Usefulness of MR imaging-derived thermometry and dosimetry in determining the threshold for tissue damage induced by thermal surgery in rabbits

PURPOSE

To investigate in vivo the feasibility of using magnetic resonance (MR) imaging-derived temperature and thermal dose measurements to find the threshold of thermal tissue damage.

MATERIALS AND METHODS

Sonications were delivered in rabbit thigh muscles at varying powers. Temperature-sensitive MR images obtained during the sonications were used to estimate the temperature and thermal dose. The temperature, thermal dose, and applied power were then correlated to the occurrence of tissue damage observed on postsonication images. An eight-element phased-array transducer was used to produce spatially flat temperature profiles that allowed for averaging to reduce the effects of noise and the voxel size.

RESULTS

The occurrence of tissue damage correlated well with the MR imaging-derived temperature and thermal dose measurements but not with the applied power. Tissue damage occurred at all locations with temperatures greater than 50.4 degrees C and thermal doses greater than 31.2 equivalent minutes at 43.0 degrees C. No tissue damage occurred when these values were less than 47.2 degrees C and 4.3 equivalent minutes.

CONCLUSION

MR imaging thermometry and dosimetry provide an index to predict the threshold for tissue damage in vivo. This index offers improved online control over minimally invasive thermal treatments and should allow for more accurate target volume coagulation.

Multi-component apparent diffusion coefficients in human brain: relationship to spin-lattice relaxation

In vivo measurements of the human brain tissue water signal decay with b-factor over an extended b-factor range up to 6,000 s/mm(2) reveal a nonmonoexponential decay behavior for both gray and white matter. Biexponential parametrization of the decay curves from cortical gray (CG) and white matter voxels from the internal capsule (IC) of healthy adult volunteers describes the decay process and serves to differentiate between these two tissues. Inversion recovery experiments performed in conjunction with the extended b-factor signal decay measurements are used to make separate measurements of the spin-lattice relaxation times of the fast and slow apparent diffusion coefficient (ADC) components. Differences between the spin-lattice relaxation times of the fast and slow ADC components were not statistically significant in either the CG or IC voxels. It is possible that the two ADC components observed from the extended b-factor measurements arise from two distinct water compartments with different intrinsic diffusion coefficients. If so, then the relaxation results are consistent with two possibilities. Either the spin-lattice relaxation times within the compartments are similar or the rate of water exchange between compartments is "fast" enough to ensure volume averaged T(1) relaxation yet "slow" enough to allow for the observation of biexponential ADC decay curves over an extended b-factor range. Magn Reson Med 44:292-300, 2000.

Three-dimensional optical flow method for measurement of volumetric brain deformation from intraoperative MR images

A three-dimensional optical flow method to measure volumetric brain deformation from sequential intraoperative MR images and preliminary clinical results from five cases are reported. Intraoperative MR images were scanned before and after dura opening, twice during tumor resection, and immediately after dura closure. The maximum cortical surface shift measured was 11 mm and subsurface shift was 4 mm. The computed deformation field was most satisfactory when the skin was segmented and removed from the images before the optical flow computation.

Virtual pancreatoscopy of mucin-producing pancreatic tumors

We used computer-based virtual endoscopy techniques as a novel approach to clarify the three-dimensional (3D) surgical anatomy of the pancreas and of mucin-producing pancreatic tumors. Thirteen cases (18 lesions) of mucin-producing pancreatic tumors were investigated by virtual pancreatoscopy. Virtual endoscopic images were generated with virtual endoscopy software application on UNIX workstations. We created surface-rendered virtual endoscopic images derived from a computer reconstruction of the cross-sectional magnetic resonance imaging data. Virtual endoscopy could visualize the surfaces of the pancreatic duct and the bile duct, and also demonstrated all cystic tumors. The surfaces of malignant mucin-producing pancreatic tumors were illustrated as being more irregular than those of benign lesions. The virtual endoscopic technique could demonstrate not only a surface-rendered endoscopic image of the tumors but also a 3D reconstructed image of the pancreas. The relationship to anatomic structures located outside the surfaces is continuously maintained and displayed at the same time. Virtual pancreatoscopy was useful for surgical planning of minimally invasive resection of the pancreas.

Evaluation of carrier agents for hyperpolarized xenon MRI

Several biocompatible carrier agents, in which xenon is highly soluble and has a long T(1), were tested, and injected in living rats. These included saline, Intralipid suspension, perfluorocarbon emulsion and (129)Xe gas-filled liposomes. The T(1) of (129)Xe in these compounds ranged from 47 to 116 s. Vascular injection of these carrier agents was tolerated well, encouraging their use for further experiments in live animals. In vivo spectra, obtained from gas-filled liposomes and perfluorocarbon solutions, suggest that these carrier agents have potential for use in angiography and perfusion imaging.

Magnetic resonance image-guided thermal ablations

Magnetic resonance imaging (MRI)-based monitoring has been shown in recent years to enhance the effectiveness of minimally or noninvasive thermal therapy techniques, such as laser, radiofrequency, microwave, ultrasound, and cryosurgery. MRI's unique soft-tissue contrast and ability to image in three dimensions and in any orientation make it extremely useful for treatment planning and probe localization. The temperature sensitivity of several intrinsic parameters enables MRI to visualize and quantify the progress of ongoing thermal treatment. MRI is sensitive to thermally induced tissue changes resulting from the therapies, giving the physician a method to determine the success or failure of the treatment. These methods of using MRI for planning, guiding, and monitoring thermal therapies are reviewed.

MRI detection of the thermal effects of focused ultrasound on the brain

This study tested the hypothesis that MRI thermometry can be correlated with the different degrees of tissue damage observed after focused ultrasound (US) exposure of brain. The brains of 6 rabbits were sonicated to calibrate the MRI proton resonant shift with temperature. In addition, 13 rabbits were sonicated at acoustic powers ranging from 3.5 to 17.5 W. The experiments were performed in a 1.5-T MRI scanner with the temperature-sensitive phase imaging used during the sonications of 4-5 different locations in each rabbit. MR images were obtained 2 h and 2 days after the sonications, depending on when the animals were sacrificed. Whole brain histologic evaluation was performed by sectioning the brain and performing a microscopic investigation. The MRI-derived temperature elevation was found to correlate well with the degree of tissue damage. In addition to the common histology findings, apoptotic cells were observed in the lesions. The T1-weighted contrast enhanced and T2-weighted scans both detected the brain damage. The applied acoustic power did not correlate well with the degree of damage. As a conclusion, the results showed that the measurement of temperature elevations by MRI during sonications can improve the accuracy and safety of clinical US brain surgery.

Temperature monitoring in fat with MRI

The aim of the study was to test the hypothesis that fast spin echo T(1)-weighted images can be used to quantify the temperature in fat during thermal therapy in vivo. An MR compatible positioning device was used to manipulate focused ultrasound transducers in an MRI scanner. This system was used to sonicate fat tissue around the kidneys of 12 rabbits at various power levels for 10 to 20 sec. The scan parameters of T(1)-weighted fast spin echo (FSE) sequence were varied to optimize signal intensity characteristics while maintaining short scan times. An invasive optical probe was used to calibrate the temperature related signal intensity changes. For the T(1)-weighted FSE sequence, the signal intensity decreased with the temperature elevation at the rate of 0.97+/-0.02%/ degrees C. The single focused transducer produced a contrast-to-noise ratio more than 10 at power levels below the tissue damage threshold. The signal intensity was linearly dependent on the power, despite the measured temperatures being well above the coagulation threshold. This study demonstrates that T(1)-weighted FSE MRI sequences can be used to quantify the temperature elevation in fat in vivo during short focused ultrasound exposures. This can be very important for breast tumor surgery, fat ablation, and for treating deep seated tumors through superficial fat layers. Magn Reson Med 43:901-904, 2000.

Serial magnetic resonance imaging in multiple sclerosis: correlation with attacks, disability, and disease stage

Serial MRI and clinical testing was performed on 45 well-defined untreated multiple sclerosis patients in different categories of disease (relapsing-remitting, progressive, stable). Up to 24 MRIs were scheduled over a 1-year period for each patient. Clinical evaluation was performed monthly and at times of attacks using the Expanded Disability Status Scale (EDSS) and the Ambulation Index (AI). MRI scans were performed both with and without gadolinium enhancement. MRI lesion volume was determined by computerized analysis and gadolinium-enhancing lesions were counted by radiologists. We observed an increase in lesion volume over 1 year in all patient groups except those classified clinically as stable. In relapsing-remitting patients there were correlations between increases in the number of gadolinium enhancing lesions and increases in EDSS and the occurrence of attacks. In chronic progressive patients, increases in lesion volume were correlated with both increases in EDSS and AI. These results demonstrate a linkage between MRI and clinical disease that depends both on the stage of MS and the MRI measures used and support the use of MRI as a surrogate marker of clinical disability in the study of multiple sclerosis.

Virtual CT cystoscopy: color mapping of bladder wall thickness

RATIONALE AND OBJECTIVES

To improve the conspicuity of bladder tumors in a virtual environment, we developed an algorithm for color mapping the thickness of the bladder wall. The purpose of this study was to demonstrate the feasibility of this algorithm as a component of virtual CT cystoscopy.

METHODS

Five subjects with a history of superficial transitional-cell carcinoma of the bladder underwent helical CT scanning after insufflation of the bladder with air. Source images were transformed into three-dimensional models, and the thickness of the bladder wall was demarcated by using a new computer algorithm and a fixed color scale. Results were compared with those obtained by conventional cystoscopy.

RESULTS

Three tumors, one site of benign wall thickening, and normal wall thickness were correctly identified by using axial source images and virtual cystoscopy with color mapping.

CONCLUSIONS

Color mapping of bladder wall thickness is feasible and demonstrates both normal and thickened urothelium. Its value in identification of small or sessile tumors will require further trials.

An image processing strategy for the quantification and visualization of exercise-induced muscle MRI signal enhancement

Exercise increases the skeletal muscle water signal in T2-weighted images. Potential medical applications of MR studies of exercise-induced muscle signal intensity changes are the assessment of myopathies, sport training regimens, and physical therapy approaches following surgeries. We developed an automated image processing technique that provides volumetric analysis and visualization of exercise-related T2-weighted image intensity changes. The image processing was applied to the segmentation and quantification of activated muscle volumes. Qualitative assessment of muscle activation is demonstrated with three-dimensional surface rendering. Quantitative determination of active muscle volume, signal intensity, and change over time is demonstrated. Visualization of the activated muscles allows functional anatomical assessment of exercise, which in turn allows detection of muscle utilization.

MR-based three-dimensional modeling of the normal pelvic floor in women: quantification of muscle mass

OBJECTIVE

Our objective was to use a combination of axial MR source images and three-dimensional (3D) models to describe the anatomy of the normal pelvic floor in young nulliparous women and to measure the volume of the levator ani.

SUBJECTS AND METHODS

Ten healthy nulliparous female volunteers (average age, 27 years) underwent T2-weighted MR imaging of the pelvis. Three-dimensional color-coded models of the pelvic bones and organs and the three major components of the levator ani--puborectalis, iliococcygeus, and coccygeus--were created. Source images were used to measure muscle width and signal intensity and to identify ligamentous structures. Using 3D models, we measured the volume of the levator ani, the angle of the levator plate, the posterior urethrovesical angle, and the distance of the bladder neck from the symphysis pubis and the pubococcygeal line.

RESULTS

In all volunteers, the signal intensity of the puborectalis exceeded that of the obturator externus. The average volume of the levator ani was 46.6 ml, the average width of the levator hiatus was 41.7 mm, and the average posterior urethrovesical angle was 143.5 degrees. Vaginal shape in the volunteers followed no recognizable pattern.

CONCLUSION

Muscle morphology, signal intensity, and volume is relatively uniform among healthy young women.

Adaptive, template moderated, spatially varying statistical classification

A novel image segmentation algorithm was developed to allow the automatic segmentation of both normal and abnormal anatomy from medical images. The new algorithm is a form of spatially varying statistical classification, in which an explicit anatomical template is used to moderate the segmentation obtained by statistical classification. The algorithm consists of an iterated sequence of spatially varying classification and nonlinear registration, which forms an adaptive, template moderated (ATM), spatially varying statistical classification (SVC). Classification methods and nonlinear registration methods are often complementary, both in the tasks where they succeed and in the tasks where they fail. By integrating these approaches the new algorithm avoids many of the disadvantages of each approach alone while exploiting the combination. The ATM SVC algorithm was applied to several segmentation problems, involving different image contrast mechanisms and different locations in the body. Segmentation and validation experiments were carried out for problems involving the quantification of normal anatomy (MRI of brains of neonates) and pathology of various types (MRI of patients with multiple sclerosis, MRI of patients with brain tumors, MRI of patients with damaged knee cartilage). In each case, the ATM SVC algorithm provided a better segmentation than statistical classification or elastic matching alone.

Temperature mapping using the water proton chemical shift: self-referenced method with echo-planar spectroscopic imaging

An echo-planar spectroscopic imaging method of temperature mapping is proposed. This method is sufficiently faster than the so-called 3D magnetic resonance spectroscopic imaging (3D-MRSI) method and does not require image subtractions, unlike the conventional phase mapping method when an internal reference signal is detectable. The water proton chemical shift measured by using the tissue lipid as an internal reference clearly visualized the temperature change in a porcine liver sample in vitro. It was also demonstrated that the internally referenced echo-planar spectroscopic imaging method could markedly reduce a temperature error caused by a simple, translational motion between scans compared with the phase-mapping method.

Disruption of attention to novel events after frontal lobe injury in humans

OBJECTIVE

To investigate whether frontal lobe damage in humans disrupts the natural tendency to preferentially attend to novel visual events in the environment.

METHODS

Nine patients with chronic infarctions in the dorsolateral prefrontal cortex (DLPFC) and 23 matched normal controls participated in a study in which subjects viewed repetitive background stimuli, infrequent target stimuli, and novel visual stimuli (for example, fragmented or "impossible" objects). Subjects controlled viewing duration by a button press that led to the onset of the next stimulus. They also responded to targets by pressing a foot pedal. The amount of time spent looking at the different kinds of stimuli, and the target detection accuracy and speed served as dependent variables.

RESULTS

Overall, normal controls spent significantly more time than frontal lobe patients looking at novel stimuli. Analysis of responses across blocks showed that initially frontal lobe patients behaved like normal controls by directing more attention to novel than background stimuli. However, they quickly began to distribute their viewing time evenly between novel and background stimuli, a pattern that was strikingly different from normal controls. By contrast, there were no differences between frontal lobe patients and normal controls for viewing duration devoted to background and target stimuli, target detection accuracy, or reaction time to targets. Frontal lobe patients did not differ from normal controls in terms of age, education, estimated IQ, or mood, but were more apathetic as measured by self report and informants' judgments. Attenuated responses to novel stimuli significantly correlated with degree of apathy.

CONCLUSIONS

This study demonstrates that DLPFC injury selectively impairs the natural tendency to seek stimulation from novel and unusual stimuli. These data provide the first quantitative behavioural demonstration that the human frontal lobes play a critical part in directing and sustaining attention to novel events. The impairment of novelty seeking behaviour may contribute to the characteristic apathy found in patients with frontal lobe injury.

Superior temporal gyrus volume abnormalities and thought disorder in left-handed schizophrenic men

OBJECTIVE

Studies of schizophrenia have not clearly defined handedness as a differentiating variable. Moreover, the relationship between thought disorder and anatomical anomalies has not been studied extensively in left-handed schizophrenic men. The twofold purpose of this study was to investigate gray matter volumes in the superior temporal gyrus of the temporal lobe (left and right hemispheres) in left-handed schizophrenic men and left-handed comparison men, in order to determine whether thought disorder in the left-handed schizophrenic men correlated with tissue volume abnormalities.

METHOD

Left-handed male patients (N = 8) with DSM-III-R diagnoses of schizophrenia were compared with left-handed comparison men (N = 10) matched for age, socioeconomic status, and IQ. Magnetic resonance imaging (MRI) with a 1.5-T magnet was used to obtain scans, which consisted of contiguous 1.5-mm slices of the whole brain. MRI analyses (as previously defined by the authors) included the anterior, posterior, and total superior temporal gyrus in both the left and right hemispheres.

RESULTS

There were three significant findings regarding the left-handed schizophrenic men: 1) bilaterally smaller gray matter volumes in the posterior superior temporal gyrus (16% smaller on the right, 15% smaller on the left); 2) a smaller volume on the right side of the total superior temporal gyrus; and 3) a positive correlation between thought disorder and tissue volume in the right anterior superior temporal gyrus.

CONCLUSIONS

These results suggest that expression of brain pathology differs between left-handed and right-handed schizophrenic men and that the pathology is related to cognitive disturbance.

Spin-Echo planar spectroscopic imaging for fast lipid characterization in bone marrow

Lipid characterization of bone marrow in vivo with proton magnetic resonance spectroscopy was performed using Spin-Echo Planar Spectroscopic Imaging sequences. The methods are shown capable of rapidly generating two-dimensional chemical shift imaging data sets suitable for measuring lipid indices that reflect unsaturation levels among triglycerides, as demonstrated in oil phantoms and bone marrow from a healthy volunteer. The volume coverage, spatial resolution, acquisition speed, and spectral characteristics of Spin-Echo Planar Spectroscopic Imaging should make it attractive for clinical studies of diseases affecting normal lipid chemical composition.

Changes in serum levels of ICAM and TNF-R correlate with disease activity in multiple sclerosis

OBJECTIVE

To study the change in serum levels of soluble intercellular adhesion molecule 1 (sICAM-1) and soluble tumor necrosis factor receptors (sTNF-Rs) in MS patients in relation to clinical disease activity and changes on brain MRI.

BACKGROUND

Circulating forms of adhesion molecules or soluble receptors may be released from cells as a consequence of activation and may be useful markers for inflammation.

METHODS

During a prospective longitudinal study over 1 year, 40 patients with MS underwent frequent imaging of the brain (22 MR images per patient) at the time of blood sampling as well as monthly neurologic examinations, and scoring on Kurtzke's Expanded Disability Status Scale (EDSS) and ambulation index (AI).

RESULTS

Patients with relapsing-progressive disease had the highest levels of sICAM-1 whereas patients with progressive disease had the highest levels of sTNF-Rs. Fluctuations in sICAM-1 correlated with the occurrence of attacks in patients with relapsing and relapsing-progressive disease. In patients with relapsing-progressive MS, an increase in sICAM-1 level preceded the appearance of new gadolinium (Gd) enhancing lesions on MRI. In patients with progressive disease, an increase in sTNF-R p55 level preceded the appearance of new Gd enhancing lesions on MRI, whereas a decrease in sICAM-1 levels correlated with the appearance of new Gd enhancing lesions.

CONCLUSIONS

These results demonstrate a linkage between sICAM-1 and sTNF-R levels and disease activity in MS. Furthermore, patients with progressive disease appear to have a different immunologic stage of disease in which immune changes are tightly linked with changes on MRI. The demonstration of a correlation in individual patients between immunologic events and changes in disease activity has implications for monitoring patients undergoing treatment and for monitoring disease progression.

T(1) of (129)Xe in blood and the role of oxygenation

In previous experiments by the authors, in which hyperpolarized (129)Xe was dissolved in fresh blood samples, the T(1) was found to be strongly dependent on the oxygenation level, the values increasing with oxygenation: T(1) was about 4 s in deoxygenated samples and about 13 s in oxygenated samples. C. H. Tseng et al. (1997, J. Magn. Reson. 126, 79-86), on the other hand, recently reported extremely long T(1) values using hyperpolarized (129)Xe to create a "blood foam" and found that oxygenation decreased T(1). In their experiments, the continual and rapid exchange of hyperpolarized (129)Xe between the gas phase (within blood-foam bubbles) and the dissolved phase (in the skin of the bubbles) necessitated a complicated analysis to extract the effective blood T(1). In the present study, the complications of hyperpolarized (129)Xe exchange dynamics have been avoided by using thermally polarized (129)Xe dissolved in whole blood and in suspensions of lysed red blood cells (RBC). During T(1) measurements in whole blood, the samples were gently and continuously agitated, for the entire course of the experiment, to avert sedimentation. Oxygenation was found to markedly increase the T(1) of (129)Xe in blood, as originally measured, and it shifts the RBC resonance to a higher frequency. Carbon monoxide has a similar but somewhat stronger effect.

Thermal dosimetry of a focused ultrasound beam in vivo by magnetic resonance imaging

Magnetic resonance imaging (MRI) thermometry has been utilized for in vivo evaluation of thermal exposure induced by a focused ultrasound beam. A simulation study of the focused ultrasound beam was conducted to select imaging parameters for reducing the error due to the spatial and temporal averaging of MRI. Temperature imaging based on the proton resonance frequency shift was utilized to obtain the temperature distribution during sonication in the skeletal muscle of eight rabbits. MRI-derived temperature information was then used to calculate the thermal dose distribution induced by the sonication and to estimate the coagulated tissue volume. The tissue changes were also evaluated directly by taking the T2-weighted and the contrast agent enhanced T1-weighted MR images. Errors in the temperature and thermal dose measurements were found to be minimal using the following parameters: slice thickness = 3 mm, voxel dimension = 0.6 mm, and scan time per image = 3.4 s. The estimated dimensions of the coagulated tissue volume were in good agreement with the tissue damages seen on the contrast agent enhanced T1-weighted images. The tissue damage seen on the histology was closely matched to the ones seen on the T2-weighted images. This study showed that MRI thermometry has significant potential for both monitoring the thermal exposure and evaluating the tissue damage. This would allow real-time control of the sonication parameters to optimize clinical treatments.

Computer-based imaging and interventional MRI: applications for neurosurgery

Advances in computer technology and the development of open MRI systems definitely enhanced intraoperative image-guidance in neurosurgery. Based upon the integration of previously acquired and processed 3D information and the corresponding anatomy of the patient, this requires computerized image-processing methods (segmentation, registration, and display) and fast image integration techniques. Open MR systems equipped with instrument tracking systems, provide an interactive environment in which biopsies and minimally invasive interventions or open surgeries can be performed. Enhanced by the integration of multimodal imaging these techniques significantly improve the available treatment options and can change the prognosis for patients with surgically treatable diseases.

Legg-Calvé-Perthes disease: MR imaging evaluation during manual positioning of the hip--comparison with conventional arthrography

PURPOSE

To evaluate the use of magnetic resonance (MR) imaging during manual positioning of the hip, or multipositional MR imaging, in an open-magnet configuration to study femoral head containment, articular congruency, and femoral head deformity in Legg-Calvé-Perthes disease.

MATERIALS AND METHODS

In 12 children with advanced Legg-Calvé-Perthes disease, multipositional MR imaging and conventional arthrography were compared in the assessment of containment, femoroacetabular congruency, and femoral head deformity. Images of the hips in several positions were compared subjectively and objectively.

RESULTS

MR imging correlated well with arthrography for overall subjective assessment of severity of disease (r = 0.71, P = .01), with good interobserver agreement (kappa = 0.65, P < .001). MR images demonstrated all cases of hinge abduction shown arthrographically. However, MR imaging failed to depict one case of femoral head flattening. MR imaging correlated well with arthrography in the objective evaluation of joint fluid and lateral subluxation (r = 0.80, P < .01). MR imaging correlated poorly with arthrography in the measurement of sphericity of the femoral head.

CONCLUSION

Multipositional MR imaging with an open-magnet configuration was comparable to arthrography for demonstration of femoral head containment and congruency of the articular surfaces of the hip. In the evaluation of deformity, it performed less well.