MRgFUS ablation of a recurrent tenosynovial giant cell tumor in the foot using ExAblate 2100 system in combination with patient immobilization device.

INTRODUCTION: Magnetic Resonance-guided Focused Ultrasound (MRgFUS) treatment for certain anatomy locations can be extremely challenging due to patient positioning and potential motion. This present study describes the treatment of a recurrent tenosynovial giant cell tumor of the plantar forefoot using the ExAblate 2100 system in combination with patient immobilization device. METHODS: Prior to the treatment, several patient immobilization devices were investigated. Vacuum cushions were selected and tested for safety and compatibility with the treatment task and the MR environment. RESULTS: During the treatment, one vacuum cushion immobilized the patient's right leg in knee flexion and allowed the bottom of the foot to be securely positioned on the treatment window. Another vacuum cushion supported the patient upper body extended outside the scanner bore. 19 sonications were successfully executed. The treatment was judged to be successful. No immediate complications were observed. CONCLUSIONS: MRgFUS treatment of a recurrent tenosynovial giant cell tumor of the right plantar forefoot was successful with the use of patient immobilization vacuum cushions. IMPLICATIONS FOR PRACTICE: The immobilization system could be utilized to aid future MRgFUS treatment of lesions in challenging anatomic locations. Various sizes of the vacuum cushions are available to potentially better accommodate other body parts and treatment configurations.

Practical implementation of robust MR-thermometry during clinical MR-guided microwave ablations in the liver at 1.5 T.

Practical non-invasive equipment modifications and effective acquisition methods to achieve robust and reliable real-time MR thermometry for monitoring of clinical hepatic microwave ablations were implemented. These included selection of the microwave generator location (inside versus outside the MR scan room), the number of radiofrequency chokes added to the microwave generator's coaxial lines, and the use of copper wool to maximize their electrical grounding. Signal-to-noise ratio (SNR) of MR thermometry images of a small fluid-filled phantom acquired during activation of microwave antenna were used to evaluate image quality as a function of each modification. SNR measurements corresponding to both locations of the microwave generator were comparable and so it was located outside the MR scan room. For this location, addition of one RF choke on the power and four chokes on the sensor coaxial lines was found to be optimal, corresponding to a 68% increase in SNR. Furthermore, image quality strongly depended on the proper electrical grounding of the power and sensor lines. SNR ratio (relative to SNR of baseline images) during activation of microwave generator was found to be 0.49 ±â€¯0.28 without adequate grounding, and 0.88 ±â€¯0.08 with adequate grounding (p = 0.002, Student's t-test). These SNR measurements were sufficiently sensitive to detect issues related to equipment performance and hence formed part of the quality assurance testing performed prior to each clinical treatment. Incorporating these non-invasive approaches resulted in significant improvements to image quality and, importantly while maintaining the clinical integrity of the microwave system which is of paramount importance in a highly regulated healthcare environment.

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.

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.

Implementation and validation of three automated methods for measuring ultrasound maximum depth of penetration: application to ultrasound quality control.

Systematic measurements of maximum depth of penetration (DOP) of ultrasound (US) scanners are essential for quality control (QC). Conventionally, DOP measurements are performed visually and as such they could be affected by various external factors, scanner control settings, and operator related errors. Automated methods should be free of the issues associated with interoperator dependence and are an attractive alternative to the visual DOP measurements. We implement and test three automated methods for measuring DOP. The methods base their measurements on signal to noise (SNR) analysis of uniform US phantom images. Two of the methods use pairs of phantom images. The third one uses a single phantom image and an "in-air" image. The validation tests included precision, sensitivity, repeatability, and usability in routine QC application. Methods based on pairs of phantom images measure the DOP with precision +/-0.2 cm or better. Precision of the single phantom image method is +/-0.05 cm, and that method is also the most sensitive of the three. All three methods are demonstrated to be repeatable among different users. Since the images for the DOP computation are collected free-hand the sensitivity to hand-transducer motion during image acquisition was also tested. Unlike the single-phantom-image based method, the methods using image pairs were found to be very sensitive to transducer motion and therefore less convenient for clinical QC applications. In conclusion, the single-phantom-image method is best suited for routine QC in a real-life clinical practice.

The calibration of experimental self-developing Gafchromic HXR film for the measurement of radiation dose in computed tomography.

A prototype, self-developing Gafchromic HXR film has sensitivity an order of magnitude larger than that of the commercially available Gafchromic XR film used in interventional radiological applications. The higher sensitivity of the HXR film allows the possibility of acquisition of high-resolution calibrated dose profiles within the diagnostic range of exposure levels, below 10 R (87.7 mGy). We employed a commercially available, optical flatbed scanner for digitization of the film and image analysis software to determine the response of the HXR films to ionizing radiation. Spatial uniformity and temporal repeatability of the flatbed scanner were determined and used in optimization of the digitization protocol. The HXR film postexposure density growth and sensitivity to ambient light were determined using multiple scans of two simultaneously exposed sheets, one stored in light-tight conditions and the other continuously illuminated with white light. A calibrated step wedge of the HXR film was obtained by simultaneous irradiation of a portion of a film strip and a calibrated ionization chamber using a radiographic x-ray tube with beam characteristics matched to a typical CT scanner (8 mm Al HVL, 120 kVp). Repeated digitization of the calibration film was used to determine the precision of the film response measurements. The precision, as measured by the standard deviation of multiple measurements, was better than 1% over the full dynamic range of film response. This precision was measured using exposures ranging from 0.5 to 12 R (4.4 to 105.3 mGy). This exposure range is highly relevant to x-ray computed tomography. Preliminary radiation dose profiles demonstrate the utility of this technique.

Comparison of ultrasound quality assurance phantom measurements from matched and mixed scanner-transducer combinations.

Our goal in this work was to compare the results of common phantom tests made using matched and mixed ultrasound (US) scanner-transducer combinations. Sets of common US quality assurance (QA) measurements were made using matched US scanner-transducer combinations (i.e., transducers purchased for use with a particular scanner), as well as unmatched (mixed) combinations. Measurements of vertical and horizontal distance accuracy, and depth of penetration were performed using three common transducer types. Means, standard deviations, and differences between the mean mix and match measurements divided by the standard deviation (match-mix difference, or MMD), and two-sided, paired t-tests were computed for the groups of mixed and matched measurements. MMDs for vertical and horizontal distance accuracy test results were less than 0.87 in all cases, well below our threshold value of 2.0, which indicates that a significant difference exists. MMDs for the depth of penetration measurements were less than 1.50, again below the threshold value. These results suggest that all of the mixed and matched data sets were very similar. The more sensitive t-tests indicate statistically significant differences in only 2 of the 18 pairs of data sets. In conclusion, this study suggests that QA measurements generated by mixed or matched scanner-transducer combinations are very comparable. The ability to obtain QA phantom test data from mixed scanner-transducer combinations reduces the time required for US QA testing.

The effects of irreversible JPEG compression on an automated algorithm for measuring carotid artery intima-media thickness from ultrasound images.

Our ultrasound practice has begun to investigate automated measurements of carotid artery intima-media thickness (IMT) as an indicator of subtle atherosclerosis. Since our clinical ultrasound images are irreversibly compressed, we investigated the effects of this compression on our IMT measurements. We obtained 10 ultrasound images of normal carotid arteries. These were compressed using JPEG to ratios of 5:1, 10:1, 15:1, 20:1, and 30:1. IMT measurements made from all compressed and uncompressed images were compared. For compression ratios ?10:1, IMT deviations between compressed and uncompressed images were ?0.03 mm. Higher than 10:1, the overall IMT deviations were small (0.01 +/- 0.04 mm), although one 25% deviation was measured. Comparison of other parameters yielded similar results. This initial study indicates that compression at 10:1 using baseline JPEG should have little effect on IMT measurements made using the current algorithm, and that compression to 20:1 or 30:1 may be feasible.

Requirements for an enterprise digital image archive.

This report describes several image archival problems facing the authors' department and the results of their attempt to define the requirements for an enterprise digital image archive. The problems identified include the costs of supporting multiple distinct archives, the increased complexity of supporting multiple archive interfaces, the differences in data handling policies and resulting variations in data integrity, and variability in support for nonimage data. The authors also describe the data collected including image volumes and trends and imaging device trends. Finally, the resulting specification for an enterprise digital image archive, including storage and retrieval performance and interface requirements are presented.

Determining the sharpness of electronic image displays: an evaluation of three methods.

The authors evaluated 3 methods developed to assess the level of monitor cathode ray tube (CRT) sharpness. Results include a comparison of 2 observer-based methods to a charged coupled device (CCD) digital camera-based method for the purposes of CRT equipment comparison, acceptance testing, and routine CRT quality control. Three methods designed to measure a monitor's sharpness were evaluated on a single 20-inch CRT monitor. We defined signal-to-noise ratio (SNR) to be the overall signal difference measured by each method from the highest to lowest values divided by the average standard deviation of the measurements. Comparing the results of the 3 methods, the authors found that the digital CCD camera-based method provided a much higher SNR than the 2 observer-based methods and, therefore, is the preferred of the 3 methods for measuring the sharpness of CRT displays. The SNR values for the CCD, Cx and line target methods were 151.5, 4.9, and 4.3, respectively. The Cx target observer-based method (a novel target and scoring routine dubbed the "Cx" target because of its appearance) had a higher SNR than the line target observer-based method. The average time and standard deviation required to score the Cx and the line targets were 5.45 +/- 2.15 minutes and 8.34 +/- 2.95 minutes, respectively. The observer-based method results (and variability) versus the camera-based method results (and variability) indicate strong linear relationships. Exploring this finding and the optimization of the camera-based method are the subjects of future research.

Digital radiology equipment acquisition and installation procedures: a team approach at Mayo Clinic, Rochester, MN.

Digital imaging system integration is a complex process. A project team and a defined process for system planning, evaluation, and implementation can improve the chance for success. In this presentation, our project team relates their experiences.

Cathode ray tube quality control and acceptance testing program: initial results for clinical PACS displays.

The use of picture archiving and communication systems (PACS) and primary soft-copy interpretation in radiology is growing rapidly. The authors present a cathode ray tube (CRT) acceptance test and quality control (QC) program developed over a 5-year period on the basis of experience with multiple PACS and CRT vendors. The CRT QC procedures address monitor cleanliness and setup, qualitative image quality, and quantitative luminance and color measurements. Required materials include a photometer with luminance and color probes and 100%-video, flat-field window and test images from the Society of Motion Picture Test Engineers (SMPTE). Luminance was found to change over time for all gray-scale CRTs examined, which necessitated quarterly recalibration. The phosphor color of these monitors was also found to change, but changes were consistent and slow enough to warrant only annual measurements. Color measurements were found to be especially useful at initial setup and for CRT replacement. Use of this program allowed standardization of absolute luminance of individual CRTs, matching of phosphor color for multimonitor workstations, and systematic tracking of image artifacts. Implementation of a QC program is strongly recommended owing to the dynamic nature of CRT displays.

Initial evaluation of a continuous speech recognition program for radiology.

The aims of this work were to measure the accuracy of one continuous speech recognition product and dependence on the speaker's gender and status as a native or nonnative English speaker, and evaluate the product's potential for routine use in transcribing radiology reports. IBM MedSpeak/Radiology software, version 1.1 was evaluated by 6 speakers. Two were nonnative English speakers, and 3 were men. Each speaker dictated a set of 12 reports. The reports included neurologic and body imaging examinations performed with 6 different modalities. The dictated and original report texts were compared, and error rates for overall, significant, and subtle significant errors were computed. Error rate dependence on modality, native English speaker status, and gender were evaluated by performing ttests. The overall error rate was 10.3 +/- 3.3%. No difference in accuracy between men and women was found; however, significant differences were seen for overall and significant errors when comparing native and nonnative English speakers (P = .009 and P = .008, respectively). The speech recognition software is approximately 90% accurate, and while practical implementation issues (rather than accuracy) currently limit routine use of this product throughout a radiology practice, application in niche areas such as the emergency room currently is being pursued. This methodology provides a convenient way to compare the initial accuracy of different speech recognition products, and changes in accuracy over time, in a detailed and sensitive manner.

Effects of x-ray spectra on the DQE of a computed radiography system.

The effect of incident x-ray beam quality on the measured detective quantum efficiency (DQE) of a computed radiography system was investigated. The incident x-ray beams used had peak tube potentials of 70, 95, and 120 kVp, were filtered with various thicknesses of a "patient equivalent phantom" (PEP), aluminum, and copper, and provided a consistent exposure to the storage phosphor. For each peak tube potential and filter combination, the one-dimensional modulation transfer function and noise power spectrum were measured and the square of the incident signal-to-noise ratio was estimated. The spatial frequency dependent DQE was calculated from these data. The DQE was integrated to provide an overall estimate of the efficiency and frequency response of the computed radiography system for the various x-ray beams. There was found to be a wide range of integral DQE (IDQE) values for the peak tube potential and filter combinations used. For example, the IDQE ranged from 3.0 to 0.9 mm(-2) using the peak tube potential and filter combinations 70 kVp with 5.1 cm PEP and 120 kVp with 30.3 cm PEP, respectively. Finally, peak tube potential and filter combinations 70 kVp with 10.2 cm PEP and 120 kVp with 20.2 cm PEP were chosen as standard x-ray beams that will be used at our facility to measure the DQE of digital radiographic imaging systems for evaluation and acceptance testing.

An investigation of the effects of mammographic acquisition parameters on a semiautomated quantitative measure of breast cancer risk.

The aim of this work was to investigate the effect of mammographic acquisition parameter variations on the estimation of percent density (PD) produced by a particular semiautomated algorithm. The PD algorithm requires the user to specify a threshold pixel value segmenting breast tissue of greater and lesser density. A whole breast specimen was imaged using a variety of acquisition techniques, and the image data were processed as prescribed by the PD algorithm. PD estimates for all possible values of the user threshold were calculated for all the images. The image data were normalized so that PD varied between 30% and 80% over a fixed threshold range of 23, and a PD value of 50% was obtained for a threshold value of 195. PD differences between all the images and a baseline standard mammographic acquisition technique were calculated. We also estimated PD differences caused by small (3%) variations in operator selection of the threshold value. We found that the largest differences in PD involved changes in the density control of the mammography unit, and changes in the detector (either film type or computed radiography). The maximum PD differences due to technique were all less than 10%, with root-mean-square (RMS) variations less than 4%. PD differences due to operator variation were 24% (maximum) and 6.1% (RMS). These findings suggest that PD differences due to mammographic technique will be considerably less than those inherent to the technique, due to operator variation. All of these estimates are likely larger than differences seen in practice since optimization of the threshold by the operator was not considered in this analysis.

Ultrasound grayscale image compression with JPEG and wavelet techniques.

The purpose of the study was to evaluate the effects of lossy compression on grayscale ultrasound images to determine how much compression can be applied while still maintaining images that are acceptable for diagnostic purposes. The study considered how the acquisition technique (video frame-grabber versus directly acquired in digital form) influences how much compression can be applied. For directly acquired digital images, the study considered how text (that is burned into the image) affects the compressibility of the image. The lossy compression techniques that were considered include JPEG and a Wavelet algorithm using set partitioning in hierarchical trees (SPIHT).

Image quality evaluation of a desktop computed radiography system.

The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the Lumisys ACR-2000 desktop computed radiography (CR) reader were measured and compared to equivalent measurements acquired from a Fuji AC-3 CR system. The one-dimensional (1D) MTF was measured from an image of a sharp edge and the 1D NPS was derived from a 2D NPS measured from a uniform field exposure. The energy dependent ideal input signal to noise ratio of the incident x-ray beams was estimated using published x-ray spectra and attenuation coefficients. Measurements were acquired using Agfa, Fuji, and Kodak storage phosphor plates and it was concluded that use of the Fuji plates resulted in the highest system DQE for the ACR-2000. The DQE was measured using exposures of 0.10, 1.0, and 10.0 mR from 70 and 120 kVp x-ray beams filtered with aluminum. The DQE of the Lumisys ACR-2000 was lower than that of the Fuji AC-3.

Electronic imaging impact on image and report turnaround times.

We prospectively compared image and report delivery times in our Urgent Care Center (UCC) during a film-based practice (1995) and after complete implementation of an electronic imaging practice in 1997. Before switching to a totally electronic and filmless practice, multiple time periods were consistently measured during a 1-week period in May 1995 and then again in a similar week in May 1997 after implementation of electronic imaging. All practice patterns were the same except for a film-based practice in 1995 versus a filmless practice in 1997. The following times were measured: (1) waiting room time, (2) technologist's time of examination, (3) time to quality control, (4) radiology interpretation times, (5) radiology image and report delivery time, (6) total radiology turn-around time, (7) time to room the patient back in the UCC, and (8) time until the ordering physician views the film. Waiting room time was longer in 1997 (average time, 26:47) versus 1995 (average time, 15:54). The technologist's examination completion time was approximately the same (1995 average time, 06:12; 1997 average time, 05:41). There was also a slight increase in the time of the technologist's electronic verification or quality control in 1997 (average time, 7:17) versus the film-based practice in 1995 (average time, 2:35). However, radiology interpretation times dramatically improved (average time, 49:38 in 1995 versus average time 13:50 in 1997). There was also a decrease in image delivery times to the clinicians in 1997 (median, 53 minutes) versus the film based practice of 1995 (1 hour and 40 minutes). Reports were available with the images immediately upon completion by the radiologist in 1997, compared with a median time of 27 minutes in 1995. Importantly, patients were roomed back into the UCC examination rooms faster after the radiologic procedure in 1997 (average time, 13:36) than they were in 1995 (29:38). Finally, the ordering physicians viewed the diagnostic images and reports in dramatically less time in 1997 (median, 26 minutes) versus 1995 (median, 1 hour and 5 minutes). In conclusion, a filmless electronic imaging practice within our UCC greatly improved radiology image and report delivery times, as well as improved clinical efficiency.