Showing papers in "Journal of Magnetic Resonance Imaging in 2000"

Magnetic resonance temperature imaging for guidance of thermotherapy

Abstract: Continuous thermometry during a hyperthermic procedure may help to correct for local differences in heat conduction and energy absorption, and thus allow optimization of the thermal therapy. Noninvasive, three-dimensional mapping of temperature changes is feasible with magnetic resonance (MR) and may be based on the relaxation time T(1), the diffusion coefficient (D), or proton resonance frequency (PRF) of tissue water. The use of temperature-sensitive contrast agents and proton spectroscopic imaging can provide absolute temperature measurements. The principles and performance of these methods are reviewed in this paper. The excellent linearity and near-independence with respect to tissue type, together with good temperature sensitivity, make PRF-based temperature MRI the preferred choice for many applications at mid to high field strength (>/= 1 T). The PRF methods employ radiofrequency spoiled gradient-echo imaging methods. A standard deviation of less than 1 degrees C, for a temporal resolution below 1 second and a spatial resolution of about 2 mm, is feasible for a single slice for immobile tissues. Corrections should be made for temperature-induced susceptibility effects in the PRF method. If spin-echo methods are preferred, for example when field homogeneity is poor due to small ferromagnetic parts in the needle, the D- and T(1)-based methods may give better results. The sensitivity of the D method is higher that that of the T(1) methods provided that motion artifacts are avoided and the trace of D is evaluated. Fat suppression is necessary for most tissues when T(1), D, or PRF methods are employed. The latter three methods require excellent registration to correct for displacements between scans.

Sensitivity and power deposition in a high-field imaging experiment.

Abstract: Image signal-to-noise ratio and power dissipation are investigated theoretically up to 400 MHz. While the text is mathematical, the figures give insights into predictions. Hertz potential is introduced for probe modeling where charge separation cannot be ignored. Using a spherical geometry, the potential from current loops that would produce a homogeneous static B1 field is calculated; at high frequency it is shown to create an unnecessarily inhomogeneous field. However, a totally homogeneous field is shown to be unattainable. Boundary conditions are solved for circularly polarized fields, and strategies for limited shimming of the sample B1 field are then presented. A distinction is drawn between dielectric resonance and spatial field focusing. At high frequency, the region of maximum specific absorption is shown to move inside the sample and decrease. From the fields in both rotating frames, the signal-to-noise ratio is derived and compared with the traditional, low-frequency formulation. On average, it is mostly found to be slightly larger at high frequency. Nevertheless, the free induction decay is sometimes found to be annulled.

Safety of Strong, Static Magnetic Fields

Abstract: Issues associated with the exposure of patients to strong, static magnetic fields during magnetic resonance imaging (MRI) are reviewed and discussed. The history of human exposure to magnetic fields is reviewed, and the contradictory nature of the literature regarding effects on human health is described. In the absence of ferromagnetic foreign bodies, there is no replicated scientific study showing a health hazard associated with magnetic field exposure and no evidence for hazards associated with cumulative exposure to these fields. The very high degree of patient safety in strong magnetic fields is attributed to the small value of the magnetic susceptibility of human tissues and to the lack of ferromagnetic components in these tissues. The wide range of susceptibility values between magnetic materials and human tissues is shown to lead to qualitatively differing behaviors of these materials when they are exposed to magnetic fields. Mathematical expressions are provided for the calculation of forces and torques.

Heating Around Intravascular Guidewires by Resonating RF Waves

Abstract: We examined the unwanted radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional magnetic resonance imaging (MRI). A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during an FFE sequence [average specific absorption rate (SAR) = 3.9 W/kg] was measured with a Luxtron fluoroscopic fiber. Starting from 26 degrees C, the guidewire tip reached temperatures up to 74 degrees C after 30 seconds of scanning. Touching the guidewire may cause sudden heating at the point of contact, which in one instance caused a skin burn. The excessive heating of a linear conductor like the guidewire can only be explained by resonating RF waves. The capricious dependencies of this resonance phenomenon on environmental factors have severe consequences for predictability and safety guidelines.

Radiofrequency energy-induced heating during MR procedures: a review.

Abstract: During an MR procedure, most of the transmitted RF power is transformed into heat within the patient's tissue as a result of resistive losses. Not surprisingly, the primary bioeffects associated with the RF radiation used for MR procedures are directly related to the thermogenic qualities of this electromagnetic field. This review article discusses the characteristics of RF energy-induced heating associated with MR procedures, with an emphasis on thermal and other physiologic responses observed in human subjects.

Safety of approved MR contrast media for intravenous injection.

Abstract: In the last 10 years, the use of intravenous contrast media in magnetic resonance (MR) has become well-established clinical practice. Contrast media provide critical additional diagnostic information in many instances. The gadolinium chelates constitute the largest group of MR contrast media and are considered to be very safe. These agents are thought to be safer than nonionic iodinated contrast agents. Unlike x-ray agents, the gadolinium chelates are not nephrotoxic. Minor adverse reactions, including nausea (1%-2% for all agents) and hives (<1% for all agents), occur in a very low percent of cases. Health care personnel should be aware of the (extremely uncommon) potential for severe anaphylactoid reactions in association with the use of MR contrast media and be prepared should complications arise. The four gadolinium chelates currently available worldwide, gadopentetate dimeglumine, gadoteridol, gadodiamide, and gadoterate meglumine, cannot be differentiated on the basis of adverse reactions. Far fewer patients have been examined to date with the two other agents that have widespread approval, mangafodipir trisodium and ferumoxides. These latter two agents are considered to be very safe but have a higher percentage of associated adverse reactions (7%-17% with mangafodipir trisodium and 15% with ferumoxides). This review discusses the safety issues involved with administration of intravenous contrast media in MR imaging, focusing on the six agents (four gadolinium chelates, one manganese chelate, and the last a large iron particle) with widespread use world-wide.

Contrast-enhanced 3D MRA using SENSE.

Abstract: Sensitivity encoding (SENSE) was used to improve the performance of three-dimensional contrast-enhanced magnetic resonance angiography (3D CE-MRA). Utilizing an array of receiver coils for sensitivity encoding, the encoding efficiency of gradient-echo imaging was increased by factors of up to three. The feasibility of the approach was demonstrated for imaging of the abdominal vasculature. On the one hand, using a SENSE reduction factor of two, the spatial resolution of a breath-hold scan of 17 seconds was improved to 1.0 × 2.0 × 2.0 mm3. On the other hand, using threefold reduction, time-resolved 3D CE-MRA was performed with a true temporal resolution of 4 seconds, at a spatial resolution of 1.6 × 2.1 × 4.0 mm3. CE-MRA with SENSE was performed in healthy volunteers and patients and compared with a standard protocol. Throughout, diagnostic quality images were obtained, showing the ability of sensitivity encoding to enhance spatial and/or temporal resolution considerably in clinical angiographic examinations. J. Magn. Reson. Imaging 2000;12:671–677. © 2000 Wiley-Liss, Inc.

Dynamic image interpretation of MRI of the breast

Abstract: Dynamic breast MRI provides information on both lesion cross-sectional morphology and functional lesion features such as vascularity/perfusion and vessel permeability. This review gives an overview of the historical background of dynamic contrast-enhanced breast MRI. It explains the technique's pathophysiological basis, describes the various technical approaches that have been pursued and the corresponding interpretation guidelines that have been proposed (including their respective diagnostic accuracies), and presents established and evolving clinical applications of the "dynamic approach" to breast MRI. J. Magn. Reson. Imaging 2000;12:965-974.

Review of patient safety in time-varying gradient fields.

Abstract: In magnetic resonance, time-varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (i.e., tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%-100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reilly's predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5-T magnetic fields.

Magnetization transfer and multicomponent T2 relaxation measurements with histopathologic correlation in an experimental model of MS.

Abstract: Magnetization transfer and multicomponent T2 imaging techniques were implemented to study guinea pig in vivo. A chronic-progressive model of experimental allergic encephalomyelitis (EAE) was produced, and the inflammatory component of the disease was manipulated using antibodies against integrin. The magnetization transfer ratio (MTR) and T2 relaxation properties were measured in normal-appearing white matter (NAWM) with histological comparisons. Significant reductions in both the mean MTR and the myelin water percentage were measured in NAWM of EAE guinea pig brain. However, the MTR and myelin water percentage appear to measure different aspects of pathology in NAWM in EAE. Reductions in the MTR were prevented or reversed with suppression of inflammation. However, modulation of inflammatory activity was not reflected in the measurement of the myelin water percentage. Since the amount of myelin is not expected to vary with inflammatory-related changes, these observations support our hypothesis that the MTR is sensitive to physiological changes to myelin induced by inflammation, while the short T2 component is a more specific indicator of myelin content in tissue. Pathologic features other than demyelination may be important in the determination of the MTR. J. Magn. Reson. Imaging 2000;11:586‐595. © 2000 Wiley-Liss, Inc.

Improved 3D quantitative mapping of blood volume and endothelial permeability in brain tumors.

Abstract: We describe a new method to allow simultaneous mapping of endothelial permeability and blood volume in intracranial lesions. The technique is based on a tumor leakage profile during the first pass (fp) of contrast bolus calculated from the time-dependent plasma-contrast concentration function (PCCF) in three-dimensional (3D) T1-weighted dynamic studies. The performance of the method has been evaluated by comparing results with those obtained from more conventional methods in patients with primary brain neoplasms. The new permeability maps (k(fp)) are visually compatible with those calculated using a conventional multicompartment model (k(tran)). Quantitatively, the new maps are free from overestimation of k(tran) due to first-pass effects. The new blood volume maps, which segment out the contamination of contrast leakage, agree closely with maps derived from susceptibility studies. The new method is fast, robust, and easy to perform. The method is suitable for use in clinical environments and is likely to be of benefit where longitudinal assessment of treatment response is required.

Auditory Noise Associated With MR Procedures: A Review

Abstract: This review article discusses the various types of acoustic noise produced during the operation of MR systems, describes the characteristics of the acoustic noise, and presents information regarding noise control techniques. In addition, the problems related to acoustic noise for patients and healthcare workers are discussed.

Non-contrast-enhanced MR angiography using 3D ECG-synchronized half-Fourier fast spin echo.

Abstract: A non-contrast-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) technique, which acquires images in a reasonably short scanning time and requires no contrast agent, is developed. An electrocardiographically (ECG) synchronized 3D half-Fourier fast spin-echo (FSE) technique with an appropriate ECG delay time for every slice encoding in 3D terms was used to examine the thoracic and iliac regions in 16 healthy volunteers at both 0.5 and 1.5 T. Prior to each 3D fresh blood imaging (FBI) experiment, an ECG preparation (ECG-prep) scan was acquired, and an appropriate ECG triggering time was selected for 3D FBI acquisition to optimize visualization of the vessel of interest. In the thoracic and abdominal regions, good-quality 3D MRA images were obtained. Furthermore, the weighted subtraction of two images in different phases provides contrast enhancement between arteries and veins.

Reconstruction of blood flow patterns in a human carotid bifurcation: a combined CFD and MRI study.

Abstract: The carotid bifurcation is a common site for clinically significant atherosclerosis, and the development of this disease may be influenced by the local hemodynamic environment. It has been shown that vessel geometry and pulsatile flow conditions are the predominant factors that determine the detailed blood flow patterns at the carotid bifurcation. This study was initiated to quantify the velocity profiles and wall shear stress (WSS) distributions in an anatomically true model of the human carotid bifurcation using data acquired from magnetic resonance (MR) imaging scans of an individual subject. A numerical simulation approach combining the image processing and computational fluid dynamics (CFD) techniques was developed. Individual vascular anatomy and pulsatile flow conditions were all incorporated into the computer model. It was found that the geometry of the carotid bifurcation was highly complex, involving helical curvature and out-of-plane branching. These geometrical features resulted in patterns of flow and wall shear stress significantly different from those found in simplified planar carotid bifurcation models. Comparisons between the predicted flow patterns and MR measurement demonstrated good quantitative agreement.

Local hyperthermia with MR-guided focused ultrasound: spiral trajectory of the focal point optimized for temperature uniformity in the target region.

Abstract: The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum amount of time. We describe a noninvasive approach to this goal based on magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR imager and that has an adjustable focal length in the third dimension. Absorption of FUS gives rise to a highly localized thermal buildup, which then spreads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum FUS power. Differences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, FUS heating is applied only at the region's boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entire target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia as a spatially uniform rise of the signal intensity in T(2)-weighted images.

Neuroendocrine tumors of the pancreas: Spectrum of appearances on MRI

Abstract: We reviewed our 8.5 year experience with magnetic resonance imaging (MRI) in the demonstration of neuroendocrine tumors of the pancreas using precontrast fat-suppressed T1-weighted, fat-suppressed T2-weighted, and serial post-gadolinium T1-weighted images, to describe the spectrum of appearances of these tumors. All MR examinations of patients with histologically proven neuroendocrine tumors were retrospectively reviewed. Histological type, tumor location, tumor diameter, signal intensity on precontrast images, enhancement patterns, and presence and appearance of metastases were determined. Twenty-two patients had histologically proved neuroendocrine tumors detected by MRI over the 8.5 year period. Histological types were gastrinoma (n = 8), insulinoma (n = 3), glucagonoma (n = 2), somatostatinoma (n = 1), VIPoma (n = 1), ACTHoma (n = 1), carcinoid (n = 1), and five untyped tumors. Primary tumors ranged in diameter from 1 to 6.2 cm. There was one histopathology-proven false-positive neuroendocrine tumor. The positive predictive value for MRI in the detection of these tumors was 96%. The most common appearance on precontrast images was low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, which was observed in tumors in 18 of 22 patients. Moderate or intense early enhancement of all or portions of the primary tumors was observed in tumors in 19 of 22 patients either as uniform homogeneous, ring, or diffuse heterogeneous enhancement. Enhancement was minimal on these images in the other three patients. Gastrinomas enhanced in a ring pattern in 7 of 8 patients whereas the majority (9 of 11 patients) of noninsulinoma-nongastrinoma and untyped tumors enhanced in a diffuse heterogeneous fashion. Liver metastases were present in 13/22 patients including 3/8 with gastrinoma and 9/11 with noninsulinoma-nongastrinoma tumors. Most neuroendocrine tumors of the pancreas are low signal intensity on fat-suppressed T1-weighted images and moderately high in signal intensity on fat-suppressed T2-weighted images, although variations do exist. Tumors most often enhance in an early moderately intense fashion. Gastrinomas are often different in appearance than other neuroendocrine tumors in that they usually enhance in a ring fashion whereas nongastrinoma-noninsulinoma tumors usually enhance in a heterogeneous fashion.

Three-dimensional, time-resolved (4D) relative pressure mapping using magnetic resonance imaging.

Abstract: We describe here a method for generating relative pressure maps from magnetic resonance velocity data in three spatial and one temporal dimension (4D). The relative pressure map calculated for pulsatile flow in a compliant phantom was shown to be consistent with independent pressure transducer measurements. The feasibility of performing 4D pressure mapping in vivo is also demonstrated. J. Magn. Reson. Imaging 2000;12: 321‐329. © 2000 Wiley-Liss, Inc.

Artery and vein separation using susceptibility-dependent phase in contrast-enhanced MRA

Abstract: In magnetic resonance angiography, contrast agents are frequently used to help highlight arteries over background tissue. Unfortunately, enhancing veins hamper the visualization of arteries when data are collected over a long period of time after the arterial phase of the contrast agent. To overcome this problem, we have developed a novel imaging and postprocessing method that is capable of eliminating veins by utilizing the susceptibility difference between veins and surrounding tissue. This method was applied in the peripheral vasculature where the vessels are predominantly parallel to the main field and where the blood oxygen level-dependent effect is most pronounced. Results are presented for both long (15.8 msec) and short echo times (7.8 msec) and for sequential and centrally reordered acquisition schemes. The short echo scan approach appears to be the most promising, making it possible to obtain good suppression of the venous signal even when the timing is not perfect or when repeat scans are necessary.

Reduced power multislice MDEFT imaging.

Abstract: A novel method is presented for acquiring multislice T1-weighted images. The method utilizes non-slice-selective inversion pulses followed by a series of slice-selective excitations. k-space is divided into a number of segments equal to the number of slices. Successive segments of k-space are assigned to successive slice-selective pulses, and the order in which the slices are excited is manipulated to ensure that images of each slice have identical contrast and point spread function (PSF). This method is applied to the MDEFT experiment, a particular version of the inversion recovery experiment. The implications of this acquisition scheme on the PSF are examined, and it is shown that, provided the k-space modulation function does not change sign, a good PSF is achieved. For a given maximum number of slices, the total experimental duration depends only on TR and the number of phase-encoding steps. A method of accelerating the experiment by multiply exciting each slice is described. An experimental demonstration of the proposed sequences is given by imaging the human head at 3 T.

Pixel analysis of MR perfusion imaging in predicting radiation therapy outcome in cervical cancer.

Abstract: The purpose of this study was to assess heterogeneity of tumor microcirculation determined by dynamic contrast-enhanced magnetic resonance (MR) imaging and its prognostic value for tumor radiosensitivity and long-term tumor control using pixel-by-pixel analysis of the dynamic contrast enhancement. Sixteen patients with advanced cervical cancer were examined with dynamic contrast-enhanced MR imaging at the time of radiation therapy. Pixel-by-pixel statistical analysis of the ratio of post- to precontrast relative signal intensity (RSI) values in the tumor region was performed to generate pixel RSI distributions of dynamic enhancement patterns. Histogram parameters were correlated with subsequent tumor control based on long-term cancer follow-up (median follow-up 4.6 years; range 3.8-5.2 years). The RSI distribution histograms showed a wide spectrum of heterogeneity in the dynamic enhancement pattern within the tumor. The quantity of low-enhancement regions (10th percentile RSI < 2.5) significantly predicted subsequent tumor recurrence (88% vs. 0%, P = 0.0004). Discriminant analysis based on both 10th percentile RSI and pixel number (reflective of tumor size) further improved the prediction rate (100% correct prediction of subsequent tumor control vs. recurrence). These preliminary results suggest that quantification of the extent of poor vascularity regions within the tumor may be useful in predicting long-term tumor control and treatment outcome in cervical cancer. J. Magn. Reson. Imaging 2000;12:1027-1033.

Quantification of endothelial permeability, leakage space, and blood volume in brain tumors using combined T1 and T2* contrast-enhanced dynamic MR imaging

Abstract: This study describes a method for imaging brain tumors that combines T1-weighted (T1W) and T2*-weighted (T2*W) dynamic contrast-enhanced acquisitions Several technical improvements have been made to produce high-quality three-dimensional mapping of endothelial permeability surface area product (k) and leakage space (vl), based on T1W data Tumor blood volume maps are obtained from T2*W images with a complete removal of residual relaxivity effects The method was employed in 15 patients with brain tumors (5 gliomas, 5 meningioma, and 5 acoustic schwannoma) Mean values of vl were significantly greater in acoustic schwannomas (53% +/- 9%) than in meningiomas (34% +/- 7%) or gliomas (22% +/- 4%) Mean values of vl in meningioma were significantly greater than those of gliomas Mean values of rCBV correlated closely with k There was also a positive correlation between k and vl for pixels with low k values This relationship was weaker in areas of high k The highest mean ratios of k to vl (k(ep)) were seen in two patients with glioblastoma, one patient with transitional cell meningioma, and one patient with angioblastic meningioma Pixel-by-pixel comparison showed a strong correlation between rCBV and k in 11 of 15 patients However, decoupling between pixel-wise rCBV and k was found in four patients who had lesions with moderate k and vl elevation but no increase of rCBV Results from this study suggest that in assessing the angiogenic activities in brain tumors it is advisable to monitor simultaneously changes in tumor blood volume, vessel permeability, and leakage space of tumor neovasculature

Crohn's disease evaluation: comparison of contrast-enhanced MR imaging and single-phase helical CT scanning.

Abstract: The purpose of this study was to evaluate the use of gadolinium and barium-enhanced magnetic resonance (MR) imaging in detecting intestinal and extraintestinal Crohn's disease and compare MRI with contrast-enhanced helical computed tomography (CT). Twenty-six patients with Crohn's disease underwent imaging examinations, including gadolinium-enhanced, fat suppressed fast multiplanar spoiled gradient-recalled (FMPSPGR) MR imaging with oral 2% barium sulfate and rectal water and with helical CT using i.v. and positive (13) or negative (13) intestinal contrast material. MR images and CT scans were reviewed separately by two radiologists for bowel wall thickness and enhancement, presence of abscess, phlegmon, and fistula. MR images and CT scans were then compared side by side. Surgical, endoscopic, and histopathologic findings and results of barium studies were reviewed to determine the location and severity of involvement of intestinal Crohn's disease. Depiction of mural thickening and/or enhancement was superior on the MR images, which showed 55 (85%) and 52 (80%) of 65 abnormal bowel segments for the two observers, compared with helical CT, which showed 39 (60%) and 42 (65%; P 0.05) of 28 segments. In the side-by side comparison, MR imaging was preferred over helical CT for depicting normal bowel wall (MR 71%, CT 4%, equal 25%; P < 0.001), mural thickening (MR 41%, CT 11% equal 48%; P < 0.01), mural enhancement (MR 89%, equal 11%; P < 0.001), and overall GI tract evaluation (MR 52%, CT 10%, equal 38%; P < 0.001). Gadolinium-enhanced MR imaging with oral dilute barium sulfate and rectal water depicts intestinal and extraintestinal changes of Crohn's disease and shows promise as a clinically useful tool.

Simultaneous MRI measurement of blood flow, blood volume, and capillary permeability in mammary tumors using two different contrast agents.

Abstract: A technique for the simultaneous measurement of three vascular parameters: blood flow (Fr), blood volume (vb), and the capillary permeability-surface area product (PSr )i n breast tumors using dynamic contrast-enhanced magnetic resonance imaging (MRI) is presented. Features of the technique include measurement of precontrast tumor T1, rapid temporal sampling, measurement of the arterial input function, and use of a distributed parameter tracer kinetic model. Parameter measurements are compared that were determined using two contrast agents of different molecular weights, gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA; 0.6 kDa) and Gadomer-17 (17 kDa), in 18 spontaneous canine mammary tumors. Measurements of Fr and vb corresponded well with literature values, and the mean PSr measured using GdDTPA was a factor of 15 higher than that measured using Gadomer-17. J. Magn. Reson. Imaging 2000;12: 991‐1003. © 2000 Wiley-Liss, Inc.

Nonlinear smoothing for reduction of systematic and random errors in diffusion tensor imaging.

Abstract: Calculation and sorting of the eigenvectors of diffusion sing diffusion tensor imaging has previously been shown to be sensitive to noise levels in the acquired data. This sensitivity manifests as random and systematic errors in the diffusion eigenvalues and derived parameters such as indices of anisotropy. An optimized application of nonlinear smoothing techniques to diffusion data prior to calculation of the diffusion tensor is shown to reduce both random and systematic errors, while causing little blurring of anatomical structures. Conversely, filtering applied to calculated images of fractional anisotropy is shown to fail in reducing systematic errors and in recovering anatomical detail. Using both real and simulated brain data sets, it is demonstrated that this approach has the potential to allow acquisition of data that would otherwise be too noisy to be of use. J. Magn. Reson. Imaging 2000;11:702-710. (C) 2000 Wiley-Liss, Inc.

Takayasu arteritis: diagnosis with breath-hold contrast-enhanced three-dimensional MR angiography.

Abstract: The purpose of the study was to determine the diagnostic accuracy of breath-hold contrast-enhanced three-dimensional (3D) magnetic resonance (MR) angiography in Takayasu arteritis. Thirty patients suspected of having Takayasu arteritis were examined with MR angiography and conventional angiography. Takayasu arteritis was diagnosed in 20 of these patients. MR angiography was performed using a 1.5-T system after bolus injection of 0.1 mmol/kg of gadodiamide. MR angiography clearly depicted various vascular lesions in the aorta and its major branches in all 20 patients with Takayasu arteritis. It also depicted pulmonary artery lesions in 10 (50%) of the 20 patients. MR angiography accurately depicted 323 (98%) of 330 arteries, but 7 (2%) stenotic arteries were overestimated as occluded. The sensitivity and specificity of MR angiography for the diagnosis of Takayasu arteritis were both 100%. Breath-hold contrast-enhanced 3D MR angiography clearly depicts various vascular lesions in both the systemic and pulmonary arteries in Takayasu arteritis, thus allowing a definitive diagnosis of Takayasu arteritis.

Turbo STIR magnetic resonance imaging as a whole-body screening tool for metastases in patients with breast carcinoma: preliminary clinical experience.

Abstract: This study was undertaken to assess the utility of whole-body turbo short tau inversion recovery (STIR) magnetic resonance imaging (MRI) to detect metastases to liver, brain, and bone as a single examination in women with breast cancer. Seventeen patients with biopsy-proven breast cancer and suspected metastatic disease attending over a 12-month period referred for both conventional imaging and whole-body MRI were included in the study. Three patients were found to be free of metastases at both conventional and MR imaging. Appendicular or axial skeletal metastases were identified in 11 of 17 patients, with correlation between findings at whole-body MRI and scintigraphy in 15 of the 17 patients. Five patients had evidence of hepatic metastases on whole-body MRI, of which metastases were identified in only three patients at CT despite contrast enhancement. Four patients had brain abnormalities (metastases in three patients, meningioma in one patient) detected on both whole-body and dedicated brain MRI. Preliminary clinical experience suggests that turbo STIR whole-body MRI may represent a convenient and cost-effective method of total body screening for metastases in patients with breast carcinoma. J. Magn. Reson. Imaging 2000;11:343–350. © 2000 Wiley-Liss, Inc.

BOLD-f MRI response to single-pulse transcranial magnetic stimulation (TMS).

Abstract: Five healthy volunteers were studied using interleaved transcranial magnetic stimulation/functional magnetic resonance imaging (TMS/fMRI) and an averaged single trial (AST) protocol. Blood oxygenation level-dependent (BOLD)-fMRI response to single TMS pulses over the motor cortex was detectable in both the ipsilateral motor cortex under the TMS coil and the contralateral motor cortex, as well as bilaterally in the auditory cortex. The associated BOLD signal increase showed the typical fMRI hemodynamic response time course. The brain's response to a single TMS pulse over the motor cortex at 120% of the level required to induce thumb movement (1.0%-1.5% signal increase) was comparable in both level and duration to the auditory cortex response to the sound accompanying the TMS pulse (1.5% -2.0% signal increase).

Image metric-based correction (autocorrection) of motion effects: analysis of image metrics.

Abstract: Magnetic resonance (MR) imaging of the shoulder necessitates high spatial and contrast resolution resulting in long acquisition times, predisposing these images to degradation due to motion. Autocorrection is a new motion correction algorithm that attempts to deduce motion during imaging by calculating a metric that reflects image quality and searching for motion values that optimize this metric. The purpose of this work is to report on the evaluation of 24 metrics for use in autocorrection of MR images of the rotator cuff. Raw data from 164 clinical coronal rotator cuff exams acquired with interleaved navigator echoes were used. Four observers then scored the original and corrected images based on the presence of any motion-induced artifacts. Changes in metric values before and after navigator-based adaptive motion correction were correlated with changes in observer score using a least-squares linear regression model. Based on this analysis, the metric that exhibited the strongest relationship with observer ratings of MR shoulder images was the entropy of the one-dimensional gradient along the phase-encoding direction. We speculate (and show preliminary evidence) that this metric will be useful not only for autocorrection of shoulder MR images but also for autocorrection of other MR exams. J. Magn. Reson. Imaging 2000;11:174–181. © 2000 Wiley-Liss, Inc.

NC100150 Injection, a preparation of optimized iron oxide nanoparticles for positive-contrast MR angiography.

Abstract: A preparation of monocrystalline iron oxide nanoparticles with an oxidized starch coating, currently in clinical trials (NC100150 Injection; CLARISCAN), was characterized by magnetization measurements, relaxometry, and photon correlation spectroscopy. By combining the results with a measure of iron content, one can obtain the size and magnetic attributes of the iron cores, including the relevant correlation times for outer sphere relaxation (tau(SO) and tau(D)), and information about the interaction of the organic coating with both core and solvent. The results are 6.43 nm for the iron oxide core diameter, a magnetic moment of 4.38x10(-17) erg/G, and a water-penetrable coating region of oxidized oligomeric starch fragments and entrained water molecules. The latter extends the hydrodynamic diameter to 11.9 nm and lowers the average diffusivity of solvent about 64% (which increases tau(D) accordingly). The nanoparticles show little size-polydispersity, evidenced by the lowest value of r(2)/r(1) at 20 MHz reported to date, an asset for magnetic resonance angiography.

Pre‐MRI Procedure Screening: Recommendations and Safety Considerations for Biomedical Implants and Devices

Abstract: Maintaining a safe MR environment is a daily challenge for MR healthcare workers, especially in consideration of the increasing number of clinical MR applications and the large and growing variety of biomedical implants and devices that are currently used in patients. This review article presents policies and procedures that should be used to screen all patients and individuals before allowing them to enter the magnetic resonance (MR) environment. Information pertaining to MR safety and the relative risk factors for implants, devices and materials is discussed. A comprehensive pre-MRI procedure screening form that is recommended for use by MR facilities is also included.