Verification of a hyperthermia model method using MR thermometry

Ultrasound Hyperthermia Technology for Radiosensitization

Hyperthermia therapy (HT) raises tissue temperature to 40-45°C for up to 60 min. Hyperthermia is one of the most potent sensitizers of radiation therapy (RT). Ultrasound-mediated HT for radiosensitization has been used clinically since the 1960s. Recently, magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU), which has been approved by the United States Food and Drug Administration for thermal ablation therapy, has been adapted for HT. With emerging clinical trials using MRgHIFU HT for radiosensitization, there is a pressing need to review the ultrasound HT technology. The objective of this review is to overview existing HT technology, summarize available ultrasound HT devices, evaluate clinical studies combining ultrasound HT with RT and discuss challenges and future directions.

Optimization of Single Voxel MR Spectroscopy Sequence Parameters and Data Analysis Methods for Thermometry in Deep Hyperthermia Treatments

OBJECTIVE

The difference in the resonance frequency of water and methylene moieties of lipids quantifies in magnetic resonance spectroscopy the absolute temperature using a predefined calibration curve. The purpose of this study was the investigation of peak evaluation methods and the magnetic resonance spectroscopy sequence (point-resolved spectroscopy) parameter optimization that enables thermometry during deep hyperthermia treatments.

MATERIALS AND METHODS

Different Lorentz peak-fitting methods and a peak finding method using singular value decomposition of a Hankel matrix were compared. Phantom measurements on organic substances (mayonnaise and pork) were performed inside the hyperthermia 1.5-T magnetic resonance imaging system for the parameter optimization study. Parameter settings such as voxel size, echo time, and flip angle were varied and investigated.

RESULTS

Usually all peak analyzing methods were applicable. Lorentz peak-fitting method in MATLAB proved to be the most stable regardless of the number of fitted peaks, yet the slowest method. The examinations yielded an optimal parameter combination of 8 cm³ voxel volume, 55 millisecond echo time, and a 90° excitation pulse flip angle.

CONCLUSION

The Lorentz peak-fitting method in MATLAB was the most reliable peak analyzing method. Measurements in homogeneous and heterogeneous phantoms resulted in optimized parameters for the magnetic resonance spectroscopy sequence for thermometry.

A method to convert MRI images of temperature change into images of absolute temperature in solid tumours

PURPOSE

During hyperthermia (HT), the therapeutic response of tumours varies substantially within the target temperature range (39-43 °C). Current thermometry methods are either invasive or measure only temperature change, which limits the ability to study tissue responses to HT. This study combines manganese-containing low temperature sensitive liposomes (Mn-LTSL) with proton resonance frequency shift (PRFS) thermometry to measure absolute temperature in tumours with high spatial and temporal resolution using MRI.

METHODS

Liposomes were loaded with 300 mM MnSO(4). The phase transition temperature (T(m)) of Mn-LTSL samples was measured by differential scanning calorimetry (DSC). The release of manganese from Mn-LTSL in saline was characterised with inductively coupled plasma atomic emission spectroscopy. A 2T GE small animal scanner was used to acquire dynamic T1-weighted images and temperature change images of Mn-LTSL in saline phantoms and fibrosarcoma-bearing Fisher-344 rats receiving hyperthermia after Mn-LTSL injection.

RESULTS

The T(m) of Mn-LTSL in rat blood was 42.9 ± 0.2 °C (DSC). For Mn-LTSL samples (0.06 mM-0.5 mM Mn(2+) in saline) heated monotonically from 30 °C to 50 °C, a peak in the rate of MRI signal enhancement occurred at 43.1° ± 0.3 °C. The same peak in signal enhancement rate was observed during heating of fibrosarcoma tumours (N = 3) after injection of Mn-LTSL, and the peak was used to convert temperature change images into absolute temperature. Accuracies of calibrated temperature measurements were in the range 0.9-1.8 °C.

CONCLUSION

The release of Mn(2+) from Mn-LTSL affects the rate of MR signal enhancement which enables conversion of MRI-based temperature change images to absolute temperature.

MR-based thermometry of laser induced thermotherapy: temperature accuracy and temporal resolution in vitro at 0.2 and 1.5 T magnetic field strengths

PURPOSE

To evaluate MR-thermometry using fast MR sequences for laser induced interstitial thermotherapy (LITT) at 0.2 and 1.5 T systems.

METHODS & MATERIALS

In-vitro experiments were performed using Agarose gel mixture and lobes of porcine liver. MR-thermometry was performed by means of longitudinal relaxation time (T1) and proton resonance frequency shift (PRF) methods under acquisition of amplitude and phase shift images. Four different sequences were used for T1 thermometry: A gradient-echo (GRE), a True Fast Imaging with Steady Precession (TRUFI), a Saturation Recovery Turbo-FLASH (SRTF), and an Inversion Recovery Turbo-FLASH (IRTF) sequence (FLASH-Fast Low Angle Shot). PRF was measured with four sequences: Two fast-spoiled GRE sequences (one as WIP sequence), a Turbo-FLASH (TFL) sequence (WIP sequence), and a multiecho-TrueFISP sequence. Temperature was controlled and verified using a fiber-optic Luxtron device. The temperature was correlated with the MR measurement.

RESULTS

All sequences showed a good linear correlation R(2) = 0.97-0.99 between the measured temperature and the MR-thermometry measurements. The only exception was the TRUFI sequence in the Agarose phantom that showed a non-linear calibration curve R(2) = 0.39-0.67. At 1.5 T, the Agarose experiments revealed similar temperature accuracies of 4-6°C for all sequences excluding TRUFI. During experiments with the liver, the PRF sequences showed better performance than the T1, with accuracies of 5-12°C, contrary to the T1 sequences at 14-18°C. The accuracy of the Siemens PRF-FLASH sequence was 5.1°C. At 0.2 T, the Agarose experiments provided the highest accuracy of 3.3°C for PRF measurement. At the liver experiments the T1 sequences SRTF and FLASH revealed the best accuracies at 6.4 and 7.0°C.

CONCLUSION

The accuracy and speed of MR temperature measurements are sufficient for controlling the temperature-based tumor destruction. For 0.2 T systems SRTF and FLASH sequences are recommended. For 1.5 T systems SRTF and FLASH are the most accurate.

Induction of luciferase activity under the control of an hsp70 promoter using high-intensity focused ultrasound: combination of bioluminescence and MRI imaging in three different tumour models

The in vivo temporal changes of luciferase activity were investigated under the control of an hsp70 promoter in three tumour models after the application of different intensities of high-intensity focused ultrasound (HIFU). Three cell lines, SCCVII, NIH3T3 and M21 were stably transfected with a plasmid containing the hsp70 promoter and luciferase reporter gene, and tumours were subcutaneously initiated into mice. At a size of 1300 ± 234 mm(3), the tumours were exposed to five intensities of continuous HIFU (802-1401-2157-3067-4133 W/cm(2)) for 20 sec. Bioluminescence and MR imaging were performed to assess luciferase activity and signal intensity changes in the tissue. The MRI scan protocol was pre- and post-contrast T1-wt-SE, T2-wt-FSE, DCE-MRI, diffusion-wt STEAM sequence, T2 relaxation time determination obtained on a 1.5-T GE MRI scanner. The NIH3T3 tumours showed the highest luciferase activity of 328.1 ± 7.1 fold at 24 h at a HIFU intensity of 3067 W/cm(2), the M21 tumours of 3.2 ± 0.6 fold 8 hours and the SCCVII tumours 2.9 ± 0.9 fold 4 hours post-HIFU at 2157 W/cm(2). The greatest increase in T2 signal intensity and T2 relaxation time of 20.7 ± 3.4% was seen in the SCCVII tumours. The highest contrast medium uptake of 10.1 ± 1.1% was noted in the M21 tumours, and 14.8 ± 1.9% in the SCCVII tumours. In all tumours, a significant increase in the diffusion coefficient was seen with increased HIFU intensity, the highest of which was 40.3 ± 4.1% in the SCCVII tumours. The three tumour cell lines stably transfected with the hsp70/luciferase gene showed differential luciferase activity, which peaked at different times after the application of HIFU and was dependant on tumour type and HIFU energy deposition.

Comprehensive analysis of the Cramer-Rao bounds for magnetic resonance temperature change measurement in fat-water voxels using multi-echo imaging

OBJECT

The aim of this paper is to characterize the noise propagation for MRI temperature change measurement with emphasis on finding the best echo time combinations that yield the lowest temperature noise.

MATERIALS AND METHODS

A Cramer-Rao lower-bound (CRLB) calculation was used to estimate the temperature noise for a model of the MR signal in fat-water voxels. The temperature noise CRLB was then used to find a set of echo times that gave the lowest temperature change noise for a range of fat-water frequency differences, temperature changes, fat/water signal ratios, and T2* values. CRLB estimates were verified by Monte Carlo simulation and in phantoms using images acquired in a 1.5 T magnet.

RESULTS

Results show that regions exist where the CRLB predicts minimal temperature variation as a function of the other variables. The results also indicate that the CRLB values calculated in this paper provide excellent guidance for predicting the variation of temperature measurements due to changes in the signal parameters. For three echo scans, the best noise characteristics are seen for TE values of 20.71, 23.71, and 26.71 ms. Results for five and seven echo scans are also presented in the text.

CONCLUSION

The results present a comprehensive analysis of the effects of different scan parameters on temperature noise, potentially benefiting the selection of scan parameters for clinical MRI thermometry.

Correction of breathing-induced errors in magnetic resonance thermometry of hyperthermia using multiecho field fitting techniques

PURPOSE

Breathing motion can create large errors when performing magnetic resonance (MR) thermometry of the breast. Breath holds can be used to minimize these errors, but not eliminate them. Between breath holds, the referenceless method can be used to further reduce errors by relying on regions of nonheated fatty tissue surrounding the heated region. When the surrounding tissue is heated (i.e., for a hyperthermia treatment), errors can result due to phase changes of the small amounts of water in the tissue. Therefore, an extension of the referenceless method is proposed which fits for the field in fatty tissue independent of temperature change and extrapolates it to the water-rich regions.

METHODS

Nonheating experiments were performed with male volunteers performing breath holds on top of a phantom mimicking a breast with a tumor. Heating experiments were also conducted with the same phantom while mechanically simulated breath holds were performed. A nonheating experiment was also performed with a healthy female breast. For each experiment, a nonlinear fitting algorithm was used to fit for temperature change and B0 field inside of the fatty tissue. The field changes were then extrapolated into water-rich (tumor) portions of the image using a least-squares fit to a fifth-order equation, to correct for field changes due to breath hold changes. Similar results were calculated using the image phase, to mimic the use of the referenceless method.

RESULTS

Phantom results showed large reduction of mean error and standard deviation. In the non-heating experiments, the traditional referenceless method and our extended method both corrected by similar amounts. However, in the heating experiments, the average deviation of the temperature calculated with the extended method from a fiber optic probe temperature was approximately 50% less than the deviation with the referenceless method. The in vivo breast results demonstrated reduced standard deviation and mean.

CONCLUSIONS

In this paper, we have developed an extension of the referenceless method to correct for breathing errors using multiecho fitting methods to fit for the B0 field in the fatty tissue and using measured field changes as references to extrapolate field corrections into a water-only (tumor) region. This technique has been validated in a number of situations, and in all cases, the correction method has been shown to greatly reduce temperature error in water-rich regions. The method has also been shown to be an improvement over similar methods that use image phase changes instead of field changes, particularly when temperature changes are induced.

Reliability of temperature and SAR measurements at oesophageal tumour locations

INTRODUCTION

For treatment of oesophageal cancer, neo-adjuvant locoregional hyperthermia (HT) has been applied in combination with chemotherapy (ChT) +/- radiotherapy (RT) at the institute. Until now, 26 patients were treated within a completed phase I study combining HT with ChT and 29 patients within an ongoing phase II study combining HT with ChT + RT.

METHODS

HT was given with the 70 MHz AMC-4 waveguide system. Initially, oesophageal temperatures were measured using multi-sensor thermocouple probes (TCs) inside a nasogastric tube (NT), but the question arose whether these measurements were reliable enough to quantify the achieved tumour temperatures accurately. Presently, TCs are mounted on the outside of an inflatable balloon catheter (BC) for better intra-luminal fixation and better contact with the tumour. During 14 treatment sessions in four patients TCs inside a NT and mounted on a BC were used simultaneously. Data from these 14 treatment sessions were used to compare temperature and Specific Absorption Rate (SAR) measurements ('DeltaT-measurements') using NTs or BCs. To determine the predictive value of the local SAR for the tumour temperatures achieved during treatment, the relation between the initial DeltaT and steady state temperature (SST) was evaluated.

RESULTS

There was a strong correlation between the temperature measured in the NT (Ttube) and the temperature measured with a BC (Tballoon): R = 0.88 +/- 0.13. However, Ttube was on average approximately 1 degrees C higher than Tballoon and there was a large variation between the different treatments in the relation between both measurements, rendering Ttube a probably unreliable measure for tumour temperatures. The correlation between the DeltaT measured in the NT (DeltaTtube) and with a BC (DeltaTballoon) was rather weak: R = 0.46 +/- 0.25. The correlation between the initial DeltaT and the SST was much stronger for the BC measurements, R = 0.78 +/- 0.19, than for the NT measurements, R = 0.61 +/- 0.23. Thus, DeltaTballoon has a higher predictive value for the achieved tumour temperatures than DeltaTtube. Both DeltaT and SST were generally higher for the NT measurements than for the BC measurements, suggesting an over-estimation of tumour temperatures. Averaged over all treatments in the phase I trial using a NT (20 treatments) or a BC (45 treatments), T90 was significantly higher when measured with a NT.

CONCLUSION

Oesophageal temperature and SAR (DeltaT) measurements inside a NT are less reliable than BC measurements. These artefacts are due to bad thermal contact with the tumour tissue and are, therefore, not specific for thermocouple thermometry. For reliable temperature or SAR measurements inside lumina or cavities good thermal contact must be assured, e.g. by using a balloon catheter.

FDTD simulations to assess the performance of CFMA-434 applicators for superficial hyperthermia

INTRODUCTION

Contact flexible microstrip applicators (CFMA), operating at 434 MHz, are applied at the Academic Medical Center (AMC) for superficial hyperthermia (e.g. chest wall recurrences and melanoma). This paper investigates the performance of CFMA, evaluating the stability of the specific absorption rate (SAR) distribution, effective heating depth (EHD) and effective field size (EFS) under different conditions.

METHODS

Simulations were performed using finite differences and were compared to existing measurement data, performed using a rectangular phantom with a superficial fat-equivalent layer of 1 cm and filled with saline solution. The electrode plates of the applicators measure approximately 7 x 20, 29 x 21 and 20 x 29 cm(2). Bolus thickness varied between 1 and 2 cm. The impact of the presence of possible air layers between the rubber frame and the electrodes on the SAR distribution was investigated.

RESULTS

The EHD was approximately 1.4 cm and the EFS ranged between approximately 60 and approximately 300 cm(2), depending on the applicator type. Both measurements and simulations showed a split-up of the SAR focus with a 2 cm water bolus. The extent and location of air layers has a strong influence on the shape and size of the iso-SAR contours with a value higher than 50%, but the impact on EFS and EHD is limited.

CONCLUSION

Simulations, confirmed by measurements, showed that the presence of air between the rubber and the electrodes changes the iso-SAR contours, but the impact on the EFS and EHD is limited.

Hyperthermia MRI temperature measurement: evaluation of measurement stabilisation strategies for extremity and breast tumours

PURPOSE

MR thermometry using the proton resonance frequency shift (PRFS) method has been used to measure temperature changes during clinical hyperthermia treatment. However, frequency drift of the MRI system can add large errors to the measured temperature change. These drifts can be measured and corrected using oil references placed around the treatment region. In this study, the number and position of four or more oil references were investigated to obtain a practical approach to correct frequency drift during PRFS thermometry in phantoms and in vivo.

MATERIALS AND METHODS

Experiments were performed in a 140 MHz four antenna mini-annular phased array (MAPA) heat applicator (for treatment of extremity tumours) and an applicator for heating of the breast, with symmetric and asymmetric positioning of the oil references, respectively. Temperature change PRFS images were obtained during an hour or more of measurement with no application of heat. Afterwards, errors in calculating temperature change due to system drift were quantified with and without various oil reference correction arrangements.

RESULTS

Results showed good temperature correction in phantoms and in a human leg, with average errors of 0.28 degrees C and 0.94 degrees C respectively. There was further improvement in the leg when using eight or more oil references, reducing the average error to 0.44 degrees C, while the phantoms showed no significant improvement.

CONCLUSIONS

These results indicate that oil reference correction performs well in vivo, and that eight references can improve the correction by up to 0.5 degrees C compared to four references.

Absolute temperature imaging using intermolecular multiple quantum MRI

PURPOSE

A review of MRI temperature imaging methods based on intermolecular multiple quantum coherences (iMQCs) is presented. Temperature imaging based on iMQCs can provide absolute temperature maps that circumvent the artefacts that other proton frequency shift techniques suffer from such as distortions to the detected temperature due to susceptibility changes and magnetic field inhomogeneities. Thermometry based on iMQCs is promising in high-fat tissues such as the breast, since it relies on the fat signal as an internal reference. This review covers the theoretical background of iMQCs, and the necessary adaptations for temperature imaging using iMQCs.

MATERIALS AND METHODS

Data is presented from several papers on iMQC temperature imaging. These studies were done at 7T in both phantoms and in vivo. Results from phantoms of cream (homogeneous mixture of water and fat) are presented as well as in vivo temperature maps in obese mice.

RESULTS

Thermometry based on iMQCs offers the potential to provide temperature maps which are free of artefacts due to susceptibility and magnetic field inhomogeneities, and detect temperature on an absolute scale.

CONCLUSIONS

The data presented in the papers reviewed highlights the promise of iMQC-based temperature imaging in fatty tissues such as the breast. The change in susceptibility of fat with temperature makes standard proton frequency shift methods (even with fat suppression) challenging and iMQC-based imaging offers an alternative approach.

On verification of hyperthermia treatment planning for cervical carcinoma patients

PURPOSE

The aim of this study was to verify hyperthermia treatment planning calculations by means of measurements performed during hyperthermia treatments. The calculated specific absorption rate (SAR(calc)) was compared with clinically measured SAR values, during 11 treatments in seven cervical carcinoma patients.

METHODS

Hyperthermia treatments were performed using the 70 MHz AMC-4 waveguide system. Temperatures were measured using multisensor thermocouple probes. One invasive thermometry catheter in the cervical tumour and two non-invasive catheters in the vagina were used. For optimal tissue contact and fixation of the catheters, a gynaecological tampon was inserted, moisturized with distilled water (4 treatments), or saline (6 treatments) for better thermal contact. During one treatment no tampon was used. At the start of treatment the temperature rise (DeltaT(meas)) after a short power pulse was measured, which is proportional to SAR(meas). The SAR(calc) along the catheter tracks was extracted from the calculated SAR distribution and compared with the DeltaT(meas)-profiles.

RESULTS

The correlation between DeltaT(meas) and SAR(calc) was on average R = 0.56 +/- 0.28, but appeared highly dependent on the wetness of the tampon (preferably with saline) and the tissue contact of the catheters. Correlations were strong (R approximately 0.85-0.93) when thermal contact was good, but much weaker (R approximately 0.14-0.48) for cases with poor thermal contact.

CONCLUSION

Good correlations between measurements and calculations were found when tissue contact of the catheters was good. The main difficulties for accurate verification were of clinical nature, arising from improper use of the gynaecological tampon. Poor thermal contact between thermocouples and tissue caused measurement artefacts that were difficult to correlate with calculations.

Steering in locoregional deep hyperthermia: evaluation of common practice with 3D-planning

PURPOSE

In Rotterdam, fifteen years of clinical experience with deep hyperthermia has sublimated in empirical treatment guidelines. In this paper, a hyperthermia treatment planning system (HTPS) is employed to investigate the effect of these guidelines on global power distribution, their effectiveness and the rationale behind each guideline.

MATERIALS AND METHODS

Four guidelines were investigated. The first two prescribe steering actions for balancing intraluminal temperatures and alleviating complaints of deep-seated pain or pressure. The third guideline handles superficial complaints of pain or heat sensation. The last guideline states that frequency should be increased from 77 MHz upwards in case of multiple, opposite, painful regions uncontrollable by the previous steering actions. For all steering actions it is assumed that input power is increased until complaints occur. Sigma Hyperplan was used to calculate specific absorption rate (SAR) distributions for five patient models with locally advanced cervical cancer. Absorbed power ratios of different regions of interest were evaluated to illustrate steering efficacy and complaint reduction.

RESULTS AND CONCLUSIONS

Phase steering is effective in shifting the central power distribution to the periphery, and is an appropriate method to balance temperatures or to handle deep-seated complaints. Reduction of amplitude is the proper action to alleviate superficial complaints of heat or pressure. Compression of the SAR distribution, mainly in the lateral direction, is predicted with increasing frequency. Hence, for complaints in the lower back or on the sides, a frequency increase should be considered. We conclude that the results of the HTPS are in close agreement with the empirical steering guidelines.

Microwave thermal imaging of scanned focused ultrasound heating: phantom results

We are developing a microwave tomographic imaging system capable of monitoring thermal distributions based on the temperature dependence of the recovered dielectric properties. The system has been coupled to a high intensity focused ultrasound (HIFU) therapy device which can be mechanically steered under computer control to generate arbitrarily shaped heating zones. Their integration takes advantage of the focusing capability of ultrasound for the therapy delivery and the isolation of the microwave imaging signal from the power deposition source to allow simultaneous treatment monitoring. We present several sets of phantom experiments involving different types of heating patterns that demonstrate the quality of both the spatial and temporal thermal imaging performance. This combined approach is adaptable to multiple anatomical sites and may have the potential to be developed into a viable alternative to current clinical temperature monitoring devices for HIFU, such magnetic resonance (MR) imaging.

From the RSNA refresher courses: MR imaging in hyperthermia

There is growing clinical evidence that the combination of radiation therapy and hyperthermia, when delivered at moderate temperatures (40 degrees-45 degrees C) for sustained times (30-90 minutes), is of benefit with regard to palliative relief of cancer, tumor response, local control, and survival. Adequate measurement of the temperature distribution achieved with the hyperthermia is a key element in successful therapy. Thermal dosimetry, even invasive dosimetry, is a complex topic when applied to the heterogeneous tissue of a tumor and associated organ systems. Imaging in hyperthermia therapy is performed primarily for estimation and control of temperature. Magnetic resonance (MR) imaging has unique parameter dependences that make it possible to monitor hyperthermia therapy by detection of proton resonant frequency changes or diffusion coefficient changes. In addition, MR imaging can be used to assess vascular parameters that not only allow selection of suitable patients for therapy but may also allow demonstration of response to therapy. Finally, as the use of thermally sensitive liposomes for delivery of chemotherapeutic agents is developed, MR imaging may allow determination of local drug dose.

Artefacts in intracavitary temperature measurements during regional hyperthermia

For adequate hyperthermia treatments, reliable temperature information during treatment is essential. During regional hyperthermia, temperature information is preferably obtained non-invasively from intracavitary or intraluminal measurements to avoid implant risks for the patient. However, for intracavitary or intraluminal thermometry optimal tissue contact is less natural as for invasive thermometry. In this study, the reliability of intraluminal/intracavitary measurements was examined in phantom experiments and in a numerical model for various extents of thermal contact between thermometry and the surroundings. Both thermocouple probes and fibre optic probes were investigated. Temperature rises after a 30 s power pulse of the 70 MHz AMC-4 hyperthermia system were measured in a tissue-equivalent phantom using a multisensor thermocouple probe placed centrally in a hollow tube. The tube was filled with (1) air, (2) distilled water or (3) saline solution that mimics the properties of tissue, simulating situations with (1) bad thermal contact and no power dissipation in the tube, (2) good thermal contact but no power dissipation or (3) good thermal contact and tissue representative power dissipation. For numerical simulations, a cylindrical symmetric model of a thermocouple probe or a fibre optic probe in a cavity was developed. The cavity was modelled as air, distilled water or saline solution. A generalised E-Field distribution was assumed, resulting in a power deposition. With this power deposition, the temperature rise after a 30 s power pulse was calculated. When thermal contact was bad (1), both phantom measurements and simulations with a thermocouple probe showed very high temperature rises (>0.5 degrees C), which are artefacts due to self-heating of the thermocouple probe, since no power is dissipated in air. Simulations with a fibre optic probe showed almost no temperature rise when the cavity was filled with air. When thermal contact was good, but no power was dissipated in the tube (2), artefacts due to self-heating were not significant and the observed temperature rises were very low ( approximately 0-0.1 degrees C). For the situation, with tissue representative power dissipation (3), a temperature rise of approximately 0.23 degrees C was observed for both measurements and simulations. A clinical example of a regional hyperthermia treatment of a patient with a cervix uteri carcinoma showed that the artefacts observed in the case of bad thermal contact also affect the steady-state temperature measurements. Good tissue contact must be assured for reliable intraluminal or intracavitary measurements.

Hyperthermia induces T1 relaxation and blood flow changes in tumors. A MRI thermometry study in vivo

Regional hyperthermia in combination with chemotherapy and/or radiotherapy has proven to be an effective treatment concept for locally advanced deep-seated tumors. Simultaneous MR-imaging could be a promising approach for therapy optimization. Purpose of this study was the in vivo investigation of temperature induced longitudinal relaxation time (T(1)) and blood flow changes in a tumor model. Using a 1.5 Tesla MR system, the T(1) sensitivity on temperature and the onset of tissue changes at temperatures >44 degrees C were investigated in muscle samples. Also, fourteen Syrian Golden Hamsters with amelanotic melanoma A-MEL-3 were examined during heating of the tumors. Temperature induced blood flow and T(1) changes were determined continuously during hyperthermia. Changes of T(1) correlated linearly with temperature over a wide range (27-44 degrees C) in the tissue sample. Tissue changes became notable above 44 degrees C. In the tumor model, relative changes of T(1) (unlike blood flow) showed linear correlation with temperature over the entire range of hyperthermia relevant temperatures (32-44 degrees C). For a low thermal dose, T(1) allows the assessment of temperature changes in tumors in vivo. At higher thermal doses, in addition to temperature changes, T(1) also shows tissue changes. Both temperature and tissue changes supply important information for hyperthermia.

Engineering aspects of hyperthermia therapy

The continuing accrual of positive results in clinical cancer trials of adjunctive, synergistic hyperthermia therapy remains a strong motivation for the development of improved hyperthermia equipment and software. Indeed, the lack of needed engineering tools can be viewed as the major stumbling block to hyperthermia's effective clinical implementation. Developing clinically effective systems will be difficult, however, because (a) it requires solving several complex engineering problems, for which (b) setting appropriate design and evaluation goals is currently difficult owing to a lack of critical biological, physiological, and clinical knowledge, two tasks which must (c) be accomplished within a complicated social/political structure.

An on-line phase measurement system for quality assurance of the BSD 2000. Part I: technical description of the measurement system

The hyperthermia system BSD 2000 with the ring applicator Sigma 60 utilizes the principle of a phase controlled group radiation source. The accuracy of the phase relationship between the four receiving HF signals is crucial for the position of the electric field inside the applicator. Therefore, essential significance falls to the phase control of the system. An automatic phase measuring technique has been developed to register immediately the phase position of the four channels of the BSD 2000 with respect to a reference signal. The system improves the insurance of the technical safeguarding. In the first part of this work, the technical realization of the measurement system is described and first measurements with the system are given. In the second part, results with respect to the quality assurance of the BSD 2000 system are presented.

Perturbations in hyperthermia temperature distributions associated with counter-current flow: numerical simulations and empirical verification

Two numerical techniques are used to calculate the effect of large vessel counter-current flow on hyperthermic temperature distributions. One is based on the Navier-Stokes equation for steady-state flow, and the second employs a convective-type boundary condition at the interface of the vessel walls. Steady-state temperature fields were calculated for two energy absorption rate distributions (ARD) in a cylindrical tissue model having two pairs of counter-current vessels (one pair with equal diameter vessels and another pair with unequal diameters). The first assumed a uniform ARD throughout cylinder; the second ARD was calculated for a tissue cylinder inside an existing four antenna Radiofrequency (RF) array. A tissue equivalent phantom was constructed to verify the numerical calculations. Temperatures induced with the RF array were measured using a noninvasive magnetic resonance imaging technique based on the chemical shift of water. Temperatures calculated using the two numerical techniques are in good agreement with the measured data. The results show: 1) the convective-type boundary condition technique reduces computation time by a factor of ten when compared to the fully conjugated method with little quantitative difference (approximately 0.3 degree C) in the numerical accuracy and 2) the use of noninvasive magnetic resonance imaging (thermal imaging) to quantitatively access the temperature perturbations near large vessels is feasible using the chemical shift technique.

Accuracy of MR phase mapping for temperature monitoring during interstitial laser coagulation (ILC) in the liver at rest and simulated respiration

The chemical shift or proton-resonance frequency (phase mapping) can be used to measure temperature changes. As a subtraction technique, it requires scans at exactly the same location, making it prone to respiration-induced artifacts. The accuracy of magnetic resonance (MR) phase mapping for temperature monitoring of interstitial laser coagulation (ILC) was therefore investigated in two ex vivo models with simulated respiration. MR temperatures were calibrated to interstitially measured temperature. Gradual cooling of a homogenous medium (gel) was monitored for four starting temperatures (room temperature, 40 degrees C, 50 degrees C, and 60 degrees C) during 30 min. Temperature increases were measured during ILC in ex vivo porcine liver with Nd:YAG for 6 min with 5 Watt. Experiments were performed at rest and with simulated respiratory motion (both n = 5). In liver, accuracy did not decrease with respiration simulation (P = 0.32), whereas a significant decline was found in the gel model (P = 0.002). In all experiments a small drift over time was observed between temperature determined with MR and thermoprobes. Correction for temperature-independent phase-shift at a reference location did not enhance agreement. Temperatures could be determined correctly by MR in the moving liver within a range of +/-3.5 degrees C after 6 min of laser application (95% confidence interval), justifying further pre-clinical studies.

MRI thermometry in phantoms by use of the proton resonance frequency shift method: application to interstitial laser thermotherapy

In this work the temperature dependence of the proton resonance frequency was assessed in agarose gel with a high melting temperature (95 degrees C) and in porcine liver in vitro at temperatures relevant to thermotherapy (25-80 degrees C). Furthermore, an optically tissue-like agarose gel phantom was developed and evaluated for use in MRI. The phantom was used to visualize temperature distributions from a diffusing laser fibre by means of the proton resonance frequency shift method. An approximately linear relationship (0.0085 ppm degrees C(-1)) between proton resonance frequency shift and temperature change was found for agarose gel, whereas deviations from a linear relationship were observed for porcine liver. The optically tissue-like agarose gel allowed reliable MRI temperature monitoring, and the MR relaxation times (T1 and T2) and the optical properties were found to be independently alterable. Temperature distributions around a diffusing laser fibre, during irradiation and subsequent cooling, were assessed with high spatial resolution (voxel size = 4.3 mm3) and with random uncertainties ranging from 0.3 degrees C to 1.4 degrees C (1 SD) with a 40 s scan time.

Design and test of a new multi-amplifier system with phase and amplitude control

The clinical relevance of the radiofrequency regional hyperthermia (RF-RHT) as an adjuvant cancer therapy grows continuously. Simulation studies for optimization of RF-RHT based on the annular phased array systems have shown a significant improvement of power deposition patterns with increasing number of channels. However, this probably requires higher phase accuracy and amplitude stability than are provided by presently used clinical systems, e.g. BSD-2000. Measurements performed on the BSD-200 electronic revealed phase inaccuracies up to +/- 20 degrees and errors in the power registration of +/- 20 W (up to +/- 50 W in the low power range). These errors are further enhanced by the mismatching of the external load (antenna applicator) and thermal instabilities. To achieve the required phase accuracy and long-term stability in the prototype of a new amplifier system, single-sideband (SSB) mixing in combination with direct digital synthesizers (DDS), in-phase and quadrature-phase (IQ) processing and phase-lock loop (PLL) were used. In the DDS's the actual phase of the output signal of each channel is calculated in real-time. No analogue control loop is involved that may cause thermal offset or drift problems. Each DDS operates at a low intermediate frequency (IF) of 1 MHz. To transform the phase information of this IF signal into the desired RF band, SSB mixing-up is performed. A second frequency source, operating as a local oscillator (LO) in the RF band, is required for this technique. Also, the frequency adjustment of the desired RF signal is performed in the LO. These phase and frequency adjustment units are followed by the high efficiency AB-class solid state amplifier unit. The phase and power level stability of the amplifier are controlled by means of digital PLL structures in conjunction with look-up tables. For this control test signals are coupled out by means of directional couplers. The phase control is based on very sensitive phase comparison. These digital control loops are programmable and allow the implementation of different control algorithms. The achieved long-term accuracy (95% confidence interval) is +/- 1-3 W for output power levels ranging from 10-100 W, and +/- 1 degree for phase differences between each channel and a reference signal at a constant power level, and +/- 1.5 degrees for phase difference values at variable power levels between 10-100 W. In conclusion, the new amplifier system is smaller and more efficient than presently available commercial systems.

Research and development of hyperthermia machines for present and future clinical needs

This article describes the clinical problems encountered with the use of hyperthermia equipment and the requisites in the development of more advanced systems. A summary of the trends in the development of hyperthermia equipment is presented. In addition, a description from the physical point of view is included for the design of new applicators for deep heating.

Quality assurance for radiofrequency regional hyperthermia

Today most treatments with regional hyperthermia are applied using radiofrequency systems with 'focus' steering by amplitude and phase control. This paper deals with quality assurance procedures developed to ensure controlled and safe treatments in such systems. Our results show how the deviations between requested and observed phase and amplitude vary with frequency, and how these deviations depend on both the geometry of the object (phantom) inside the system and the power level applied. The results also indicate that the investigated systems' internal quality assurance procedures were inadequate and that additional procedures should be applied. Since the system parameters depend on patient and treatment specific conditions it is concluded that there is a need for QA measurements before or during treatment. This paper deals specifically with the commercial BSD-2000 system from BSD Medical Corp. in Salt Lake City, Utah, as installed in Bergen, but the procedure outlined can be applied to other phase and amplitude-controlled RF-RHT systems with only minimal adjustments.

Hyperthermia treatment planning and temperature distribution reconstruction: a case study

While a great deal of effort has been applied toward solving the technical problems associated with modelling clinical hyperthermia treatments, much of that effort has focused on only estimating the power deposition. Little effort has been applied toward using the modelled power depositions (either electromagnetic (EM) or ultrasonic) as inputs to estimate the hyperthermia induced three-dimensional temperature distributions. This paper presents a case report of a patient treated with hyperthermia at the Duke University Medical Center where numerical modelling of the EM power deposition was used to prospectively plan the treatment. Additionally, the modelled power was used as input to retrospectively reconstruct the transient three-dimensional temperature distribution. The modelled power deposition indicated the existence of an undesirable region of high power in the normal tissue. Based upon this result, amplitudes and phases for driving the hyperthermia applicator were determined that eliminated the region of high power and subsequent measurements confirmed this. The steady-state and transient three-dimensional temperature distributions were reconstructed for four out of the seven treatments. The reconstructed steady-state temperatures agreed with the measured temperatures; root-mean-square error ranged from 0.45 to 1.21 degrees C. The transient three-dimensional tumour temperature was estimated assuming that the perfusion was constant throughout the treatment. Using the computed three-dimensional transient temperature distribution, the hyperthermia thermal dose was computed. The equivalent minutes at 43 degrees C achieved by 50% (T50Eq43) of the tumour volume was computed from the measured data and the three-dimensional reconstructed distribution yielding T50Eq43 = 40.6 and 19.8 min respectively.