An investigation of the effects from a urethral warming system on temperature distributions during cryoablation treatment of the prostate: a phantom study.

Introduction of urethral warmers to aid cryosurgery in the prostate has significantly reduced the incidence of urethral sloughing; however, the incidence rate still remains as high as 15%. Furthermore, urethral warmers have been associated with an increase of cancer recurrence rates. Here, we report results from our phantom-based investigation to determine the impact of a urethral warmer on temperature distributions around cryoneedles during cryosurgery. Cryoablation treatments were simulated in a tissue mimicking phantom containing a urethral warming catheter. Four different configurations of cryoneedles relative to urethral warming catheter were investigated. For each configuration, the freeze-thaw cycles were repeated with and without the urethral warming system activated. Temperature histories were recorded at various pre-arranged positions relative to the cryoneedles and urethral warming catheter. In all configurations, the urethral warming system was effective at maintaining sub-lethal temperatures at the simulated surface of the urethra. The warmer action, however, was additionally demonstrated to potentially negatively impact treatment lethality in the target zone by elevating minimal temperatures to sub-lethal levels. In all needle configurations, rates of freezing and thawing were not significantly affected by the use of the urethral warmer. The results indicate that the urethral warming system can protect urethral tissue during cryoablation therapy with cryoneedles placed as close as 5mm to the surface of the urethra. Using a urethral warming system and placing multiple cryoneedles within 1cm of each other delivers lethal cooling at least 5mm from the urethral surface while sparing urethral tissue.

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.

Phantom testing of peripheral artery. Absolute blood flow measurement with digital arteriography.

A technique to measure absolute arterial blood flow in ml/min has been developed utilizing software programs with high-speed digital recording for the off-line analysis of intra-arterial injections of contrast medium. Measurements of pulsatile flow in a phantom for a physical flow model showed that calculations with the final upgrades were within 10% of known flow, using a recording rate of 30 frames/s, a diameter tubing of 5 mm, and flow rates of 300 to 400 ml/min. Quantitative absolute flow of a peripheral artery such as the internal carotid artery may be obtained during routine cerebral arteriography for comparison with anatomic data.

An evaluation of the signal and noise characteristics of four CCD-based film digitizers.

Film digitizers are common devices in radiology departments involved with picture archive and communication systems (PACS) and teleradiology. In this paper, we studied the performance of film digitizers based on charge-coupled device detectors (CCD digitizers), and compared this with the performance of a laser digitizer (the de facto standard). Our focus was on the assessment of signal, noise and useful optical density range performance. A function (L* delta D) derived from the Rose model was used to evaluate these parameters in absolute terms, based their predicted ability to detect objects of specific size and optical density difference with respect to background. We studied CCD digitizers from four different vendors and found that none was able to reliably operate up to the maximum density of 3.0 required to digitize plain radiographs, while the laser digitizer was capable of this task. Our analysis also indicated that two of the four CCD digitizers were adequate for digitizing laser-printed cross-sectional images in certain cases. Finally, our analysis indicated that digitization of SMPTE pattern films along with visual assessment of the 5% and 95% contrast patches was not sufficient for determining the utility of film digitizers for clinical tasks. Computation of the L* delta D function provides a useful means of assessing the performance of film digitizers (e.g., for acceptance testing and quality control), and this technique may be adaptable for evaluation of other digital imaging modalities.

Reliability of water proton chemical shift temperature calibration for focused ultrasound ablation therapy.

Our purpose in this work was to assess the reliability of the calibration coefficient for magnetic resonance water proton chemical shift temperature mapping. Over a six month period, the calibration coefficient was measured 15 times in several different phantoms. A highly linear relationship between water proton chemical shift and temperature change was found. The average temperature calibration coefficient determined from all studies was 0.009+/-0.001 ppm/degrees C. Four of the 15 studies were conducted on the same day using the same phantom. The average temperature calibration coefficient of these four studies was 0.0096+/-0.0001 ppm/degrees C.

Magnetic resonance elastography: non-invasive mapping of tissue elasticity.

Magnetic resonance elastography (MRE) is a phase-contrast-based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and the generation of images that depict tissue elasticity or stiffness. This is significant because palpation, a physical examination that assesses the stiffness of tissue, can be an effective method of detecting tumors, but is restricted to parts of the body that are accessible to the physician's hand. MRE shows promise as a potential technique for 'palpation by imaging', with possible applications in tumor detection (particularly in breast, liver, kidney and prostate), characterization of disease, and assessment of rehabilitation (particularly in muscle). We describe MRE in the context of other recent techniques for imaging elasticity, discuss the processing algorithms for elasticity reconstruction and the issues and assumptions they involve, and present recent ex vivo and in vivo results.

Hand dose measurements in interventional radiology.

Measurements of radiation dose to the hand were conducted using TLD ring badges for individual interventional radiology cases. Results from over 30 examinations (including transhepatic cholangiograms and biliary and nephrostomy procedures) conducted by four radiologists using identical equipment show an average hand dose of 1.5 mGy (150 mrad) per procedure. Hand dose varied inversely with distance from the patient. Due to variable hand positions during clinical examinations, fluoroscopic time was not found to be a good indicator of hand dose.

Spatial presaturation: a method for suppressing flow artifacts and improving depiction of vascular anatomy in MR imaging.

In clinical magnetic resonance (MR) imaging, the diagnostic quality of examinations is often degraded by streaklike flow artifacts that obscure anatomic details and reduce contrast. In addition, vascular structures are often not depicted clearly because the desired flow voids are obliterated by spurious intraluminal signals. On the basis of analysis of the physical mechanism of flow artifact formation, the authors developed a new technique for suppressing these artifacts. This applies interleaved, spectrally shaped radio frequency pulses to selectively saturate spins located in regions outside the image volume. In phantom, volunteer, and clinical imaging studies, the technique has proved to be effective by yielding a striking reduction in flow artifacts and markedly improving the reliability with which arterial and venous structures are imaged. The method has few drawbacks: It is applicable to most MR pulse sequences and, in principle, can be implemented on most imagers. It is particularly helpful for high-resolution surface coil studies of the neck, mediastinal imaging, gated cardiac imaging, and for detecting thrombus and other intravascular lesions such as dissections.

Section thickness and contiguity phantom for MR imaging.

Magnetic resonance (MR) imaging systems are uniquely able to acquire data simultaneously for the reconstruction of images from multiple sections, that is, for volumes up to 20 cm or more in length and for axial, coronal, and sagittal planes. A phantom has been developed that allows one to determine if the sections are of the desired thickness throughout the volume, if the sections are contiguous or the section spacing is as specified, if the central section is at the appropriate location, if the profile of the section is as specified by the manufacturer, if the signal strength is uniform for all sections, that the signal strength decreases in a consistent manner for second and subsequent echo images, and that gross spatial distortions are not present. Such measurements are essential for acceptance testing and quality control purposes over reasonably long volumes. The details of this new phantom are described, and results of its use with both low-and high-field-strength MR imaging systems are presented.

Flow artifact reduction in MRI: a review of the roles of gradient moment nulling and spatial presaturation.

In the past, flow artifacts and inconsistent depiction of vascular anatomy have represented significant problems in clinical MRI. These difficulties are now generally well addressed by the techniques of gradient moment nulling and spatial presaturation. Gradient moment nulling (GMN) is an effective method for eliminating flow artifacts in gradient echo images, while presaturation is more applicable to the same task in spin echo acquisitions. The GMN technique also has useful applications in spin echo imaging such as combating the effects of tissue and CSF motion in long TE sequences. In contrast to presaturation, however, GMN is not suitable for suppressing artifacts due to pulsatile blood flow in spin echo images.

Adaptive technique for high-definition MR imaging of moving structures.

An adaptive technique for measuring and correcting the effects of patient motion during magnetic resonance image acquisition was developed and tested. A set of algorithms that can reverse the effects of object displacements and phase shifts was used. These algorithms essentially transfer the frame of reference of the image reconstruction from the static frame of the imager couch to the moving "visceral frame." An accurate record of tissue motion during image acquisition is required. To achieve this, the authors used specially encoded "navigator" echoes that are interleaved with the imaging sequence. Postprocessing of the navigator echo data provides a highly detailed record of the displacements and phase shifts that occur during imaging. Phantom studies demonstrated that the technique can directly correct image degradation caused by motion. In contrast to conventional artifact reduction techniques, such as ordered phase encoding and gradient moment nulling, this new method has a unique capacity to reduce motion unsharpness. Preliminary in vivo studies have demonstrated that the technique can markedly improve images degraded by voluntary motion and shows promise for addressing the problem of respiratory motion in thoracoabdominal imaging.

Radio-frequency survey at the bore of a 1.5-T MR imager.

A survey at the bore of a 1.5-T magnetic resonance (MR) imager assessed radio-frequency (RF) exposure. With variable pulse sequences and loading conditions, the RF power density at relevant occupational positions was below measurable limits (less than the threshold limit value for occupational workers of 1 mW/cm2). Exposure to RF fields is below safe limits for personnel who routinely work within an MR imaging suite.

Phase alignment of multiple surface coil data for reduced bandwidth and reconstruction requirements.

Multiple element surface coils are often used in clinical MRI to increase the image signal-to-noise ratio (S/N). Use of multicoils typically requires increased net sampling bandwidth and data processing for each coil element. A phase-alignment technique is described which combines the signals from all coil elements before image reconstruction, greatly relaxing the technical requirements of the standard multicoil methods. Hardware and software implementations allow reduction of the reconstruction requirement to that of a single coil. The hardware implementation additionally allows a significant reduction in the net sampling bandwidth. The method is applicable to high speed MRI techniques, as demonstrated in phantoms and volunteers.

Optimization of a contrast-detail-based method for electronic image display quality evaluation.

The authors previously reported a general technique based on contrast-detail methods to provide an overall quantitative evaluation of electronic image display quality. The figure-of-merit reflecting overall display quality is called maximum threshold contrast or MTC. In this work we have optimized the MTC technique through improvements in both the test images and the figure-of-merit computation. The test images were altered to match the average luminance with that observed for clinical computed radiographic images. The figure-of-merit calculation was altered to allow for contrast-detail data with slopes not equal to -1. Preliminary experiments also were conducted to demonstrate the response of the MTC measurements to increased noise in the displayed image. MTC measurements were obtained from five observers using the improved test images displayed with maximum monitor luminance settings of 30-, 50-, and 70-ft-Lamberts. Similar measurements were obtained from two observers using test images altered by the addition of a low level of image noise. The noise-free MTC and MTC difference measurements exhibited standard deviations of 0.77 and 1.55, respectively. This indicates good measurement precision, comparable or superior to that observed using the earlier MTC technique. No statistically significant image quality differences versus maximum monitor luminance were seen. The noise-added MTC measurements were greater than the noise-free values by an average of 4.08 pixel values, and this difference was statistically significant. This response is qualitatively correct, and is judged to indicate good sensitivity of the MTC measurement to increased noise levels.

Half-value-layer increase owing to tungsten buildup in the x-ray tube: fact or fiction.

The half-value layer (HVL) of an x-ray beam is generally believed to increase with x-ray tube use. This increase in HVL has previously been attributed to the hardening of the x-ray beam as a result of a buildup of tungsten on the x-ray tube glass window. Radiographs and HVL measurements were obtained to determine the effect of tungsten deposited on the x-ray tube windows. This work, along with the HVL data from approximately 200 functioning x-ray tubes used for all applications that were monitored for more than 8 years, indicated there is no significant increase in HVL with diagnostic x-ray tube use.

US-guided percutaneous biopsy: use of a screw biopsy stylet to aid needle detection.

Insertion of a screw biopsy stylet into a thin-walled biopsy needle greatly enhances detection of the needle during ultrasound-guided percutaneous biopsy. This technique is helpful when precise needle-tip localization is needed for biopsies of small lesions.

Cerebrospinal fluid-iophendylate contrast on gradient-echo MR images.

The effect on the signal intensities of cerebrospinal fluid (CSF) and iophendylate (Pantopaque) and on CSF-iophendylate contrast was studied in vitro with a small-nutation-angle (alpha) gradient refocused magnetic resonance (MR) imaging technique (GRASS) as alpha, repetition time (TR), and echo time (TE) were varied. CSF signal intensity was consistently greater than that of iophendylate. Therefore, retained intraspinal iophendylate may be considered in the differential diagnosis of focal areas of low signal intensity at the periphery of the spinal canal on GRASS images. At constant TE and TR, an increase in alpha from 6 degrees to 45 degrees increased the signal intensities of CSF and iophendylate but decreased CSF-iophendylate contrast. At constant alpha and TR, an increase in TE from 13 to 28 msec decreased the signal intensities of CSF and iophendylate but increased contrast. At constant alpha and TE, an increase in TR from 50 to 400 msec increased the signal intensities of CSF and iophendylate, as well as contrast. Clinical examples of the contrast behavior of retained intraspinal iophendylate on both spin-echo and GRASS images corroborate the experimental findings. Retained intraspinal iophendylate may mimic the appearance of intra-or extra-dural lesions, magnetic susceptibility artifact, and flow on gradient-echo MR images of the spine.

Adaptive technique for three-dimensional MR imaging of moving structures.

The authors describe an adaptive motion correction method for three-dimensional magnetic resonance (MR) imaging. Three-dimensional imaging offers many advantages over two-dimensional multisection imaging but is susceptible to image corruption due to motion. Thus, it has been of limited use in the imaging of mobile structures, and the relatively long imaging times required have hindered its use in patients who tend to move during imaging. The authors' technique uses interleaved "navigator" echoes to provide a measure of displacement for each image echo in the acquisition and then uses this information to allow correction of the image data. The theory for signal corruption due to motion and the correction scheme that follows from it are presented. This method can produce excellent results when the motion is correctly modeled.