Segmental lumbar mobility in individuals with low back pain: in vivo assessment during manual and self-imposed motion using dynamic MRI.

BACKGROUND: Altered spinal mobility is thought to be related to current or past episodes of low back pain; however evidence of that relationship in younger subjects has not been established. The purpose of this study was to compare lumbar segmental mobility in asymptomatic and symptomatic subjects during posterior to anterior (PA) manual spinal mobilization and a self-initiated prone press-up (PU) maneuver. We hypothesized that persons with central low back pain would have an altered lumbar segmental mobility pattern compared to those without pain. METHOD: Forty-five individuals (age 32.1 +/- 8.5) with non-specific low back pain and 20 persons (age 31.1 +/- 7.0) without low back pain participated. Each subject underwent dynamic imaging of the lumbar spine during a PA mobilization procedure and while performing a PU. Segmental motion was quantified as the change in the intervertebral angle between the resting and end-range vertebral positions. RESULTS: The symptomatic group had a larger percentage of subjects with evidence of single level segmental hypermobility than the asymptomatic group during the PA (40.0% vs. 5%) and PU (26.7% vs. 15%) procedures. Single lumbar motion-segment analysis revealed hyper-mobility in symptomatic subjects at L5 - S1 (Chi-square = 10.0, p < or = 0.01) and L4 - L5 (Chi-square = 4.18, p < or = 0.05) during the PA test. CONCLUSION: Persons with non-specific low back pain have a tendency to demonstrate single level lumbar segmental hypermobility when compared to age specific asymptomatic subjects.

MRI-guided interstitial ultrasound thermal therapy of the prostate: a feasibility study in the canine model.

The feasibility of MRI-guided interstitial ultrasound thermal therapy of the prostate was evaluated in an in vivo canine prostate model. MRI compatible, multielement interstitial ultrasound applicators were developed using 1.5 mm diameter cylindrical piezoceramic transducers (7 to 8 MHz) sectored to provide 180 degrees of angular directional heating. Two in vivo experiments were performed in canine prostate. The first using two interstitial ultrasound applicators, the second using three ultrasound applicators in conjunction with rectal and urethral cooling. In both experiments, the applicators were inserted transperineally into the prostate with the energy directed ventrally, away from the rectum. Electrical power levels of 5-17 W per element (approximately 1.6-5.4 W acoustic output power) were applied for heating periods of 18 and 48 min. Phase-sensitive gradient-echo MR imaging was used to monitor the thermal treatment in real-time on a 0.5 T interventional MRI system. Contrast-enhanced T1-weighted images and vital-stained serial tissue sections were obtained to assess thermal damage and correlate to real-time thermal contour plots and calculated thermal doses. Results from these studies indicated a large volume of ablated (nonstained) tissue within the prostate, extending 1.2 to 2.0 cm from the applicators to the periphery of the gland, with the dorsal margin of coagulation well-defined by the applicator placement and directionality. The shape of the lesions correlated well to the hypointense regions visible in the contrast-enhanced T1-weighted images, and were also in good agreement with the contours of the 52 degrees C threshold temperature and t43 > 240 min. This study demonstrates the feasibility of using directional interstitial ultrasound in conjunction with MRI thermal imaging to monitor and possibly control thermal coagulation within a targeted tissue volume while potentially protecting surrounding tissue, such as rectum, from thermal damage.

In vivo MR thermometry of frozen tissue using R2* and signal intensity.

Cryoablation is one of several minimally invasive treatments that may be suitable for a targeted treatment of prostate cancer. Because efficacy is improved when a sufficiently cold end temperature is reached, the purpose of this work was to demonstrate an image-based thermometry method that could provide temperature maps throughout the frozen tissue. In five in vivo canine prostate cryoablation experiments performed under magnetic resonance imaging guidance, two MR parameters were measured and correlated to temperature: R2* and changes in signal intensity. R2* is elevated approximately linearly as tissue temperature decreases below the freezing point, while the signal intensity decreases exponentially. In vivo temperature maps with isotherms at -5 degrees C, -15 degrees C, and -30 degrees C are demonstrated.

Truly hybrid X-ray/MR imaging: toward a streamlined clinical system.

RATIONALE AND OBJECTIVE: We have installed an improved X-ray/MR (XMR) truly hybrid system with higher imaging signal-to-noise ratio (SNR) and versatility than our first prototype. In our XMR design, a fixed anode X-ray fluoroscopy system is positioned between the two donut-shaped magnetic poles of a 0.5T GE Signa-SP magnet (SP-XMR). This paper describes the methods for increased compatibility between the upgraded x-ray and MR systems that have helped improve patient management. MATERIALS AND METHODS: A GE OEC 9800 system (GE OEC Salt Lake City, UT) was specially reconfigured for permitting X-ray fluoroscopy inside the interventional magnet. A higher power X-ray tube, a new permanent tube mounting system, automatic exposure control (AEC), remote controlled collimators, choice of multiple frame rates, DICOM image compatibility, magnetically shimmed X-ray detector, X-ray compatible MR coil, and better RF shielding are the highlights of the new system. A total of 23 clinical procedures have been conducted with SP-XMR guidance of which five were performed using the new system. RESULTS: The 70% increased power for fluoroscopy, and a new 6 times higher power single frame imaging mode, has improved imaging capability. The choice of multiple imaging frame rates, AEC, and collimator control allow reduction in X-ray exposure to the patient. The DICOM formatting has permitted easy transfer of clinical images over the hospital PACS network. The increased MR compatibility of the detector and the X-ray transparent MR coil has enabled faster switching between X-ray and MR imaging modes. CONCLUSION: The improvements introduced in our SP-XMR system have further streamlined X-ray/MR hybrid imaging. Additional clinical procedures could benefit from the new SP-XMR imaging.

Line scan diffusion imaging of the spine.

BACKGROUND AND PURPOSE: Recent findings suggest that diffusion-weighted imaging might be an important adjunct to the diagnostic workup of disease processes in the spine, but physiological motion and the challenging magnetic environment make it difficult to perform reliable quantitative diffusion measurements. Multi-section line scan diffusion imaging of the spine was implemented and evaluated to provide quantitative diffusion measurements of vertebral bodies and intervertebral disks. METHODS: Line scan diffusion imaging of 12 healthy study participants and three patients with benign vertebral compression fractures was performed to assess the potential of line scan diffusion imaging of the spinal column. In a subgroup of six participants, multiple b-value (5-3005 s/mm(2)) images were obtained to test for multi-exponential signal decay. RESULTS: All images were diagnostic and of high quality. Mean diffusion values were (230 +/- 83) x 10(-6) mm(2)/s in the vertebral bodies, (1645 +/- 213) x 10(-6) mm(2)/s in the nuclei pulposi, (837 +/- 318) x 10(-6) mm(2)/s in the annuli fibrosi and ranged from 1019 x 10(-6) mm(2)/s to 1972 x 10(-6) mm(2)/s in benign compression fractures. The mean relative intra-participant variation of mean diffusivity among different vertebral segments (T10-L5) was 2.97%, whereas the relative difference in mean diffusivity among participants was 7.41% (P <.0001). The estimated measurement precision was <2%. A bi-exponential diffusion attenuation was found only in vertebral bodies. CONCLUSION: Line scan diffusion imaging is a robust and reliable method for imaging the spinal column. It does not suffer as strongly from susceptibility artifacts as does echo-planar imaging and is less susceptible to patient motion than are other multi-shot techniques. The different contributions from the water and fat fractions need to be considered in diffusion-weighted imaging of the vertebral bodies.

Dynamic magnetic resonance guided treatment of developmental dysplasia of the hip.

This study demonstrates the feasibility and advantages of near real-time, multiplanar, dynamic magnetic resonance image-assisted treatment of patients with developmental dysplasia of the hip. Pathoanatomy and dynamic blocks to reduction are visualized with anatomic clarity not otherwise possible. Continuous imaging allows accurate assessment and maintenance of optimum positioning throughout the casting procedure. Patient charges for this new technique are less than standard methods of treatment, and the child receives no ionizing radiation.

Changes in posterior lumbar disk contour abnormality with flexion-extension movement in subjects with low back pain and degenerative disk disease.

OBJECTIVE: To determine whether posterior lumbar disk contour dimensions differ in the flexed seated, upright seated, and extended seated positions. DESIGN: Two subgroups of subjects with degenerative disk disease were compared: those with central posterior disk bulge (at L4-5 or L5-S1 levels) and those with a dark nucleus pulposus without posterior disk bulge (L3-4, L4-5, and/or L5-S1 levels). SETTING: Academic medical center. PARTICIPANTS: Eight subjects with a central disk bulge and 9 subjects with a dark nucleus pulposus on magnetic resonance imaging. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Quantitative comparisons of posterior disk contour between neutral, flexed, and extended sitting positions. RESULTS: Of 8 subjects with central disk bulge, spinal flexion (from the neutral position) produced a decreased disk contour in all subjects, whereas spinal extension (from the neutral position) produced an increased disk contour in 6 subjects, a decreased disk contour in 1 subject, and no measurable change in 1 subject. Changes in posterior disk contour in subjects with a dark nucleus pulposus were variable. Approximately half increased and half decreased, but no relation to position was determined. CONCLUSIONS: The results of this pilot study suggest a consistent pattern of decreased posterior disk contour with spinal flexion and increased posterior disk contour with spinal extension in subjects with central disk bulge, but not in those with a dark nucleus pulposus.

MRI-guided thermal ablation therapy: model and parameter estimates to predict cell death from MR thermometry images.

Solid tumors and other pathologies can be treated using laser thermal ablation under interventional magnetic resonance imaging (iMRI) guidance. A model was developed to predict cell death from magnetic resonance (MR) thermometry measurements based on the temperature-time history, and validated using in vivo rabbit brain data. To align post-ablation T2-weighted spin-echo MR lesion images to gradient-echo MR images, from which temperature is derived, a registration method was used that aligned fiducials placed near the thermal lesion. The outer boundary of the hyperintense rim in the post-ablation MR lesion image was used as the boundary for cell death, as verified from histology. Model parameters were simultaneously estimated using an iterative optimization algorithm applied to every interesting voxel in 328 images from multiple experiments having various temperature histories. For a necrotic region of 766 voxels across all lesions, the model provided a voxel specificity and sensitivity of 98.1 and 78.5%, respectively. Mislabeled voxels were typically within one voxel from the segmented necrotic boundary with median distances of 0.77 and 0.22 mm for false positives (FP) and false negatives (FN), respectively. As compared to the critical temperature cell death model and the generalized Arrhenius model, our model typically predicted fewer FP and FN. This is good evidence that iMRI temperature maps can be used with our model to predict therapeutic regions in real-time during treatment.

Reduction of blurring in view angle tilting MRI.

Magnetic resonance imaging (MRI) in the presence of metallic objects suffers from slice-selection errors and in-plane distortions. View angle tilting (VAT) corrects for in-plane distortions by adding a gradient on the slice-select axis during readout, but can suffer from image blurring. This work demonstrates that the major source of blurring is a slice profile modulation of the data, and proposes several solutions to prevent such blurring. Multiple high-bandwidth readouts are demonstrated to reduce the blurring while improving the signal-to-noise ratio (SNR) over a single high-bandwidth readout.

X-ray compatible radiofrequency coil for magnetic resonance imaging.

The range of RF coils that can be used in combined X-ray/MR (XMR) systems is limited because many conventional coils contain highly X-ray attenuating materials that are visible in the X-ray images and potentially obscure patient anatomy. In this study, an X-ray compatible coil design that has minimal X-ray attenuation in the field of view (FOV) of the X-ray image is presented. In this design, aluminum is used for the loop conductor and discrete elements of the coil are eliminated from the X-ray FOV. A surface coil and an abdominal phased array coil were built using the X-ray compatible design. X-ray attenuation and MR imaging properties of the coils were evaluated and compared to conventional coils. The X-ray compatible phased array coil was used to image patients during two interventional procedures in the XMR system. The X-ray compatible coils allowed for fluoroscopic X-ray image acquisition, without degradation by the coil, while maintaining excellent MR imaging qualities.

Effects of spatial and temporal resolution for MR image-guided thermal ablation of prostate with transurethral ultrasound.

PURPOSE: To describe approaches for determining optimal spatial and temporal resolutions for the proton resonance frequency shift method of quantitative magnetic resonance temperature imaging (MRTI) guidance of transurethral ultrasonic prostate ablation. MATERIALS AND METHODS: Temperature distributions of two transurethral ultrasound applicators (90 degrees sectored tubular and planar arrays) for canine prostate ablation were measured via MRTI during in vivo sonication, and agree well with two-dimensional finite difference model simulations at various spatial resolutions. Measured temperature distributions establish the relevant signal-to-noise ratio (SNR) range for thermometry in an interventional MR scanner, and are reconstructed at different resolutions to compare resultant temperature measurements. Various temporal resolutions are calculated by averaging MRTI frames. RESULTS: When noise is added to simulated temperature distributions for tubular and planar applicators, the minimum root mean squared (RMS) error is achieved by reconstructing to pixel sizes of 1.9 and 1.7 mm, respectively. In in vivo measurements, low spatial resolution MRTI data are shown to reduce the noise without significantly affecting thermal dose calculations. Temporal resolution of 0.66 frames/minute leads to measurement errors of more than 12 degrees C during rapid heating. CONCLUSION: Optimizing MRTI pixel size entails balancing large pixel SNR gain with accuracy in representing underlying temperature distributions.

MR-guided transjugular intrahepatic portosystemic shunt creation with use of a hybrid radiography/MR system.

PURPOSE: To evaluate the performance of a combined hybrid radiography/magnetic resonance (MR) unit to guide portal vein (PV) puncture during human transjugular intrahepatic portosystemic shunt (TIPS) creation. MATERIALS AND METHODS: Fourteen patients undergoing TIPS creation were studied during standard clinical applications. Patients were anesthetized and then positioned in an open MR unit containing a flat-panel radiographic fluoroscopic unit. With use of a combination of fluoroscopy and MR imaging, the PV was accessed and the TIPS procedure was performed. A noncovered nitinol stent or a covered stent-graft was placed in the TIPS tract. Number of punctures required, total procedure time, fluoroscopy time, procedural success rate, complications, and ultrasonographic and clinical follow-up were recorded. RESULTS: Clinical success was obtained in 13 of 14 patients. In one patient, extrahepatic puncture of the PV occurred, resulting in hemorrhage and requiring placement of a covered stent to control the bleeding. The mean number of punctures required to access the PV was 2.6 +/- 1.7, and the total procedure time was 2.5 hours +/- 0.6. Mean fluoroscopy time was 22.3 minutes +/- 5.5. Results of clinical and ultrasonographic follow-up compare favorably to previously published reports. CONCLUSION: TIPS creation with a combination hybrid radiography/MR unit is feasible and may reduce the number of needle passes required and radiation exposure, with similar overall outcomes compared with studies reported in the literature.

Pair of resonant fiducial markers for localization of endovascular catheters at all catheter orientations.

PURPOSE: To test wireless resonance circuits (RC) to be used as fiducial marker of endovascular catheters during MR-guided interventions. Current markers loose their signal enhancement for certain catheter orientations. The purpose of this study was to test a marker setup which overcomes this orientation problem. MATERIALS AND METHODS: The markers were constructed from a pair of two RCs. The RCs were individually tuned and the coil axes were oriented perpendicular to each other in order to decouple the two RCs. The markers were mounted on the tip of endovascular catheters and tested in vitro and in one porcine in vivo experiment. RESULTS: An intense MR signal at similar signal levels was noted at all catheter orientations. In the in vivo experiment the markers allowed for fast and reliable MR guidance of the catheters. CONCLUSION: A pair of two individually tuned and decoupled RCs is well suited for MR guidance of endovascular catheters.

Study of laser ablation in the in vivo rabbit brain with MR thermometry.

PURPOSE: To investigate the peak temperature and thermal dose (T(43)) as tissue damage indicators for thermal therapy. MATERIALS AND METHODS: The proton resonant frequency (PRF) shift thermal coefficient was calibrated on six in vivo rabbit brains during interstitial laser ablation. The peak temperature and T(43) were correlated with the lesion boundary observed on T2-weighted spin-echo (SE) MRI at 4 hours post-heating in seven thermal lesions using direct MR measurement and analysis based on a binary discriminate model. RESULTS: The peak temperature and T(43) were 48.3 +/- 1.7 degrees C and 191 +/- 219 minutes, respectively, from the direct MR measurement. The values derived by the binary discriminate analysis were 47.8 +/- 2.2 degrees C and 28 +/- 41 minutes, respectively. CONCLUSION: Our results suggest that tissue damage in rabbit brain 4 hours after thermal ablation can be predicted reliably from a threshold temperature of approximately 48 degrees C.

Triggered, navigated, multi-baseline method for proton resonance frequency temperature mapping with respiratory motion.

A technique is presented for the acquisition of temperature maps in the presence of variable respiratory motion using the proton resonance frequency (PRF) shift. The technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate temperature maps. Laser ablations were performed in an ex vivo liver phantom undergoing variable simulated respiratory motion and in vivo in four porcine livers, demonstrating a reduction of artifacts in the computed temperature maps compared with conventional single baseline techniques, both uncorrected and corrected for motion.

Referenceless PRF shift thermometry.

The proton resonance frequency (PRF) shift provides a means of measuring temperature changes during minimally invasive thermotherapy. However, conventional PRF thermometry relies on the subtraction of baseline images, which makes it sensitive to tissue motion and frequency drift during the course of treatment. In this study, a new method is presented that eliminates these problems by estimating the background phase from each acquired image phase. In this referenceless method, a polynomial is fit to the background phase outside the heated region in a weighted least-squares fit. Extrapolation of the polynomial to the heated region serves as the background phase estimate, which is then subtracted from the actual phase. The referenceless method is demonstrated on a phantom during laser heating, 0 degrees temperature rise images of in vivo human liver, interstitial laser ablation of porcine liver, and transurethral ultrasound ablation of canine prostate. A good correlation between temperature maps reconstructed with the referenceless and subtraction methods was found.

Interstitial ultrasound applicators with dynamic angular control for thermal ablation of tumors under MR-guidance.

Thermal ablation has been investigated as a treatment for a variety of cancers. Heat treatments have not gained large-scale clinical acceptance due to inconsistencies in controlling heat deposition in vivo and the lack of precise temperature measurement. Interstitial ultrasound provide a good method of controlling the radial depth of a thermal lesion and the applicator designs evaluated in this study allow for dynamic angular control of the shape of the lesion. A trisectored internally water-cooled applicator (TriAD) and a rotating catheter water-cooled applicator (RIUS) angularly controlled thermal dose to a target area. Both devices were small in diameter (1.8 mm-2.4 mm), making them clinically feasible for minimally invasive treatment in device size-sensitive tissues. A biothermal model accounting for changes in acoustic attenuation and perfusion as a function of thermal dose was used to evaluate and predict applicator performance. The MR susceptibility artifact of the applicators was examined with MR temperature imaging (MRTI) sequences at 1.5 T and 0.5 T. Ex vivo experiments in turkey and beef muscle with realtime MRTI correlated well with results from the biothermal model. These results display the feasibility of thermally treating tumors with controllable interstitial ultrasound applicators under real-time MRTI and bracket the applicators' predicted performance in vivo.

MRI-temperature mapping during ultrasound prostate ablation using fat for phase estimation.

Referenceless proton resonance frequency (PRF) shift thermometry provides a means to measure temperature changes during minimally invasive thermotherapy that is inherently robust to motion and tissue displacement. In this study, the method is expanded to allow background phase estimation from fatty tissue. A correction scheme for temperature map distortions caused by the ultrasound applicator is developed. The method is tested during thermal ablation of canine prostate using a directional transurethral ultrasound applicator.

Diffusion-weighted MRI after cryosurgery of the canine prostate. Magnetic resonance imaging.

PURPOSE: To evaluate the acute lesion created by cryosurgery with diffusion-weighted magnetic resonance imaging (DWI). MATERIALS AND METHODS: The appearance of the acute cryolesion was evaluated in four canine prostates DWI after they were warmed to original body temperature. The prostates were excised, stained with triphenyl tetrazolium chloride (TTC), photographed, prepared for hematoxylin and eosin (H&E) staining, and examined under a light microscope. RESULTS: A marked decrease in apparent diffusion coefficient of 38% was evident in the center of the previously frozen tissue, but not in all of the previously frozen tissue. Histologic results confirm differences between the iceball core and the periphery of the iceball, which have markedly different imaging characteristics on DWI. CONCLUSION: The core of the previously frozen tissue has a reduced apparent diffusion coefficient (ADC) compared to the periphery of the previously frozen tissue and previously unfrozen tissue.