitative analysis showed that electrodes were well visualized at all angles and sequences and on both MR imagers. Quantitative analysis showed that artifact-induced widening of the shaft was increased in all electrodes by: a). use of fluoroscopic sequences, GE sequences, and fat saturation, b). increasing the angle between the needle and main magnetic field, and c). high field strength (1.5 T). Expandable needles produced fewer tip artifacts but broader signal voids along the shaft compared to nonexpandable needles. Cluster electrodes produced less widening than the other electrodes.

CONCLUSION

Visibility and artifacts in all six MR compatible RF electrodes are satisfactory and these electrodes could be used for MR-guided radiofrequency ablation procedures.

Prediction of subtle thermal histopathological change using a novel analysis of Gd-DTPA kinetics

PURPOSE

To investigate Gd-DTPA kinetics as predictors of histopathological changes following focused ultrasound (FUS) thermal ablation for improved planning and assessment.

MATERIALS AND METHODS

Twenty-nine FUS lesions were created in the thigh muscle of eight rabbits under MR-guidance at 1.5 Tesla. Three rabbits were killed at four hours; and 11 lesions were analyzed with histopathology. Temperature-sensitive MRI using proton-resonant frequency-shift was used for time-dependent temperature measurements. Analysis of the uptake kinetics of Gd-DTPA was performed after Gd-DTPA injection, within 20 minutes after heating and again at two hours after heating. The resulting kinetic maps, permeability (K(trans)) and leakage space (v(e)), were correlated to peak temperatures, T(2)-weighted MR, and histopathology.

RESULTS

Images of K(trans) and v(e) reveal regions of histopathological change not visible on conventional post-therapy MR. At early times after heating, v(e) predicts the area of injury more accurately than T(2) (7 +/- 2% vs. 25 +/- 6% underestimation). A circular region of extensive structural/vascular disruption is indicated only on K(trans) maps. The sharp decrease in K(trans) at the boundary of this region occurs at 47.5 +/- 0.5 degrees C, and may be a better estimate of cell death than the conventional method of temperature threshold (55 degrees C for coagulation) used in therapy planning.

CONCLUSION

Our results suggest Gd-DTPA kinetics can predict different histopathological changes following FUS ablation and may be valuable for early prediction.

High intensity focused ultrasound ablation of kidney guided by MRI

The effectiveness of magnetic resonance imaging (MRI) to monitor therapeutic protocols of high-intensity focused ultrasound (HIFU), in freshly excised pig kidney cortex is investigated. For high quality imaging, the pulse sequence fast spin echo (FSE) T1- and T2-weighted, and proton density were evaluated. For fast imaging, the pulse sequence T1-weighted fast spoiled gradient (FSPGR) was used. The main goal was to evaluate the MRI detection of large lesions (bigger than 1 cm x 1 cm x 1 cm) that is achieved by moving the transducer in a predetermined pattern. The contrast between lesion and kidney tissue is excellent with either T1-weighted or T2-weighted FSE. With T1-weighted FSE, the best contrast is observed for recovery time (TR) between 200 ms and 400 ms. With T2-weighted FSE best contrast can be achieved for echo time (TE) between 16 and 32 ms. T2-weighted FSE was proven as the best pulse sequence to detect cavitational activity. This advantage is attributed to the significant difference in signal intensity between air spaces and necrotic tissue. Air spaces appear brighter than thermal lesions. Therefore, for therapeutic protocols created using cavitational mode, T2-weighted FSE may be the optimum pulse sequence to use. The proton density pulse sequence does not provide any advantage over the T1- and T2-weighted pulse sequences. Using T1-weighted FSPGR, acquisition time as low as 5 s could be achieved. Good contrast and signal-to-noise ratio (SNR) are achieved with TR = 100 ms and flip angle between 75 to 90 degrees. The above techniques were very successful in detecting large lesion volumes.

MRI investigation of the threshold for thermally induced blood-brain barrier disruption and brain tissue damage in the rabbit brain

The ability of MRI-derived thermometry to predict thermally induced tissue changes in the brain was tested, and the thermal thresholds for blood-brain barrier (BBB) disruption and brain tissue damage were estimated. In addition, the ability of standard MRI to detect threshold-level effects was confirmed. These safety thresholds are being investigated to provide guidelines for clinical thermal ablation studies in the brain. MRI-monitored focused ultrasound heating was delivered to 63 locations in 26 rabbits. Tissue changes were detected in T(2)-weighted imaging and T(1)-weighted imaging (with and without contrast) and with light microscopy. The probability for tissue damage as a function of the accumulated thermal dose, the peak temperature achieved, the applied acoustic energy, and the peak acoustic power was estimated with probit regression. The discriminative abilities of these parameters were compared using the areas under the receiver operator characteristic (ROC) curves. In MRI, BBB disruption was observed in contrast-enhanced T(1)-weighted imaging shortly after the ultrasound exposures, sometimes accompanied by changes in T(2)-weighted imaging. Two days later, changes in T(2)-weighted imaging were observed, sometimes accompanied by changes in T(1)-weighted imaging. In histology, tissue damage was seen at every location where MRI changes were observed, ranging from small (diameter <1.0 mm) areas of tissue necrosis to severe vascular damage and associated hemorrhagic infarct. In one location, small (diameter: 0.8 mm) damage was not detected in MRI. The thermal dose and peak temperature thresholds were between 12.3-40.1 equivalent min at 43 degrees C and 48.0-50.8 degrees C, respectively, and values of 17.5 equivalent min at 43 degrees C and 48.4 degrees C were estimated to result in tissue damage with 50% probability. Thermal dose and peak temperature were significantly better predictors than the applied acoustic energy and peak acoustic power (P < 0.01). BBB disruption was always accompanied by tissue damage. The temperature information was better than the applied acoustic power or energy for predicting the damage than the ultrasound parameters. MRI was sensitive in detecting threshold-level damage.

Feasibility of MR-guided focused ultrasound with real-time temperature mapping and continuous sonication for ablation of VX2 carcinoma in rabbit thigh

The efficiency of MRI-guided focused ultrasound (FUS) hyperthermia with continuous sonication was investigated for the treatment of VX2 carcinoma implanted in rabbit thigh muscle. Six rabbits were treated with a single session of FUS when the tumor diameter exceeded 2 cm (10-21 days after implant). The FUS treatment method was based on a spiral trajectory of the focal point that allows continuous sonication under automatic, real-time MR guidance. The total heating time was approximately 1000 sec. Efficacy of treatment was evaluated twice a week based on clinical (weight) and MRI data. Treated animals were sacrificed 5 weeks after the heating procedure and histological analysis was performed. Tumor regression was observed in each treated animal. Complete ablation of tumor, with confirmation by histological analysis, was obtained in five of six treated cases. Tumor regrowth occurred in one animal. Thermal injury was limited to the targeted region in three cases, but ablation also reached some healthy muscle around the tumor in the other three cases. A good correlation was found between postmortem histological analysis and premortem MRI data. Efficacy of MR-controlled hyperthermia using FUS heating with spiral trajectories was demonstrated for successful local control of intramuscular VX2 tumor.

Vergleich nichtinvasiver MRT-Verfahren zur Temperaturmessung für den Einsatz bei medizinischen Thermotherapien

Novel methods for hyperthermia tumor therapy, such as high-intensity focused ultrasound (HIFU) or laser-induced thermotherapy (LITT), require accurate non-invasive temperature monitoring. Non-invasive temperature measurement using magnetic resonance imaging (MRI) is based on the analysis of changes in longitudinal relaxation time (T1), diffusion coefficient (D), or water proton resonance frequency (PRF). The purpose of this study was the development and comparative analysis of the three different approaches of MRI temperature monitoring (T1, D, and PRF). Measurements in phantoms (e.g., ultrasound gel) resulted in the following percent changes: T1-relaxation time: 1.98%/degree C; diffusion coefficient: 2.22%/degree C; and PRF: -0.0101 ppm/degree C. All measurements were in good agreement with the literature. Temperature resolutions could also be measured from the inverse correlation of the data over the whole calibration range: T1: 2.1 +/- 0.6 degrees C; D: 0.93 +/- 0.2 degree C; and PRF: 1.4 +/- 0.3 degrees C. The diffusion and PRF methods were not applicable in fatty tissue. The use of the diffusion method was restricted due to prolonged echo time and anisotropic diffusion in tissue. Initial tests with rabbit muscle tissue in vivo indicated that MR thermometry via T1 and PRF procedures is feasible to monitor the local heating process induced by HIFU. The ultrasound applicators in the MR scanner did not substantially interfere with image quality.

Magnetic resonance imaging-guided laser thermal ablation of renal tumours

OBJECTIVE

To test the hypothesis that magnetic resonance imaging (MRI)-guided laser thermal ablation (LTA) of inoperable renal tumours is a safe, tolerable and potentially effective treatment.

PATIENTS AND METHODS

Nine patients (aged 56-81 years) with malignant renal tumours underwent percutaneous LTA under MRI guidance in a 0.5 T open magnet. Real-time colour thermal mapping was used to monitor tumour ablation, and the follow-up was with gadolinium-enhanced MRI at 6 weeks and (where appropriate) 3-4 months after the procedure. Tumour volume and percentage tumour enhancement before and after ablation were compared. The percentage of tumour ablated on real-time T1-weighted thermal maps was compared with that on gadolinium-enhanced follow-up MRI.

RESULTS

The mean (range) follow-up was 16.9 (3-32) months after the first ablation. The mean tumour size did not change significantly, but the mean percentage of viable tumour decreased significantly from 73.7% before to 29.5% after ablation (P = 0.012, Wilcoxon signed-ranks test). Thermal maps correlated moderately well with follow-up MRI in predicting the extent of tumour ablation (Pearson correlation coefficient 0.55). There were two minor and one major complication.

CONCLUSION

In this pilot study of patients unsuitable for surgery, MRI-guided LTA of renal tumours was safe, feasible (being well tolerated by the patient) and significantly reduced enhancing tumour volume by a mean of 45%.

Intraoperative magnetic resonance imaging and magnetic resonance imaging-guided therapy for brain tumors

Since their introduction into surgical practice in the mid 1990s, intraoperative MRI systems have evolved into essential, routinely used tools for the surgical treatment of brain tumors in many centers. Clear delineation of the lesion, "under-the-surface" vision, and the possibility of obtaining real-time feedback on the extent of resection and the position of residual tumor tissue (which may change during surgery due to "brain-shift") are the main strengths of this method. High-performance computing has further extended the capabilities of intraoperative MRI systems, opening the way for using multimodal information and 3D anatomical reconstructions, which can be updated in "near real time." MRI sensitivity to thermal changes has also opened the way for innovative, minimally invasive (LASER ablations) as well as noninvasive therapeutic approaches for brain tumors (focused ultrasound). Although we have not used intraoperative MRI in clinical applications sufficiently long to assess long-term outcomes, this method clearly enhances the ability of the neurosurgeon to navigate the surgical field with greater accuracy, to avoid critical anatomic structures with greater efficacy, and to reduce the overall invasiveness of the surgery itself.