Measurement of MRI scanner performance with the ADNI phantom

The objectives of this study are as follows: to describe practical implementation challenges of multisite, multivendor quantitative studies; to describe the MRI phantom and analysis software used in the Alzheimer's Disease Neuroimaging Initiative (ADNI) study, illustrate the utility of the system for measuring scanner performance, the ability to assess gradient field nonlinearity corrections: and to recover human brain images without geometric scaling errors in multisite studies. ADNI is a large multicenter study with each center having its own copy of the phantom. The design of the phantom and analysis software are presented as results from predistribution systematics studies and results from field experience with the phantom at 58 enrolling ADNI sites over a 3 year period. The estimated coefficients of variation intrinsic to measurements of geometry in a single phantom are in the range of 3-5 parts in 10(4). Phantom measurements accurately detect linear and nonlinear scaling in images. Gradient unwarping methods are readily assessed by phantom nonlinearity measurements. Phantom-based scaling correction reduces observed geometric drift in human images by one-third or more. Repair or replacement of phantoms between scans, however, is a confounding factor. The ADNI phantom can be used to assess both scanner performance and the validity of postprocessing image corrections in order to reduce systematic errors in human images. Reduced measurement errors should decrease measurement bias and increase statistical power for measurements of rates of change in the brain structure in AD treatment trials. Perhaps the greatest practical value of incorporating ADNI phantom measurements in a multisite study is to identify scanner errors through central monitoring. This approach has resulted in identification of system errors including sites misidentification of their own gradient hardware and the disabling of autoshim, and a miscalibrated laser alignment light. If undetected, these errors would have contributed to imprecision in quantitative metrics at over 25% of all enrolling ADNI sites.

Longitudinal study of intraneural perineurioma--a benign, focal hypertrophic neuropathy of youth

The natural history of intraneural perineurioma has been inadequately studied. The aim of this study was to characterize the clinical presentation, electrophysiologic and imaging features and outcome of intraneural perineurioma. We ask if intraneural perineurioma is a pure motor syndrome that remains confined to one nerve and should be treated by surgical resection. We examined the nerve biopsies of cases labelled perineurioma and selected those with diagnostic features. Thirty-two patients were identified; 16 children and 16 adults; 16 males and 16 females. Median age of onset of neurological symptoms was 14 years (range 0.5-55 years) and median age at evaluation was 17 years (range 2-56 years). All patients had motor deficits; however, mild sensory symptoms or signs were experienced by 27 patients; 'prickling' or 'asleep numbness' in 20, mild pain in 13 and sensory loss in 23. The sciatic nerve or its branches was most commonly affected in 15, followed by brachial plexus, radial nerve and ulnar nerve (four each). Magnetic resonance imaging demonstrated nerve enlargement (29/32), T1 isointensity (27/32), T2 hyperintensity (25/32) and contrast enhancement (20/20). Diagnoses were made based on targeted biopsy of the focal nerve enlargement identified by imaging. Neurological impairment was of a moderate severity (median Neuropathy Impairment Score was 12 points, range 2-49 points). All patients had focal involvement with 27 involving one nerve and five involving a plexus (one bilateral). Long-term follow-up was possible by telephone interview for 23 patients (median 36 months, range 2-177 months). Twelve patients also had follow-up neurologic evaluation (median 45 months, range 10-247 months). The median Neuropathy Impairment Score had changed from 12.6 to 15.4 points (P = 0.19). In all cases, the distribution of neurologic findings remained unchanged. Median Dyck Disability Score was 3 (range 2-5) indicating a mild impairment without interfering with activities of daily living. Ten patients judged their symptoms unchanged, nine slightly worse and four slightly better. We conclude intraneural perineurioma is a benign hypertrophic (non onion bulb) peripheral nerve tumour that presents insidiously in young people and is motor predominant with mild sensory involvement. It is most often a mononeuropathy, but a plexopathy can occur. Diagnosis of this condition requires clinical suspicion, imaging, targeted fascicular biopsy of the lesion and expertise of nerve pathologists. As these tumours are static or slowly progressive, remain confined to their original distribution and have low morbidity, they probably should not be resected routinely. Because intensive evaluation is needed for diagnosis, intraneural perineurioma is probably under-recognized.

3T magnetic resonance imaging of ankle and hindfoot tendon pathology

Ankle tendon pathology is relatively common in the active adult population. Magnetic resonance imaging is often the preferred advanced imaging option for the evaluation of tendon pathology of the hindfoot and ankle. The almost linear increase in signal-to-noise ratio provided by higher field strength imaging allows for improved image resolution and decreased scan times. Newer systems with faster gradients allow for optimal fast spin-echo imaging with lower echo spacing for longer echo train lengths and minimal image blurring. The ability to comfortably scan the ankle within the magnet isocenter using high-field strength-compatible extremity coils further maximizes the image resolution. It is imperative for the radiologist to be aware of necessary protocol adjustments and potential imaging artifacts unique to high-field strength imaging of the ankle. Our review outlines high-field strength magnetic resonance imaging technique and artifacts and also details the specifics of our own methods of ankle imaging.

MRI of peripheral nerves

MRI has become the modality of choice for imaging the peripheral nervous system. When technically optimized and customized for individual clinical problems, MRI can provide insight into the underlying causes of neoplastic, inflammatory, and other diseases affecting peripheral nerves with a high degree of accuracy and effectively distinguish benign from malignant processes. With high-resolution imaging techniques targeted fascicular biopsy can be planned to improve diagnostic yield and decrease the risk of surgically sampling primary nerve pathology.

3-Tesla imaging of the wrist and hand: techniques and applications

MR imaging of the wrist is complicated by its size and small critical ligamentous structures, such as the triangular fibrocartilage complex, intrinsic and extrinsic radiocarpal ligaments. 3-T magnetic resonance (MR) imaging with its nearly linear increase in signal-to-noise ratio allows for imaging the wrist with small fields of view and high spatial resolution, which in turn has potential to improve visualization of these small ligaments. Dedicated radiofrequency coils and appropriate imaging tools are required to optimize the potential of high field imaging of the wrist. Early results suggest improvement in diagnostic accuracy for the wrist when compared with 1.5 T in small studies; as larger series are acquired this will certainly become more evident. As experience with 3 T for the wrist and hand grows, it is becoming clear that it is the new standard for MR imaging of these small joints.

Diagnostic comparison of 1.5 Tesla and 3.0 Tesla preoperative MRI of the wrist in patients with ulnar-sided wrist pain

PURPOSE

The diagnostic sensitivity, specificity, and accuracy of 1.5 Tesla (T) and of 3.0T magnetic resonance imaging (MRI) are correlated with wrist arthroscopy findings in patients presenting with ulnar-sided wrist pain.

METHODS

The records and diagnostic MRI scans of 102 patients who presented between 1997 and 2006 with ulnar-sided wrist pain were evaluated. Preoperative MRI scans at 1.5T (n = 70) and 3.0T (n = 32) were evaluated by 2 experienced musculoskeletal radiologists with different levels of experience who were blinded to the arthroscopic findings. Preoperative MRI findings for the triangular fibrocartilage complex (TFCC), scapholunate, ulnotriquetral, and lunotriquetral ligaments were recorded and compared with findings at diagnostic arthroscopy. The sensitivity, specificity, and accuracy were calculated for both the 1.5T and 3.0T preoperative MRI scans. Statistical comparisons were made using chi-square test and JMP 6.0 software.

RESULTS

A tear of the TFCC was identified retrospectively on 1.5T images in 49 of 58 patients and on 3.0T images in 15 of 16 patients. Compared with the gold standard of arthroscopy, 1.5T wrist MRI in this patient population had a sensitivity of 85%, a specificity of 75%, and an accuracy of 83% for reader 1 for the detection of a tear of the TFCC. In the same patient population, 3.0T wrist MRI had a sensitivity of 94%, a specificity of 88%, and an accuracy of 91% for reader 1. For reader 2, the improvement in sensitivity for the lunotriquetral ligament between the 1.5T and 3.0T images was statistically significant.

CONCLUSIONS

The sensitivity, specificity, and accuracy of 3.0T wrist MRI for the TFCC is consistently higher compared with those of 1.5T wrist MRI. The trend suggests that 3.0T wrist MRI provides improved capability for detection of TFCC injuries. Given the available sample size, however, the confidence intervals around the point estimates are wide and overlapping. Further studies are needed to confirm or refute our results of the estimated sensitivity, specificity, and accuracy parameters.

The Alzheimer's Disease Neuroimaging Initiative (ADNI): MRI methods

The Alzheimer's Disease Neuroimaging Initiative (ADNI) is a longitudinal multisite observational study of healthy elders, mild cognitive impairment (MCI), and Alzheimer's disease. Magnetic resonance imaging (MRI), (18F)-fluorodeoxyglucose positron emission tomography (FDG PET), urine serum, and cerebrospinal fluid (CSF) biomarkers, as well as clinical/psychometric assessments are acquired at multiple time points. All data will be cross-linked and made available to the general scientific community. The purpose of this report is to describe the MRI methods employed in ADNI. The ADNI MRI core established specifications that guided protocol development. A major effort was devoted to evaluating 3D T(1)-weighted sequences for morphometric analyses. Several options for this sequence were optimized for the relevant manufacturer platforms and then compared in a reduced-scale clinical trial. The protocol selected for the ADNI study includes: back-to-back 3D magnetization prepared rapid gradient echo (MP-RAGE) scans; B(1)-calibration scans when applicable; and an axial proton density-T(2) dual contrast (i.e., echo) fast spin echo/turbo spin echo (FSE/TSE) for pathology detection. ADNI MRI methods seek to maximize scientific utility while minimizing the burden placed on participants. The approach taken in ADNI to standardization across sites and platforms of the MRI protocol, postacquisition corrections, and phantom-based monitoring of all scanners could be used as a model for other multisite trials.

Bias of cartilage T2 values related to method of calculation

PURPOSE

To determine how different methods for calculating T2 affect the resulting T2 values of patellar cartilage.

MATERIALS AND METHODS

T2-weighted images of patellar cartilage for 10 subjects were acquired using two MRI scanners. T2 values of patellar cartilage were calculated using linear, weighted and nonlinear fitting algorithms for a monoexponential decay equation. T2 values were also calculated for the superficial, middle and deep zones of the cartilage.

RESULTS

All three methods of calculation resulted in significantly different T2 values (P<.0001). The weighted calculation produced the highest T2 values, and the nonlinear calculation produced the lowest T2 values. The average difference of T2 value between the methods was under 5 ms. Similar results were found in a zonal analysis of the tissue. The nonlinear calculation of T2 consistently had the best fit to the acquired data.

CONCLUSION

The T2 value of patellar cartilage depends on the method of calculation. It is unclear if larger T2 value differences would be seen in subjects diagnosed with osteoarthritis. This study highlights the potential difficulty of comparing different studies with one another based on the method of T2 calculation.

Calorimetric calibration of head coil SAR estimates displayed on a clinical MR scanner

Calorimetric measurements were performed to determine the average specific absorption rates (SAR) resulting from MRI head examinations. The data were compared with average head coil SAR estimates displayed by the MR scanner in order to refine the imaging protocols used in imaging patients with implanted deep brain stimulators (DBS). The experiments were performed using transmit-receive (TR) head coil on clinical 1.5 T General Electric MR scanners running 11.0 M4 revision software. The average applied SAR was derived from temperature increases measured inside a head phantom, due to deposition of RF energy during MRI scanning with a spin echo imaging sequence. The measurements were repeated for varied levels of RF transmit gain (TG) and analyzed with a range of entered patient weights. The measurements demonstrate that the ratio of the actual average head SAR to the scanner-displayed value (coil correction factor) decreases for decreasing TG or for increasing patient weight and may vary between 0.3 and 2.1. An additional retrospective patient study, however, shows that not all combinations of TG and patient weight are encountered clinically and, instead, TG generally increases with the patient weight. As a result, a much narrower range of coil correction factors (e.g., typically 0.5-1.0) will be encountered in practice. The calorimetric method described in this work could aid the physicians and technologists in refinement of the model-dependent SAR estimates displayed by the MR scanner, and in selection of imaging parameters for MR head examinations within allowable SAR safety levels.

Magnetic resonance elastography of the brain

The purpose of this study was to obtain normative data using magnetic resonance elastography (MRE) (a) to obtain estimates of the shear modulus of human cerebral tissue in vivo and (b) to assess a possible age dependence of the shear modulus of cerebral tissue in healthy adult volunteers. MR elastography studies were performed on tissue-simulating gelatin phantoms and 25 healthy adult volunteers. The data were analyzed using spatiotemporal filters and a local frequency estimating algorithm. Statistical analysis was performed using a paired t-test. The mean shear stiffness of cerebral white matter was 13.6 kPa (95% CI 12.3 to 14.8 kPa); while that of gray matter was lower at 5.22 kPa (95% CI 4.76 to 5.66 kPa). The difference was statistically significant (p<0.0001).

Chronic exertional compartment syndrome of the lower extremities: improved screening using a novel dual birdcage coil and in-scanner exercise protocol

OBJECTIVE

The purpose of this study was to design and evaluate an MRI screening protocol for chronic exertional compartment syndrome (CECS) of the lower legs using an in-scanner exercise protocol and novel dual birdcage coil design for improved imaging.

MATERIALS AND METHODS

Coil and phantom studies: a custom-made dual birdcage coil designed for this protocol was evaluated for uniformity and signal-to-noise ratio (SNR) compared with a conventional phased-array receive-only torso coil and the body coil. Phantom and normal subject studies were performed to confirm coil performance. In-vivo studies: eight unaffected subjects and 42 patients with lower extremity symptoms suggestive of CECS were imaged with the dual birdcage coil and an in-scanner exercise protocol which included imaging at rest, during isometric resisted dorsi flexion, at rest (recovery), during isometric resisted plantar flexion and, again, at rest. Of 42 patients, 14 had confirmed CECS and 28 had lower extremity anomalies attributable to other causes. Ratios of relative T2-weighted signal intensities were calculated for exercise and recovery images compared to baseline after processing of images, including re-registration for motion, smoothing and segmentation to remove bone and pulsation artifacts from blood vessels.

RESULTS

Receiver operating characteristic (ROC) analysis showed a threshold for the ratio of relative T2-weighted signal intensity of 1.54 to have a sensitivity of 96%, specificity of 90% and accuracy of 96% for CECS. Patients with CECS had their peak ratio of signal intensity compared with baseline during the first recovery period after isometric dorsi flexion, whereas unaffected subjects and patients with other causes of exercise-induced lower extremity pain reached their peak values during exercise (P<0.001).

CONCLUSION

We have developed the first in-scanner MRI exercise protocol for the assessment of patients with suspected CECS. The technique shows high accuracy, sensitivity and specificity for diagnosis in this small cohort of patients with CECS. Further study may allow this non-invasive test to be used as a triage tool for invasive intracompartmental pressure measurements in patients with suspected CECS.

Preliminary assessment of one-dimensional MR elastography for use in monitoring focused ultrasound therapy

The purpose of this work is to assess a fast technique that measures tissue stiffness and temperature during focused ultrasound thermal therapy (FUS). A one-dimensional (1D) MR elastography (MRE) pulse sequence was evaluated for the purpose of obtaining rapid measurements of thermally induced changes in tissue stiffness and temperature for monitoring FUS treatments. The accuracy of the 1D measurement was studied by comparing tissue displacements measured by 1D MRE with those measured by the well-established 2D MRE pulse sequence. The reproducibility of the 1D MRE measurement was assessed, in gel phantoms and ex vivo porcine tissue, for varied FUS intensity levels (31.5-199.9 W cm(-2)) and over a range of displacements at the focus (0.1-1 microm). Temperature elevations in agarose gel phantoms were measured using 1D MRE and calibrated using fiberoptic-thermometer-based measurements. The 1D MRE displacement measurements are highly correlated with those obtained with the 2D technique (R(2) = 0.88-0.93), indicating that 1D MRE can successfully measure tissue displacement. Ten repeated trials at each FUS power level yielded a minimum detectable displacement change of 0.2 microm in phantoms and 0.4 microm in tissue (at 95% confidence level). The 1D MRE temperature measurements correlated well with temperature changes measured simultaneously with fiberoptic thermometers (R(2) = 0.97). The 1D MRE technique is capable of detecting tissue displacements as low as 0.4 microm, which is an order of magnitude smaller than 5 microm displacements expected during FUS therapy (Le et al 2005 AIP Conf. Proc.: Ther. Ultrasound 829 186-90). Additionally, 1D MRE was shown to provide adequate measurements of temperature elevations in tissue. These findings indicate that 1D MRE may be an effective tool for monitoring FUS treatments.

The compartments of the foot: a 3-tesla magnetic resonance imaging study with clinical correlates for needle pressure testing

BACKGROUND

Reliable measurement of subfascial pressures represents an essential part of compartment syndrome management. To date, there is neither consensus on the number or location of foot compartments, nor a standardized protocol for needle placement. The purpose of this study was to devise a new system using 3-Tesla MRI that assesses the number and location of these compartments.

METHODS

To document the specific location of foot compartments, high resolution 3-Tesla MRI (General Electric, Milwaukee, WI) was coupled with a dedicated transmit-receive high signal-to-noise foot/ankle coil (IGC-Medical Advances, Milwaukee, WI). Individual compartments were highlighted and mapped to T1-weighted MRI. Three-dimensional image analysis allowed standardized needle placement recommendations.

RESULTS

Six feet from healthy volunteers were imaged. From these, ten compartments were described: (1) medial, (2) central superficial, (3) central deep (adductor), (4) lateral, (5-8) interossei, (9) calcaneal, and (10) skin. Optimal needle placement and depth were identified.

CONCLUSIONS

The proposed system allowed us to assess the number and location of foot compartments. Computer image analysis enabled us to define exact points for needle insertion and depth of penetration for accurate pressure monitoring.

Anatomic compartments of the foot: a 3-Tesla magnetic resonance imaging study

There is neither consensus on the number nor agreement on the location of the anatomic compartments of the foot. This project utilized high-resolution magnetic resonance imaging (MRI) to identify foot compartments. The purpose of this study was to devise a new system using 3-Tesla (3T) MRI that assessed the number and location of these compartments. Six feet from healthy volunteers were imaged. From these, 10 compartments were described: (1) medial, (2) calcaneal, (3) lateral, (4) central superficial, (5) central deep (adductor), (6-9) interossei, and (10) skin. The 3T MRI and foot/ankle coil allowed us to assess the number and location of foot compartments.

Selective maximization of (31)P MR spectroscopic signals of in vivo human brain metabolites at 3T

PURPOSE

To develop a short TR, short TE, large flip angle (LFA), in vivo (31)P MR spectroscopy (MRS) technique at 3T that selectively maximizes the signal-to-noise ratio (SNR) of long T(1) human brain metabolites implicated in bipolar disorder.

MATERIALS AND METHODS

Two pulse sequences were evaluated for efficiency. Slice profiles acquired with the scaled, sinc-shaped, radiofrequency (RF) LFA pulses were compared to those acquired with Shinnar-Le Roux (SLR) RF LFA pulses. The SLR-based LFA pulse sequence was used to maximize the inorganic phosphate signal in a phantom, after which volunteer metabolite signals were selectively maximized and compared to their correlates acquired with conventional spin-echo methods.

RESULTS

The comparison of slice profiles acquired with sinc-shaped RF LFA pulses vs. SLR RF LFA pulses showed that SLR-based pulse sequences, with their improved excitation and slice profiles, yield significantly better results. In vivo LFA spin-echo MRS implemented with SLR pulses selectively increased the (31)P MRS signal, by as much as 93%, of human brain metabolites that have T(1) times longer than the TR of the acquisition.

CONCLUSION

The data show that the LFA technique can be employed in vivo to maximize the signal of long T(1) (31)P brain metabolites at a given TE and TR. LFAs ranging between 120 degrees and 150 degrees are shown to maximize the (31)P signal of human brain metabolites at 3T.

Evaluation of ganglion cysts using vastly undersampled isotropic projection reconstruction (VIPR)

For some atypical para-articular ganglia, the presence of a joint connection is highly controversial. The proper preoperative diagnosis and identification of this joint connection for ganglion cysts is important for patient treatment and outcome. MRI is the imaging modality of choice when evaluating such lesions, but the detection of subtle joint connections remains difficult with conventional MR protocols. We investigated the utility of a steady-state free-precession acquisition with isotropic high resolution using the vastly undersampled isotropic projection reconstruction (VIPR) pulse sequence to determine if joint connections for ganglion cysts could be seen more effectively, using the knee region as a model. We evaluated four patients: two with peroneal intraneural ganglion cysts, one with adventitial cystic disease of the popliteal artery, and one patient with a more typical extraneural (intramuscular) cyst. Both conventional MR and VIPR techniques were used. In our clinical experience, we found VIPR to be superior to conventional MR techniques in detecting and depicting joint connections in typical and atypical ganglion cysts around the knee.

Evaluation of mineral oil as an acoustic coupling medium in clinical MRgFUS

We empirically evaluate mineral oil as an alternative to the mixture of de-gassed water and ultrasound gel, which is currently used as an acoustic coupling medium in clinical magnetic resonance guided focused ultrasound (MRgFUS) treatments. The tests were performed on an ExAblate 2000 MRgFUS system (InSightec Inc., Haifa, Israel) using a clinical patient set-up. Acoustic reflections, treatment temperatures, sonication spot dimensions and position with respect to target location were measured, using both coupling media, in repeated sonications in a tissue mimicking gel phantom. In comparison with the water-gel mix, strengths of acoustic reflections from coupling layers prepared with mineral oil were on average 39% lower and the difference was found to be statistically significant (p = 3.3 x 10(-8)). The treatment temperatures were found to be statistically equivalent for both coupling media, although temperatures corresponding to mineral oil tended to be somewhat higher (on average 1.9 degrees C) and their standard deviations were reduced by about 1 degrees C. Measurements of sonication spot dimensions and positions with respect to target location did not reveal systematic differences. We conclude that mineral oil may be used as an effective non-evaporating acoustic coupling medium for clinical MRgFUS treatments.

Magnetic resonance measurements of renal blood flow and disease progression in autosomal dominant polycystic kidney disease

Whether changes in renal blood flow (RBF) are associated with and possibly contribute to cystic disease progression in autosomal dominant polycystic kidney disease (ADPKD) has not been ascertained. The Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP) was created to develop imaging techniques and analyses to evaluate progression. A total of 131 participants with early ADPKD had measurements of RBF and total kidney (TKV) and cyst (TCV) volumes by magnetic resonance and of GFR by iothalamate clearance at baseline and 1, 2, and 3 yr. The effects of age, gender, body mass index, hypertension status, mean arterial pressure (MAP), TKV, TCV, RBF, renal vascular resistance (RVR), GFR, serum uric acid, HDL and LDL cholesterol, 24-h urine volume, sodium (UNaE) and albumin (UAE) excretions, and estimated protein intake were examined at baseline on TKV, TCV, and GFR slopes. TKV and TCV increased, RBF decreased, and GFR remained stable. TKV, TCV, RVR, serum uric acid, UAE, UNaE, age, body mass index, MAP, and estimated protein intake were positively and RBF and GFR negatively correlated with TKV and TCV slopes. TKV, RBF, UNaE, and UAE were independent predictors of TKV and TCV slopes (structural disease progression). TKV, TCV, RVR, and MAP were negatively and RBF positively correlated with GFR slopes. Regression to the mean confounded the analysis of GFR slopes. TKV and RBF were independent predictors of GFR decline (functional disease progression). In ADPKD, RBF reduction (1) parallels TKV increase, (2) precedes GFR decline, and (3) predicts structural and functional disease progression.

Feasibility of simultaneous temperature and tissue stiffness detection by MRE

Temperature and tissue stiffness are two indices that can be used to monitor MRI-guided focused ultrasound thermal therapy. It would be beneficial to have both measures available to monitor treatment progression during thermal therapy. MR Elastography (MRE) has already been shown to provide tissue stiffness information; the purpose of this work is to demonstrate how temperature can be derived from the same MRE data acquisition. MRE data were acquired from 1.5% agarose phantoms and ex vivo porcine muscle tissue (from a grocery store) while they were heated slowly. The temperatures were measured using a fluorescent thermometer. The phase average from the MRE acquisition was used to calculate the phase shift induced by the proton resonance frequency shift associated with the temperature change. The results show that the phase shift due to temperature extracted from MRE data correlate well with the temperature change recorded by thermometer, yielding a temperature coefficient of -0.0096 ppm/ degrees C for the agarose phantom, and -0.0103 ppm/ degrees C for the ex vivo porcine tissue. These results indicate that it is possible to simultaneously measure both temperature and tissue shear stiffness using a new method of MRE data reconstruction.

Noninvasive treatment of uterine fibroids: early Mayo Clinic experience with magnetic resonance imaging-guided focused ultrasound

Uterine fibroids often cause symptoms of pelvic pain, pressure, and bleeding. Traditional therapies have included medical (eg, hormonal therapy) and surgical (eg, myomectomy, hysterectomy) options. Recently, uterine artery embolization was added to the treatment armamentarium. We describe an exciting new non-invasive treatment option using focused ultrasound with magnetic resonance imaging and summarize the early experience at the Mayo Clinic in Rochester, Minn, during the initial research studies of this new technology.

MR guided focused ultrasound: technical acceptance measures for a clinical system

Magnetic resonance (MR) guided focused ultrasound (MRgFUS) is a hybrid technique which offers efficient and safe focused ultrasound (FUS) treatments of uterine fibroids under MR guidance and monitoring. As a therapy device, MRgFUS requires systematic testing over a wide range of operational parameters prior to use in the clinical environment. We present technical acceptance tests and data for the first clinical MRgFUS system, ExAblate 2000 (InSightec Inc., Haifa, Israel), that has been FDA approved for treating uterine fibroids. These tests characterize MRgFUS by employing MR temperature measurements in tissue mimicking phantoms. The coronal scan plane is empirically demonstrated to be most reliable for measuring temperature elevations resulting from high intensity ultrasound (US) pulses ('sonications') and shows high sensitivity to changes in sonication parameters. Temperatures measured in the coronal plane were used as a measure of US energy deposited within the focal spot for a range of sonication parameters used in clinical treatments: spot type, spot length, output power, sonication duration, US frequency, and depth of sonication. In addition, MR images acquired during sonications were used to measure effective diameters and lengths of available sonication spot types and lengths. At a constant 60 W output power, the effective spot type diameters were measured to vary between 4.7 +/- 0.3 mm and 6.6 +/- 0.4 mm; treatment temperatures were found to decrease with increasing spot diameter. Prescribing different spot lengths was found to have no effect on the measured length or on measured temperatures. Tests of MRgFUS positioning accuracy determined errors in the direction parallel to the propagation of the US beam to be significantly greater than those in the perpendicular direction; most sonication spots were erroneously positioned towards the FUS transducer. The tests reported here have been demonstrated to be sufficiently sensitive to detect water leakage inside the FUS transducer. The data presented could be used for comparison by those conducting acceptance tests on other clinical MRgFUS systems.