Showing papers in "Journal of Magnetic Resonance Imaging in 1998"

Interactive MRI-guided radiofrequency interstitial thermal ablation of abdominal tumors: clinical trial for evaluation of safety and feasibility.

Abstract: This clinical trial was performed to evaluate the safety and feasibility of interactive MR-guided radiofrequency (RF) interstitial thermal ablation (ITA) performed entirely within the MR imager. RF-ITA was performed on 11 intra-abdominal metastatic tumors during 13 sessions. The RF electrode was placed under MR guidance on a .2-T system using rapid fast imaging with steady state precession (FISP) and true FISP images. A custom 17-gauge electrode was used and was modified in four sessions to allow circulation of iced saline for cooling during ablation. Tissue necrosis monitoring and electrode repositioning were based on rapid T2-weighted and short-inversion-time inversion recovery (STIR) sequences. Morbidity and toxicity were assessed by clinical and imaging criteria. The region of tissue destruction was visible in all 11 tumors treated, as confirmed on subsequent contrast-enhanced images. No significant morbidity was noted, and patient discomfort was minimal. In conclusion, interactive MR-guided RF-ITA is feasible on a clinical .2-T C-arm system with supplemental interventional accessories with only minor patient morbidity. The ability to completely ablate tumors with RF-ITA depends on tumor size and vascularity.

Remember true FISP? A high SNR, near 1-second imaging method for T2- like contrast in interventional MRI at .2 T

Abstract: Clinical requirements for interventional MRI (I-MRI) monitoring of needle placement or thermal ablation demand rapid (near-real-time) image acquisition rates, high spatial resolution, and T2 weighting. Experimental analysis performed earlier (see ref. 8) suggests that many sequences used for either rapid scanning or T2 weighting at high fields fail to meet both the speed (conventional spin echo [SE], turbo SE) or contrast (ie, fast low-angle shot [FLASH], fast imaging with steady state precession [FISP]) requirements when used at .2 T. In this work, we revisited a number of pulse sequences advocated primarily for higher field applications requiring T2 weighting and found that refocused steady state coherent pulse sequences, aka, true FISP sequences, performed superiorly in achieving both speed and T2 contrast requirements for I-MRI at .2 T. This work focuses on our experience with this new/old technique in the I-MRI setting and describes how one can take advantage of the low field strength and modest inhomogeneity of .2 T (and similar) systems to design pulse sequences that balance TE, TR (and hence T2 dephasing), and resonant offset frequency effects to provide images with the desired contrast and minimal artifactual field inhomogeneity “banding.” At high flip angles (∼90°), reasonably short TEs (∼5 msec) and short TRs (∼ 10 msec), we have used this method in our last 25 I-MRI procedures (biopsies and/or radiofrequency [RF] thermal ablations) and found these sequences to be extremely useful in both needle localization phases of I-MRI biopsy procedures, RF thermal ablation electrode guidance, and posttherapy imaging assessment. Design methods and clinical I-MRI cases are presented that highlight these points.

1H MRS in acute traumatic brain injury.

Abstract: The objective of this study was to demonstrate 1H MR spectroscopy (MRS) changes in cerebral metabolites after acute head trauma. Twenty-five patients (12 children, 13 adults) were examined with quantitative 1H MRS after closed head injury. Clinical grade (Glasgow Coma Scale [GCS]) and outcome (Rancho Los Amigos Medical Center Outcome Score [ROS]) were correlated with quantitative neurochemical findings. N-acetylas-partate (NAA), a neuronal and axonal marker, was reduced (P < .03−.001). In children, a reduced NAA/creatine plus phosphocreatine (Cr) level and the presence of detectable lipid/lactate predicted bad outcome (sensitivity, 89%; specificity, 89%). The first MRS examination of all patients correlated with ROS versus NAA (r = .65, P < .0001). Although most patients showed MRS abnormalities, striking heterogeneity of 1H MRS characterized the individual patients. 1H MRS identifies multiple patterns of diffuse brain injury after blunt head trauma. There was a strong correlation between MRS and outcome. Future prospective studies will be needed to determine the clinical usefulness of MRS in predicting outcome from closed head injury.

Quantification of in-plane motion of the coronary arteries during the cardiac cycle: implications for acquisition window duration for MR flow quantification.

Abstract: Motion of the coronary arteries during the heart cycle can result in image blurring and inaccurate flow quantification by MR. This condition applies particularly for longer acquisition windows that are typical of breath-hold coronary flow measurements. To determine the sensitivity of the technique to in-plane motion of different coronary arteries, the temporal variation in coronary position was measured in a plane perpendicular to the proximal portion of the vessel. The results indicated the presence of substantial displacement of the coronary arteries within the cardiac cycle, with a magnitude of motion approximately twice as large for the right as for the left coronary arteries. An estimation of the resulting vessel blurring was calculated, showing that the duration of the acquisition window for high spatial resolution coronary flow acquisitions should be less than 25 to 120 msec, depending on the specific coronary artery studied. In addition, these data specify optimal acquisition window placement for high resolution coronary angiography.

Limits to the accuracy of vessel diameter measurement in MR angiography.

Abstract: This work addresses the fundamental limits imposed by the MRI process on the accuracy with which vessel diameters and cross-sectional areas can be derived from time-of-flight (TOF) and phase-contrast (PC) MR source images. By means of simulations and in vitro experiments, it is demonstrated that, even in the absence of flow-related artifacts, severe inaccuracies in the determination of diameters or cross-sectional areas may occur solely because of the physical process of the MR image acquisition. Resolution and intraluminal saturation have strong effects on the vessel appearance and thus on the diameter estimation error. It is shown that low resolution leads to diameter overestimation or even underestimation and that intraluminal saturation causes severe underestimation, even for relatively low flip angles. Velocity and velocity encoding do not have a major influence on lumen appearance in PC images. Accurate diameter estimations can be attained only if lumen diameters constitute at least three pixels for both TOF and PC acquisitions, provided that intraluminal saturation is suppressed or avoided. Additionally, since the constitution of TOF and PC images is dissimilar, lumina should be analyzed differently to obtain accurate diameters and cross-sectional areas.

MRI evaluation of thermal ablation of tumors with focused ultrasound.

Abstract: MRI was used to target and evaluate the tissue effects of focused ultrasound ablation on tumors implanted in the skeletal muscle of rabbits in vivo. First, MRI was used to localize the tumors and plan the ultrasound therapy. Second, temperature-sensitive phase-difference images were acquired to monitor the location of the ultrasound focus and to estimate the effects of temperature rise. After the treatment, the spatial and temporal temperature profiles for defining boundaries of tissue coagulation were calculated. Finally, these boundaries were compared to T2-weighted and contrast-enhanced T1-weighted images obtained immediately after therapy. The results indicate that using MRI for planning and evaluating focused ultrasound surgery is feasible. We showed a linear relationship between applied power and shifts in the proton resonant frequency. Fluctuations in the location of the focus about the target location were on the order of the resolution of the MR images. The temperature rise and lesion size varied significantly. Regions of tissue coagulation calculated from MR data correlated well with post-therapy imaging.

Water-fat imaging with direct phase encoding (DPE)

Abstract: A method acquires three complex images, wherein their respective phase differences between water and fat components are α 0 , α 0 +α, and α 0 +2α, respectively. The method obtains from these three complex images two possible solution sets for water and fat images. For pixels with both water and fat components, one correct solution set is selected using a binary choice based on the relative Larmor frequencies of water and fat. If a pixel contains only one component, a known statistical bias is applied to identify the component and, thus, the pixel. To correct misidentified pixels, various filters may be applied to all the pixels. The water and fat solutions obtained from the three complex images are used to produce separate images of water and fat. Second pass solutions of water and fat with improved signal-to-noise ratio can be obtained by either averaging or a least square error method.

Dynamic scan plane tracking using MR position monitoring

Abstract: An MRI system measures the movements of a subject during the acquisition of a series of NMR images and automatically updates the scan parameters such that the image plane or volume tracks the movement of the anatomy of interest. An array of tracking coils fasten to the subject and an NMR measurement pulse sequence is interleaved with the image acquisitions to measure the location of the tracking cols.

A new method for imaging perfusion and contrast extraction fraction: input functions derived from reference tissues.

Abstract: This study describes a new method for analysis of dynamic MR contrast data that greatly increases the time available for data acquisition. The capillary input function, CB(t), is estimated from the rate of contrast agent uptake in a reference tissue such as muscle, based on literature values for perfusion rate, extraction fraction, and extracellular volume. The rate constant for contrast uptake (the product of perfusion rate, F, and extraction fraction, E; F-E) is then determined in each image pixel using CB(t), extracellular volume (relative to the reference tissue) measured from MR and the tissue concentration of contrast media as a function of time calculated from the MR data. The “reference tissue method” was tested using rats with mammary (n = 10) or prostate (n = 15) tumors implanted in the hindlimb. Dynamic MR images at 4.7 T were acquired before and after Gd-DTPA intravenous bolus injections to determine F·EGd-DTPA. Acquisition parameters were optimized for detection of the first pass of the contrast agent bolus, so that “first-pass analysis” could be used as the “gold standard” for determination of F·E. The accuracy of values of F·E determined using the reference tissue method was determined based on comparison with first-pass analysis. In some cases, deuterated water (D2O) was injected IV immediately after Gd-DTPA measurements, and the reference tissue method was used to calculate F, based on the rate of uptake of D2O. Comparison of rate constants for Gd-DTPA uptake and D2O uptake allowed calculation of EGd-DTPA. Values for F·EGd-DTPA, F, and EGd.DTPA were determined for selected regions and on a pixel-by-pixel basis. Values for F·E and EGd.DTPA measured using the reference tissue method correlated well (P = .90 with a standard error of ±.016, n = 15) with values determined based on first-pass contrast media uptake. The reference tissue method has important advantages: (a) A large volume of reference tissue can be used to determine the contrast agent input function with high precision. (b) Data obtained for 20 minutes after injection are used to calculate F or F·E. The greatly increased acquisition time can be used to increase the spatial resolution, field of view or SNR of measurements. The reference tissue method is most useful when the volume of tissue that must be imaged and/or the spatial resolution required precludes use of traditional first-pass methods.

Neuroimaging in epilepsy

Abstract: Neuroimaging techniques have improved the understanding, diagnosis, and management of epilepsy. By providing excellent structural information, MRI is the technique of choice in evaluating patients with epilepsy. Functional imaging techniques, including MR spectroscopy, functional MRI, positron emission tomography, and single photon emission CT, permit noninvasive assessment of the epileptic substrate, its functional status, and neuroreceptors. The MRI-based techniques will potentially assume a greater role in the cost-effective workup of the patient. Currently, newer techniques such as magnetoencephalography, magnetic source imaging, and optical imaging are research tools.

In vivo MRI thermometry using a phase-sensitive sequence : preliminary experience during MRI-guided laser-induced interstitial thermotherapy of brain tumors

Abstract: The purpose of this study was the application of the proton-resonance-frequency method to monitor laser-induced interstitial thermotherapy (LITT) in a patient with an astrocytoma WHO II. A phase-sensitive two-dimensional (2D) fast low-angle shot (FLASH) sequence was used to determine the temperature-related phase shifts during LITT. Temperature maps were displayed during therapy with a temporal resolution of 20 seconds. Irradiation was discontinued as soon as the 60 to 65 degrees C isotherm reached the margin of the tumor. A contrast-enhanced MRI study performed immediately after therapy showed a good correlation of the size of an enhancing rim around the lesion with the 60 to 65 degrees C isotherm. The preliminary results of our study indicate that MRI guidance of LITT may be improved by temperature quantification based on the proton-resonance-frequency method.

Quantification of patellar tracking using kinematic MRI

Abstract: The purpose of this study was to describe a method to quantify dynamic patellar tracking using kinematic MRI (KMRI). Twelve normal females and three patients with patellofemoral pain participated. Imaging was performed with a 1.5-T/64-MHz MR system using a fast spoiled gradient-recalled acquisition in the steady state (GRASS) pulse sequence. A nonferromagnetic positioning device permitted active, bilateral knee extension against resistance (15% bwt) from 45° knee flexion to full extension. Subjects were instructed to extend their knees at a rate of 9° per second, which allowed images to be obtained at 45°, 36°, 27°, 18°, 9°, and 0°. All images were assessed for medial/lateral patellar displacement, patellar tilt, and sulcus angle using a computer-aided system. Normal patellar motion was characterized by medial movement from 45° to 18°, followed by a reversal toward lateral displacement from 18° to full extension. The results for patellar tilt revealed a tendency toward decreasing lateral tilt as the knee extended. Sulcus angle measurements indicated that the patella was moving to a more shallow portion of the trochlear groove (superiorly) during extension.

Invited. On the feasibility of MRI-guided focused ultrasound for local induction of gene expression

Abstract: Gene therapy is a promising approach to the treatment of many forms of disease, including cancer. Of critical concern in its implementation is the ability to control the location, duration, and level of expression of the therapeutic gene. Here, we propose the use of local heat in combination with a heat-sensitive promoter to help accomplish this. Certain members of the family of heat shock protein (hsp) promoters display a regulation that depends strongly on temperature. We present a study of natural hsp70 induction in rat leg by MRI-guided focused ultrasound to investigate the hsp70 promoter as a possible candidate for use in control of gene expression with local heat. A temperature increase of 5–8°C in the focal region for 45 minutes led to a differential expression of the hsp70 mRNA between the focal region and the surrounding tissue ranging from a factor of 3 to 67.

Magnetic resonance imaging of shear wave propagation in excised tissue.

Abstract: The propagation of shear waves in ex vivo tissue samples, agar/gel phantoms, and human volunteers was investigated. A moving coil apparatus was constructed to generate low acoustic frequency shear perturbations of 50 to 400 Hz. Oscillating gradients phase-locked with the shear stimulus were used to generate a series of phase contrast images of the shear waves at different time-points throughout the wave cycle. Quantitative measurements of wave velocity and attenuation were obtained to evaluate the effects of temperature, frequency, and tissue anisotropy. Results of these experiments demonstrate significant variation in shear wave behavior with tissue type, whereas frequency and anisotropic behavior was mixed. Temperature-dependent behavior related mainly to the presence of fat. Propagation velocities ranged from 1 to 5 m/sec, and attenuation coefficients of from 1 to 3 nepers/unit wavelength, depending on tissue type. These results confirm the potential of elastic imaging attributable to the intrinsic variability of elastic properties observed in normal tissue, although some difficulty may be experienced in clinical implementation because of viscous attenuation in fat.

In vivo quantitative mapping of cardiac perfusion in rats using a noninvasive MR spin-labeling method.

Abstract: Measurement of myocardial perfusion is important for the functional assessment of heart in vivo. Our approach is based on the modification of the longitudinal relaxation time T1 induced by magnetic spin labeling of endogenous water protons. Labeling is performed by selectively inverting the magnetization within the detection slice, and longitudinal relaxation is measured using a fast gradient echo MRI technique. As a result of blood flow, nonexcited spins enter the detection slice, which leads to an acceleration of the relaxation rate. Incorporating this phenomenon in a mathematical model that describes tissue as two compartments yields a simple expression that allows the quantification of perfusion from a slice-selective and a global inversion recovery experiment. This model takes into account the difference between T1 in blood and T1 in tissue. Our purpose was to evaluate the feasibility and reproducibility of this technique to map quantitatively myocardial perfusion in vivo in rats. Quantitative maps of myocardial blood flow were obtained from nine rats, and the reproducibility of the technique was evaluated by repeating the whole perfusion experiment four times. Evaluation of regions of interest within the myocardium yielded a mean perfusion value of 3.6 ± .5 ml min−1 g-1 over all animals, which is in good agreement with previously reported literature values.

Optimized outer volume suppression for single‐shot fast spin‐echo cardiac imaging

Abstract: Among the ultrafast MRI techniques, the single-shot fast spin-echo sequence offers a robust alternative to echo planar imaging, essentially because of a much reduced sensitivity to B0 inhomogeneity. This property is particularly appealing in situations in which B0 in-homogeneities can be severe and difficult to correct, such as in cardiac imaging. With single-shot cardiac imaging, however, achieving high resolution over the necessarily large field of views without introducing back-folding artifacts is problematic. One option is to use multishot sequences. However, then issues related to cardiac gating arise. Another solution is to use, optimized presaturation slabs with quadratic phase pulses generated by the Shinnar-LeRoux algorithm. These can be set to reduce the field of view in the phase-encoding direction, resulting in a reduction in the number of phase-encoding steps. For instance, for a 1 × 2-mm spatial resolution, over a rectangular, 250 × 125-mm field of view, and using a half Fourier acquisition, an echo-train length of only 40 is required. With a 4.5-msec echo spacing, the total imaging time is ≈ 180 msec. The efficacy of this solution on phantoms and volunteers is demonstrated. Multislice short-axis examinations of the whole heart, realized within a single short breath-hold of ∼ 10 seconds, are shown. The possibility of investigating not only cardiac anatomy but also both contractility and myocardial perfusion is discussed.

Treatment planning for MRI-guided laser-induced interstitial thermotherapy of brain tumors--the role of blood perfusion.

Abstract: MR techniques have been demonstrated to allow a reliable monitoring of laser-induced interstitial thermotherapy (LITT). However, an adequate on-line control of this coagulation technique requires an exact therapy planning. The latter is mandatory to interpret the MR-monitoring data correctly to guarantee a precise laser irradiation. Moreover, it is a prerequisite for on-line decisions if modifications of the therapeutic regimen are required. In this work, we present a new simulation technique for LITT planning. The model accounts for the specific geometry of the treatment site, the exact configuration of the applicator, and the optical and thermal properties of the tissue, including changes during the heating process. The simulation results were compared with MR scans of laser-induced lesions in three patients with World Health Organization (WHO) grade II astrocytoma. Special interest was directed toward the role of blood perfusion, which was studied parametrically. Good agreement between the simulation results and the MR data was found if the appropriate blood perfusion rates were taken into account. Thus, the model can generate valid therapy plans allowing a precise on-line control of laser irradiation using MR techniques. Neglecting adequate perfusion parameters resulted in substantial errors with respect to the prediction of the final laser lesion.

In vivo 1H MRS of normal breast and breast tumors using a dedicated double breast coil.

Abstract: Image-guided localized proton MR spectroscopy (MRS) of normal breasts and breast tumors (ductal and undifferentiated carcinomas) was performed using a dedicated double breast coil. In vivo 1H MR spectra from 10 normal volunteers showed signals from water and lipids only, even in breasts with small contribution of fatty breast tissue. In the spectra from 6 of the 12 examined patients, an intense signal assigned to choline compounds was detected. The signal was also detected at lower levels in the remaining patients. This study shows that in vivo 1H MRI/MRS examinations of breast tumors can be performed within an examination time of 45 to 60 minutes. Signals from breast tumor metabolites may be detected using in vivo 1H MRS.

Monitoring and visualization techniques for MR-guided laser ablations in an open MR system.

Abstract: Our purpose was to develop temperature-sensitive MR sequences and image-processing techniques to assess their potential of monitoring interstitial laser therapy (ILT) in brain tumors (n = 3) and liver tumors (n = 7). ILT lasted 2 to 26 minutes, whereas images from T1-weighted fast-spin-echo (FSE) or spoiled gradient-recalled (SPGR) sequences were acquired within 5 to 13 seconds. Pixel subtraction and visualization of T1-weighted images or optical flow computation was done within less than 110 msec. Alternating phase-mapping of real and imaginary components of SPGR sequences was performed within 220 msec. Pixel subtraction of T1-weighted images identified thermal changes in liver and brain tumors but could not evaluate the temperature values as chemical shift-based imaging, which was, however, more affected by susceptibility effects and motion. Optical flow computation displayed the predicted course of thermal changes and revealed that the rate of heat deposition can be anisotropic, which may be related to heterogeneous tumor structure and/or vascularization.

Active MR visualization of a vascular guidewire in vivo

Abstract: The purpose of this study was development of an actively visualized .035-inch vascular guidewire for use in MR-guided interventions. The guidewire was actively visualized by inclusion of a 6-cm-long radiofrequency coil in its tip. A high contrast outline of the distal tip of the guidewire was obtained by acquiring an image with the radiofrequency coil as the receiving antenna. The position of the guidewire relative to the surrounding anatomy was determined by overlaying the guidewire image on a previously acquired road map. The guidewire was evaluated in vivo in the abdominal vessels of a rabbit and swine at 1.5 T. The built-in radiofrequency coil delivered a high contrast signal over its full length, enabling visualization of the position and curvature of the tip of the guidewire. The ability to see the curvature of the guidewire over several centimeters significantly eased manipulation into targeted vessels and represents an important advance toward MR-guided vascular interventions.

Temperature monitoring of interstitial thermal tissue coagulation using MR phase images.

Abstract: The temperature-dependent water proton frequency shift was investigated for temperature monitoring of interstitial thermal coagulation. A procedure for on-line temperature calculation was developed, and errors due to temperature-dependent susceptibility were investigated by finite element analysis and reference measurements. The temperature coefficient of magnetic susceptibility and proton chemical shift were determined for brain tissue and other substances. With the proposed procedure, the location of isotherms could be well visualized during laser-induced interstitial coagulation in vitro and in vivo. Systematic errors caused by magnetic susceptibility changes with temperature depend strongly on the characteristics of the heat source and can exceed susceptibility effects caused by physiologic tissue changes. For the laser applicators discussed here, however, a first order compensation for this effect was found to be satisfactory, because it reduces the absolute error to the range of +/- 1 degrees C. The proposed method represents a very promising approach for monitoring of the interstitial thermal coagulation.

Calibration of water proton chemical shift with temperature for noninvasive temperature imaging during focused ultrasound surgery.

Abstract: The present work was performed to calibrate water proton chemical shift change with tissue temperature in vivo to establish a method of quantitative temperature imaging during focused ultrasound surgery. The chemical shift change measured with a phase-mapping method using spoiled gradient-recalled acquisition in steady state (SPGR) (TR = 26 msec, TE = 12.8 msec, matrix = 256 x 128) was calibrated with the corresponding temperature elevation (0-50 degrees C, 32-84 degrees C in absolute temperature) measured with a copper-constantan thermocouple (.05-mm-diameter bare wires) in rabbit skeletal muscle (16 animals) under focused ultrasound exposures (10-100 W radiofrequency [RF] power, 20-second sonication). A linear calibration with a regression coefficient of (-8.76+/-.69) x 10(-3) ppm/degrees C (P < .01 [P, significance level]) was obtained. Temperature distributions during a 20-second sonication were visualized every 3.3 seconds with a 2.3-mm3 spatial resolution and 4 degrees C temperature uncertainty.

Dependence of apparent diffusion coefficients on axonal spacing, membrane permeability, and diffusion time in spinal cord white matter.

Abstract: We used a numerical simulation of water self-diffusion among permeable cylinders to predict the dependence of MR-based apparent diffusion coefficients in white matter on axonal separation, barrier permeability, and diffusion time (T). The transverse apparent diffusion coefficient (tADC), calculated with simulated diffusion-sensitizing gradients perpendicular to the axon fibers, remains a function of T down to diffusion times as short as .1 μsec for a range of diffusion barrier permeability. As the diffusion time lengthens, the response of tADC depends on axon diameter, with decreases in tADC occurring earliest, and most dramatically, for the smallest fiber diameter simulated (2 μm). For a given axonal separation, asymptotic values of ADC are determined by permeability alone and are the same for 2-μm and 11-μm fibers of equal membrane permeability. The effect of increased relative intracellular volume is manifested primarily in a decrease in tADC at short T. Increases in interaxonal spacing increase the tADC at asymptotically long diffusion times and reduce the dependence on permeability. However, at the widest plausible axonal separations, permeability remains an important determinant of tADC. These simulations may enhance interpretation of measured tADC in the context of the underlying physiologic and structural changes at the cellular level that accompany white-matter disease.

MR monitoring of MR-guided radiofrequency thermal ablation of normal liver in an animal model

Abstract: The purpose of this study was to determine the suitability of MRI to accurately detect radiofrequency (RF) thermoablative lesions created under MR guidance. In vivo RF lesions were created in the livers of six New Zealand White rabbits using a 2-mm-diameter titanium alloy RF electrode with a 20-mm exposed tip and a 50-W RF generator. This was performed using a 0.2T clinical C-arm MR imager for guidance and monitoring. Each animal was sacrificed and gross evaluation was performed. Histologic correlation was performed on the first two animals. The MR-compatible RF electrode was easily identified on rapid gradient-echo images used to guide electrode placement. A single lesion was created in each rabbit liver. Lesions ranged from approximately 10 to 17 mm in diameter (mean, 13.5 mm). T2-weighted and short T1 inversion recovery (STIR) images demonstrated lesions ranging in diameter from 12 to 18 mm (mean, 14.6 mm). Lesion dimensions determined from images closely correlated with those determined at gross examination with the discrepancy never exceeding 2 mm, for an r2 value of .87. MRI performed at the time of MR-guided RF ablation accurately demonstrated created lesions. This modality may provide a new option for the treatment of local and regional neoplastic disease.

Ultrafast MR imaging of the abdomen: Echo planar imaging and diffusion- weighted imaging

Abstract: Ultrafast MRI technique has become available with the introduction of new generation MR scanners for abdominal imaging. However, there is no consensus about the optimal imaging acquisition at the present time. Because single shot echo planar imaging (EPI) technique is based on high technology and had just applied in clinical imaging, further clinical investigation will be needed. Currently, the hypersensitivity to magnetic inhomogeneity and local magnetic susceptibility and the low spatial resolution may limit the widespread application of EPI technique. In addition to providing information for morphologic diagnosis, EPI will be more widely used for functional and qualitative diagnosis. Diffusion-weighted imaging can be used for differentiation of solid tumors according to their different cellular construction, evaluation of cystic lesions based on the different viscosity of their contents, and assessment of diffused pathologic changes in the parenchyma of solid organs. In addition to the previous parameters such as proton density and T1 and T2 values, diffusion factors may provide important information for the qualitative and dynamic evaluation of abdominal pathologic changes. Even though there are many difficulties that must be solved for diffusion-weighted imaging, a more wide application of this technique is expected through technologic improvement.

Invited. MR systems for image‐guided therapy

Abstract: The use of MRI to guide and monitor interventional procedures requires the merging of surgical and MRI environments. The ideal magnet shape for homogeneity and efficiency is spherical, but this design provides no access. Opening the sphere to provide both patient and surgeon access suggests cylindrical or biplanar magnets. Cylindrical magnets have poor surgical access but provide good imaging capabilities, which can be used in conjunction with a neighboring but distinct surgical environment. Biplanar magnets provide more and better approaches to the patient, but generally with lower field strength. Vertical biplanar systems allows surgical approaches from above but reduce the access of support staff to the patient. A hybrid magnet design, which combines the benefits of both cylindrical and biplanar magnets, can provide increased access with simultaneous approach from two sides of the patient. Application-specific magnets can target a smaller region, leading to compact magnet designs that greatly expand access for both surgical intervention as well as patient support. As the field of interventional MRI matures, the suitability of each design to specific applications will be better understood, leading to more integrated system designs tailored to the needs of image-guided therapy.

Functional MRI of the lumbar spine in erect position in a superconducting open-configuration MR system: Preliminary results

Abstract: The purpose of this study was to determine the feasibility of obtaining, and findings in, functional MRI of the lumbar spine in an erect position and with flexion and extension. Thirty subjects (including 5 volunteers) were imaged in a sitting position and while performing flexion and extension. The alternations in posterior disk margin, size of neural foramina, and central canal were evaluated. In addition, routine supine imaging was accomplished in 15 of these subjects. The foraminal size and posterior disk margins did not change appreciably from supine to upright position. With extension, there was an increased disk bulge in 27% of disks (40% of those with desiccation). Central canal size (50%) and foraminal size (27%) decreased with extension, especially at levels with disk desiccation. Images obtained with our open-configuration MR unit were diagnostically adequate, although of inferior quality compared with those obtained with a conventional unit. Our preliminary results show the feasibility of obtaining diagnostic images of the erect lumbar spine with flexion and extension. The results are in agreement with those obtained with cadaveric studies. The utility of this method in diagnostic imaging of patients with low back pain remains to be determined.

A method for simultaneous RF ablation and MRI

Abstract: Radiofrequency (RF) energy has many advantages in thermal tumor ablation protocols. With the recent development of open MRI systems, interventional MRI procedures, including thermal ablation, have become the focus of great research interest. However, the significant interference between RF generators and MR imagers has prevented simultaneous imaging and RF ablation and, until now, has limited the role of RF-based thermal therapy in interventional MRI. Here, a simple switching circuit designed with consideration of patient safety provides compatibility between open MRI systems and RF thermal lesion generators. The experimental results show that the switching circuit allows imaging during RF ablation and opens new opportunity for MR-guided thermal therapy.

MRI of patellar articular cartilage: Evaluation of an optimized gradient‐echo sequence (3D‐DESS)

Abstract: Our purpose was to evaluate the diagnostic efficacy of a gradient-echo sequence optimized for cartilage imaging in patellar cartilage abnormalities and to compare it to a standard turbo-spin-echo sequence Fifty-eight consecutive patients who underwent, within 3 months both MRI and arthroscopy or surgery, were included in the investigation Two radiologists specializing in musculoskeletal imaging independently assessed axial three-dimensional double-echo steady state (3D-DESS) gradient-echo images and sagittal proton- and T2-weighted turbo-spin-echo images with regard to retropatellar cartilage abnormalities Possible findings were: 0: normal, 1: cartilage softening, and 2: lesion of the articular surface Inter- and intraobserver variability was assessed For cartilage softening, the axial 3D-DESS sequence had a sensitivity of 73%, a specificity of 75%, and an accuracy of 70% The corresponding results for the sagittal turbo-spin-echo sequence were 53%, 65%, and 62% For surface lesions, the results for the 3D-DESS sequence were 43%, 92%, and 83% and for the turbo-spin-echo sequence were 60%, 92%, and 86% Intra- and interobserver agreement was moderate (k = 059 and 045 [DESS], 06 and 046 [turbo -spin-echo]) We conclude that the 3D-DESS sequence is moderately accurate in detecting patellar cartilage abnormalities Compared with the sagittal turbo-spin-echo sequence, the axial 3D-DESS sequence is superior in diagnosing cartilage softening but not surface lesions