Computing diffusion rates in T2-dark hematomas and areas of low T2 signal

BACKGROUND AND PURPOSE

It has been suggested that restricted diffusion is present within hematomas with intact red cell membranes; however, computing apparent diffusion coefficient (ADC) values in areas of low T2 signal can be problematic. Our purpose was to show the pitfalls of measuring diffusion within hematomas with intracellular blood products and to present a framework based on the properties of expected values for computing ADC values from regions with signal intensities close to that of the background noise (ie, T2-dark hematomas).

METHODS

Twelve patients with intracranial hematomas who had undergone diffusion imaging were retrospectively identified during a 2-year period (four intracellular oxyhemoglobin, seven intracellular deoxyhemoglobin, one intracellular methemoglobin). Regions of interest were drawn on the hematomas, the contralateral white matter, and over the background. ADC values were computed using a variety of methods: 1) using expected values incorporating the variance of the background, 2) computing the mean of the regions of interest before taking the natural log, 3) masking negative values, and 4) masking the background at 0.5% increments from 0.5 to 5.5% and including the masked voxels (an intrinsically flawed method). Two-tailed Student's t test was performed between the white matter and the hematoma ADC values.

RESULTS

There was no statistically significant difference between the hematomas and the white matter for methods 1 through 3 (P = .14, P = .23, and P = .83, respectively). Only method 4 revealed a statistically significant difference, beginning at 0.5% masking (P = .04) and becoming progressively more significant with increased masking (P = 4.14 x 10(-7) at 5.5% masking). The effect of masking was limited to the T2-dark hematomas.

CONCLUSION

There is no restriction of diffusion for in vivo hematomas with intracellular blood products. The T2 blackout effect for T2-dark hematomas on diffusion-weighted images should not be interpreted as fast diffusion. The method of expected values can be used to obtain measurements for regions with signal intensities near the background noise. Using literature values for RBC self-diffusion, we computed lower limits of diffusion for hematomas with intracellular blood products to be 0.3 x 10(-3) mm2/s.

Automated teaching file and slide database for digital images

OBJECTIVE

We developed an easy-to-use method for creating a searchable digital teaching file of CT and MR images.

CONCLUSION

We describe a method of creating a digital archive of interesting cases that is easy to implement and works on a commercially available workstation. A remote daemon polls for images transferred to the product film tool. It creates a tagged image file format (TIFF) digital archive of these images on any platform supporting file transfer protocol (FTP), operates in the background, and automatically generates a searchable index of case information in the database.

T2* and proton density measurement of normal human lung parenchyma using submillisecond echo time gradient echo magnetic resonance imaging

OBJECTIVE

To obtain T2* and proton density measurements of normal human lung parenchyma in vivo using submillisecond echo time (TE) gradient echo (GRE) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Six normal volunteers were scanned using a 1.5-T system equipped with a prototype enhanced gradient (GE Signa, Waukausha, WI). Images were obtained during breath-holding with acquisition times of 7-16 s. Multiple TEs ranging from 0.7 to 2.5 ms were tested. Linear regression was performed on the logarithmic plots of signal intensity versus TE, yielding measurements of T2* and proton density relative to chest wall muscle. Measurements in supine and prone position were compared, and effects of the level of lung inflation on lung signal were also evaluated.

RESULTS

The signal from the lung parenchyma diminished exponentially with prolongation of TE. The measured T2* in six normal volunteers ranged from 0.89 to 2.18 ms (1.43 +/- 0.41 ms, mean +/- S.D.). The measured relative proton density values ranged between 0.21 and 0.45 (0.29 +/- 0.08, mean +/- S.D.). Calculated T2* values of 1.46 +/- 0.50, 1.01 +/- 0.29 and 1.52 +/- 0.18 ms, and calculated relative proton densities of 0.20 +/- 0.03, 0.32 +/- 0.13 and 0.35 +/- 0.10 were obtained from the anterior, middle and posterior portions of the supine right lung, respectively. The anterior-posterior proton density gradient was reversed in the prone position. There was a pronounced increase in signal from lung parenchyma at maximum expiration compared with maximum inspiration. The ultrashort TE GRE technique yielded images demonstrating signal from lung parenchyma with minimal motion-induced noise.

CONCLUSION

Quantitative in vivo measurements of lung T2* and relative proton density in conjunction with high-signal parenchymal images can be obtained using a set of very rapid breath-hold images with a recently developed ultrashort TE GRE sequence.

Intracranial aneurysms: detection and characterization with MR angiography with use of an advanced postprocessing technique in a blinded-reader study

PURPOSE

To assess magnetic resonance (MR) angiography for the detection and characterization of angiographically proved intracranial aneurysms by using an advanced method of postprocessing, in a blinded-reader study.

MATERIALS AND METHODS

One hundred fifty-eight vessels were examined with catheter angiography and three-dimensional time-of-flight MR angiography in 44 patients with 63 aneurysms and 15 patients with no aneurysm at catheter angiography. Postprocessing was performed off-line with an advanced multifeature-extraction, ray-tracing algorithm. MR angiograms were interpreted independently by three neuroradiologists blinded to the catheter angiographic results for presence, location, size, and morphology of the aneurysm. Proof of diagnosis was consensus reading of catheter angiograms.

RESULTS

Mean sensitivity for detection of aneurysms was 75% (range, 70%-79%). As a screening tool (ie, detection of at least one aneurysm necessitating catheter angiography), mean sensitivity was 91% for all aneurysms and 95% for aneurysms larger than 3 mm. This method was not adequate for detection of lobulation or size of aneurysm.

CONCLUSION

MR angiography with an advanced method of postprocessing can result in highly sensitive, specific studies for the diagnosis of intracranial aneurysms that are of sufficient size to be considered for surgical treatment, but it is inadequate for characterization of aneurysms.

Off-resonance pulsed magnetization transfer in clinical MR imaging: optimization by an analysis of transients

Significant degrees of magnetization transfer (MT) have been observed in the magnetic resonance imaging of biological materials by conventional clinical imaging sequences, as well as by sequences specifically designed to enhance MT image contrast. Two aspects of these procedures distinguish them from the classic spectroscopic MT experiments using either so-called "hard" radiofrequency (RF) pulses of short duration and high power, or continuous wave RF irradiation of low power. First, clinical sequences must make exclusive use of "soft" pulses of intermediate length and power. Second, biological materials are modeled by a two-spin system involving magnetization transfer between a narrow and a broad homogeneous spectral component. Such materials are a relatively restricted group within the larger family of materials studied with MT by spectroscopy. The current paper addresses these two issues with a theory that gives a new transient analysis of the off-resonance pulsed MT problem for biological materials. It leads to predictions for optimal magnetization transfer in the context of medical imaging that have been verified by computer modeling.

OIL FLAIR: optimized interleaved fluid-attenuated inversion recovery in 2D fast spin echo

Inversion recovery may be used to suppress signal from cerebrospinal fluid, a technique which has been named "fluid attenuated with inversion recovery" (FLAIR). This report describes interleaving a slice selective inversion pulse within a rapid spin-echo sequence to obtain the desirable contrast characteristics of FLAIR in imaging times comparable to standard rapid spin echo. Additionally, the pulse repetition time is allowed to float above a defined minimum, which can further shorten scan times and dramatically ease the optimization process. The optimized interleaved sequence is referred to as OIL FLAIR.

Functional activation during an auditory comprehension task in patients with temporal lobe lesions

Functional magnetic resonance imaging (fMRI) was used to map regional brain activation during an auditory comprehension task in two normal controls and two patients with left temporal lobe lesions. Activity in the superior temporal and angular gyrus regions was detected in all normal subjects. In the patients, the spatial distribution of activation ipsilateral to the lesions differed from the pattern observed in contralateral cortex or in control subjects. These studies highlight the potential of fMRI for mapping abnormal functional anatomy in the human brain.

Functional magnetic resonance imaging of regional brain activity in patients with intracerebral arteriovenous malformations before surgical or endovascular therapy

Functional magnetic resonance (MR) imaging was performed in six patients harboring proven intracerebral arteriovenous malformations (AVMs) using a noninvasive blood oxygen level-dependent technique based on the documented discrepancy between regional increases in blood flow and oxygen utilization in response to regional brain activation. Statistical functional MR maps were generated and overlaid directly onto conventional MR images obtained at the same session. In the six patients studied, a total of 23 separate functional MR imaging activation studies were performed. Of these, two runs were discarded because of motion artifacts. All of the remaining 21 studies demonstrated activation in or near expected regions for the paradigm employed. Qualitatively reproducible regional localizations of functional activity in unexpected sites were also seen. The authors' findings indicating aberrant mapping of cortical function may be explained on the basis of the plasticity of brain function, in that the developing brain can take over function that would normally have been performed by regions of brain encompassed by the lesion. Preliminary results in this study's small number of cases also indicate that activity demonstrated within the confines of the apparent AVM nidus may help predict the development of a posttherapy deficit. The authors demonstrate that functional MR imaging can be successfully and reproducibly performed in patients with intracerebral AVMs. Notwithstanding the paucity of normative data using functional MR imaging, the author' findings support cortical reorganization associated with these congenital lesions. Blood oxygen level-dependent MR imaging is a noninvasive method used to localize areas of eloquent cortex in patients harboring AVMs; it may prove to be of value in treatment planning.

Functional magnetic resonance imaging of regional brain activity in patients with intracerebral gliomas: findings and implications for clinical management

Functional magnetic resonance imaging (fMRI) was performed in seven patients harboring intracerebral gliomas proven by histological analysis using a noninvasive blood oxygen level-dependent technique based on the documented discrepancy between regional increases in blood flow and oxygen use in response to regional brain activation. We combined fMRI with conventional magnetic resonance imaging (MRI) during motor or language task activation experiments to investigate the potential usefulness of mapping regional brain activity as part of treatment planning in patients with intracerebral gliomas, in whom preservation of areas of functioning brain tissue is critical. Statistical fMRI maps were generated and directly mapped onto conventional MRI scans obtained at the same session. Of the five patients cooperative enough to remain motionless for the study and perform the task, the location of activation in the primary sensorimotor cortex on the side of the tumor was clearly displaced compared with that in the normal contralateral hemisphere in four patients. Four of the five tumors in these patients showed fMRI activation within the periphery of (or immediately adjacent to) areas of presumed tumor based on spin-echo MRI. In some patients with neurological deficit, the extent of activation was reduced on the side of the tumor as compared with the normal hemisphere. The supplemental motor area and the ipsilateral primary motor cortex were also reproducibly activated during motor tasks. We conclude that blood oxygen level-dependent fMRI can localize areas of cortical function in patients undergoing treatment planning for gliomas so that therapy can be directed away from regions of residual function. Our preliminary data suggest that functioning cortex within or adjacent to tumor margins can be demonstrated, which may correspond to partial preservation of clinical function. Our preliminary data also suggest that there may be a quantifiable difference on fMRI between activation in tumor-bearing cortex and activation in corresponding normal cortex in the contralateral hemisphere. We postulate that the magnitude of this difference may relate to the severity of patient deficit.

Multi-slice, breathhold imaging of the lung with submillisecond echo times

The signal from the lungs is heavily attenuated by T2 and T2 decay in standard MR images of the thorax. The authors utilized the capabilities of a prototype fast gradient system to develop a multi-slice gradient echo sequence that can obtain images with an echo time of 0.7 ms. Images acquired in a single breath-hold are free from respiratory motion artifacts and clearly display signal from lung parenchyma. The use of fast gradients makes short echo times possible without the use of nonstandard RF pulses and spatial encoding techniques and their associated limitations.

Intracranial aneurysms: depiction on MR angiograms with a multifeature-extraction, ray-tracing postprocessing algorithm

PURPOSE

To compare a new method for postprocessing data, soft thresholding and depth cueing of unrestricted techniques (STANDOUT), with maximum intensity projection (MIP) for magnetic resonance (MR) angiograms of intracranial aneurysms.

MATERIALS AND METHODS

Eighteen patients with 19 intracranial aneurysms were studied. Images generated with STANDOUT were compared with those of MIP for both three-dimensional (3D) time-of-flight (TOF) and 3D phase-contrast (PC) techniques. Images were assessed for detection of aneurysms, scored for delineation of aneurysmal features and overall MR angiographic characteristics, and compared with catheter angiograms.

RESULTS

Both postprocessing techniques demonstrated aneurysms on most TOF and PC MR angiograms. Statistically significant improvement of aneurysmal features, aneurysmal neck definition, and delineation of adjacent arterial anatomy was seen on both TOF and PC MR angiograms with STANDOUT. Feature definition was most improved for smaller aneurysms. TOF and PC MR angiograms with STANDOUT showed improved elimination of artifactual intraluminal signal loss in normal large vessels, reduced artifactual narrowing of vessel caliber, and better small vessel delineation.

CONCLUSION

STANDOUT has great benefits for postprocessing of intracranial MR angiograms, regardless of the technique of data acquisition.

MR diagnosis of meniscal tears of the knee: value of fast spin-echo vs conventional spin-echo pulse sequences

OBJECTIVE

Contrast can be similar on fast spin-echo and conventional spin-echo MR images, but data acquisition is faster with fast spin-echo sequences. This study was designed to evaluate the performance of fast spin-echo sequences in the detection of meniscal tears in the knee, with the established conventional spin-echo technique used as a reference standard.

SUBJECTS AND METHODS

We imaged 66 consecutive patients (129 menisci) who were referred for MR examination with suspected meniscal tears. We used our routine two-dimensional, multisection, long repetition time/double-echo spin-echo sequence and one of two fast spin-echo sequences. The fast spin-echo parameters were chosen to minimize the loss of high-resolution detail while otherwise maintaining the sequence as close as possible to the spin-echo sequence. We then did a retrospective evaluation of the fast spin-echo images, using the spin-echo images as the gold standard.

RESULTS

Fast spin-echo images showed only 30 (65%) of the 46 meniscal tears seen on the conventional spin-echo images. In addition, four of the 30 tears seen with both sequences were diagnosed with greater confidence on the conventional spin-echo images. In the cases in which both sequences allowed a diagnosis of definite meniscal tear, the abnormalities tended to be more conspicuous on the spin-echo images.

CONCLUSION

Our results suggest that the fast spin-echo sequence should not be used for the diagnosis of meniscal tears.

MR imaging with spatial modulation of magnetization in the evaluation of chronic central pulmonary thromboemboli

PURPOSE

To assess the diagnostic value of magnetic resonance (MR) imaging with SPAMM (spatial modulation of magnetization) in the identification of chronic central pulmonary thromboemboli.

MATERIALS AND METHODS

Twelve patients with pulmonary hypertension and five healthy volunteers were prospectively studied with a 1.5-T MR imaging system. The SPAMM technique was integrated into a conventional cardiac-synchronized spin-echo (SE) sequence. Six of the 12 patients had central thromboemboli.

RESULTS

In the healthy subjects, intravascular stripes in the central pulmonary arteries disappeared as a result of flow within 100 msec after the R wave. Areas of persistent stripes were identified in seven of eight central pulmonary arteries with thromboemboli. Conversely, in the 16 central pulmonary arteries without clot, intraluminal stripes disappeared despite the presence of flow-related signal (sensitivity = 88%, specificity = 100%, accuracy = 96%).

CONCLUSION

SPAMM appears to be a simple and effective technique for differentiating central pulmonary arterial thromboemboli from flow-related signal frequently observed with pulmonary hypertension.

Blinded-reader comparison of magnetic resonance angiography and duplex ultrasonography for carotid artery bifurcation stenosis

BACKGROUND AND PURPOSE

We compared two-dimensional time-of-flight magnetic resonance angiography (MRA) and duplex ultrasonography with arteriography for the detection of 70% to 99% stenoses at the carotid artery bifurcation (ie, surgical disease according to findings of the North American Carotid Endarterectomy Trial).

METHODS

Three blinded readers independently measured stenoses on MRA in 73 vessels from 38 patients. Duplex ultrasonography was available in 66 vessels from 35 of these patients, and blinded reading was performed by one reader. Comparison was made to arteriography.

RESULTS

Magnetic resonance angiography demonstrated a sensitivity of 92.4%, specificity of 74.5%, and negative predictive value of 95.8% for 70% to 99% stenoses. Interobserver agreement was high (kappa = 0.91). Absence of signal at stenoses with evidence of distal flow usually, but not always, corresponded to surgical disease. Duplex ultrasonography demonstrated a sensitivity of 81.0%, specificity of 82.2%, and negative predictive value of 90.2% for surgical disease. There was no significant difference between MRA and duplex ultrasonography for the sensitivity or specificity in detecting 70% to 99% stenoses (P > .1, exact form of the McNemar test). MRA had no false positives or false negatives for complete occlusions of the carotid artery, whereas duplex ultrasonography missed one occlusion and falsely called two patent vessels occluded. In seven cases, both MRA and duplex ultrasonography overestimated stenoses to miscategorize them as surgical disease.

CONCLUSIONS

Although the sensitivity and specificity of MRA and duplex ultrasonography are not significantly different for distinguishing surgical and nonsurgical degrees of stenosis at the carotid bifurcation, MRA has some advantages that may make it the screening test of choice. Concordant MRA and duplex ultrasonography for surgical disease does not necessarily obviate the need for catheter arteriography.

Suspected pulmonary embolism: prospective evaluation with pulmonary MR angiography

PURPOSE

To prospectively evaluate three magnetic resonance (MR) imaging techniques for detection of pulmonary embolism.

MATERIALS AND METHODS

Eighteen patients in whom the presence of acute or chronic pulmonary emboli was suspected underwent examination with the following pulse sequences: cardiac-gated spin echo, cine spatial modulation of magnetization, and two-dimensional time-of-flight pulmonary breath-hold (PBH) MR angiography. Three radiologists independently and blindly reviewed each case and graded a total of 518 arterial segments for each pulse sequence with a continuous scale of 0%-100% for likelihood of pulmonary embolism.

RESULTS

The overall sensitivity of PBH MR angiography for detection of acute pulmonary emboli was 0.85; for chronic emboli, which were smaller in anteroposterior (AP) diameter, the overall sensitivity was 0.42. Emboli larger than 1 cm in AP diameter were typically identified with > 75% confidence with all pulse sequences.

CONCLUSION

Acute pulmonary emboli greater than 1 cm in AP diameter were as accurately identified on PBH MR angiograms obtained in 15 seconds as they were on MR images obtained with longer pulse sequences not dependent on breath holding.

Magnetic resonance arteriography of the pelvis and lower extremities

Reconstructive vascular surgery is the last resort for limb salvage in patients with vascular insufficiency. Planning the sites of arterial bypass grafts depends on an exact knowledge of the angiographic anatomy. Currently, conventional contrast angiography is the primary preoperative study for surgical planning. Recently, magnetic resonance (MR) arteriography has been shown to be an accurate technique for the evaluation of slow arterial flow in the lower extremities. The purpose of this review will be to show how MR arteriography can be used as both an adjunct to and in some cases instead of conventional angiography for the preoperative assessment of lower-extremity vascular reconstruction. The derivation of common artifacts, the many pitfalls of interpretation, and the evaluation of peripheral arterial occlusion and percentage stenosis will be discussed. MR arteriography of the lower extremities is an important advance for the noninvasive evaluation of disease of the peripheral vasculature.

Fast spin-echo imaging of the brain and spine

The recent advent and implementation of rapid spin-echo techniques has allowed increased imaging speed while maintaining spin-echo-like contrast. This review explains the basis of fast spin-echo imaging and attempts to elucidate the etiology of the differences between it and spin-echo imaging. Clinical applications and limitations of fast spin-echo imaging in the brain and spine will also be addressed.

Conventional and fast spin-echo MR imaging: minimizing echo time

Magnetic resonance imaging is frequently complicated by the presence of motion and susceptibility gradients. Also, some biologic tissues have short T2s. These problems are particularly troublesome in fast spin-echo (FSE) imaging, in which T2 decay and motion between echoes result in image blurring and ghost artifacts. The authors reduced TE in conventional spin-echo (SE) imaging to 5 msec and echo spacing (E-space) in FSE imaging to 6 msec. All magnetic gradients (except readout) were kept at a maximum, with data sampling as fast as 125 kHz and only ramp waveforms used. Truncated sinc radio-frequency pulses and asymmetric echo sampling were also used in SE imaging. Short TE (5.8 msec) SE images of the upper abdomen were compared with conventional SE images (TE = 11 msec). Also, FSE images with short E-space were compared with conventional FSE images in multiple body sites. Short TE significantly improved the liver-spleen contrast-to-total noise ratio (C/N) (7.9 vs 4.1, n = 9, P < .01) on T1-weighted SE images, reduced the intensity of ghost artifacts (by 34%, P < .02), and increased the number of available imaging planes by 30%. It also improved delineation of cranial nerves and reduced susceptibility artifacts. On short E-space FSE images, spine, lung, upper abdomen, and musculoskeletal tissues appeared crisper and measured spleen-liver C/N increased significantly (6.9 vs 4.0, n = 12, P < .01). The delineation of tissues with short T2 (eg, cartilage) and motion artifact suppression were also improved. Short TE methods can improve image quality in both SE and FSE imaging and merit further clinical evaluation.

First principles of fast spin echo

Fast spin echo (FSE), a variant of the rapid acquisition with refocused echoes pulse sequence, is now being widely considered as an alternative to conventional spin echo for proton density and T2-weighted imaging. Although the medical experience with this sequence is relatively limited, relevant aspects of the technique have been well understood in the context of spectroscopic applications for many years. This article attempts to portray the subject in an appropriate historical context. Such a viewpoint promotes a deeper understanding of the artifacts, determinants of contrast, and future evolution of FSE. Hopefully, this may not only be of benefit in the design of optimal clinical imaging protocols for current state of the art but may also be of use in fashioning the criteria by which new developments in this field may be judged.

Magnetic resonance angiography of the pelvis and lower extremities. Works in progress

Two-dimensional time-of-flight (TOF) magnetic resonance angiography (MRA) of the lower extremities has recently been rediscovered as a technique that can be used to find small vessels in the foot and leg for surgical revascularization in patients with severe peripheral vascular disease. The purpose of this article is to acquaint the reader with some of the common problems that may be encountered in this imaging technique and to show their derivations and solutions were applicable. In addition, the authors show receiver operator curve analysis of one radiologist's ability to determine the location and patency of arteries using this technique to be outstanding (AZ, 0.9737). This represents a promising new technique for imaging the arterial system in patients with severe peripheral vascular disease.

Cerebrovascular magnetic resonance angiography: a critical verification

Because simultaneous noninvasive noncontrast imaging of cervical and cerebral vasculature and brain is possible with magnetic resonance angiography (MRA) and imaging (MRI), the following study was undertaken from July 1990 to January 1992. One hundred twenty-eight patients were examined with General Electric 1.5 Tesla MRI systems. Axially acquired volumetric three-dimensional time-of-flight MRA with 0.7 mm3 voxel size with regional maximum intensity projection after processing followed a two-dimensional time-of-flight localizing sequence. These two MRA sequences combined with spin-echo parenchymal brain MRI were compared with duplex scans, contrast angiograms, and surgical findings. Blinded readings by a radiologist and vascular surgeon allowed comparison of grades of luminal diameter narrowing (normal, mild, moderate, severe, and occluded) seen on MRA to be compared with those of Doppler and contrast angiography. Excluding 12 nondiagnostically imaged internal carotid arteries (10 MRA) and limiting duplex correlation to within 5 days of the MRA examination allowed critical appraisal of 182 internal carotid arteries. Exact correlation of grade of stenosis was obtained by the radiologist in 136 (74.7%) of 182 arteries and the surgeon in 138 (75.8%) of 182 arteries. Spearman rank correlation analysis found rank correlation coefficients of 0.88 (p < 0.001) and 0.83 (p < 0.001), respectively, for the radiologist and vascular surgeon. Disagreement one category apart was found by the radiologist in 35 studies (19.3%) and the surgeon in 28 studies (15.4%). Two or more grades of disagreement were found by the radiologist in 11 studies (6%) and the surgeon in 16 studies (8.8%). Contrast angiogram-MRA agreement was found in 86% of 36 internal carotid arteries. The degree of stenosis detected by MRA was concordant with surgical findings in 39 of 40 patients. Thus MRA emerges as a useful and accurate method of obtaining cerebrovascular evaluation in clinical practice.

Common artifacts encountered in thoracic magnetic resonance imaging: recognition, derivation, and solutions

The article discusses the many types of thoracic magnetic resonance imaging artifacts, their derivations, and their current solutions. Magnetic resonance imaging artifacts of the thorax can be divided into two major categories. First, there are machine-related artifacts related to the machine's hardware: the intrinsic magnetic field, chemical shift, magnetic susceptibility, radio frequency leaks, metal artifacts, B1-homogeneity, gradient coils, truncation, aliasing, zipper artifacts, artifacts related to the surface coil profiles, pulse profiles, and crosstalk. Second, there are artifacts related to motion: voluntary patient motion, involuntary motion, and physiologic motions. Each one of these artifacts, and their solutions, will be discussed.

Thin-section, three-dimensional Fourier transform, steady-state free precession MR imaging of the brain

The authors evaluated a three-dimensional Fourier transform implementation of a very short repetition time (TR) (24 msec), steady-state free precession (SSFP) pulse sequence for clinical imaging of the brain and compared it with a conventional two-dimensional Fourier transform long TR/echo time (TE) spin-echo sequence. First, the optimal flip angle of 10 degrees for generating images with contrast similar to that of long TR/TE spin-echo images was determined. Then, 29 patients with suspected brain lesions were studied with both techniques. Although the SSFP images did not exhibit the magnetic susceptibility artifacts that plague other rapid-imaging techniques, the conspicuity of most parenchymal lesions was often less than that on the spin-echo images. Also, the visibility of paramagnetic effects, such as the low signal intensity of brain iron, was less obvious at SSFP imaging. These substantial limitations may relegate the SSFP sequence to an adjunctive role, perhaps mainly demonstration of the cystic nature of mass lesions, because of its extreme sensitivity to slow flow.

Pulmonary MR angiography utilizing phased-array surface coils

Magnetic resonance angiography of the pulmonary vasculature was evaluated in 12 subjects using breath-hold gradient echo scans and surface coils at 1.5 T. Flow-compensated GRASS, spoiled GRASS (SPGR), and WARP-SPGR sequences were utilized. Comparisons were made among flip angles of 10-60 degrees, slice thicknesses of 3-10 mm, and body coil as well as Helmholtz pair and phased-array multiple coils. With 30-40 contiguous slices encompassing the lung, intrathoracic vasculature was segmented using a UNIX/X-windows based package dubbed VIDA. Three-dimensional anatomy was visualized by a brightest voxel projection algorithm, following reduction of chest wall pixel intensities by an operator-interactive module. Both SPGR (30 degrees flip angle, 4 mm slice thickness) and WARPSPGR (15 degrees flip angle, 5 mm slice thickness) in combination with phased-array multiple coils provided the most satisfactory images, based upon observations by three radiologists and signal-to-noise ratio measurements. The MR angiograms visualized vessels as distal as sixth to seventh order branches. The technique was successfully applied to three patients with pulmonary embolism. The results of this study demonstrate that the pulmonary vascular tree can be imaged by MR angiography combining a high resolution technique utilizing phased-array multiple coils, fast gradient echo sequences with breath-holding, and postprocessing of the volumetric image data. The technique is attractive since it is noninvasive and provides a full three-dimensional portrayal of the pulmonary vasculature.

Combined chemical-shift and phase-selective imaging for fat suppression: theory and initial clinical experience

Incomplete fat suppression with a chemical-shift-selective (CHESS) or phase-selective (Dixon) technique is partially due to the olefinic fat component, which precesses at the same frequency as water. The authors developed a new method of fat suppression--the opposed-fat saturation (OP-ES) sequence--that combines both techniques to obtain superior fat saturation. Fat suppression was verified in phantom studies, which showed that the CHESS portion can eliminate most of the aliphatic fat signal except for a small residual component because of steady state effects and magnetic field imperfections. This residual component is cancelled by the olefinic fat with the phase-selective opposed portion of the sequence. Furthermore, this sequence was superior to another hybrid technique, chopper Dixon in combination with CHESS. When used in 10 healthy volunteers, the OP-FS sequence showed consistently better suppression of the subcutaneous and retroperitoneal fat compared with CHESS alone. Additional advantages for clinical abdominal imaging include its compatibility with respiratory compensation, use of a single excitation, and ease of implementation with gradient-echo imaging. Preliminary application in 10 patients illustrated other potential advantages, including clarification of fat-containing diseases and increasing conspicuity of some lesions.

Postoperative lumbar spine: contrast-enhanced chemical shift MR imaging

A modified fat-suppression pulse sequence (consisting of combined frequency-selective fat presaturation followed by a spin-echo acquisition when fat and water magnetization vectors have opposite phase) was used to optimize the conspicuity of intravenous enhancement by gadopentetate dimeglumine on magnetic resonance images in 10 patients previously operated on for lumbar discogenic disease as well as in two patients with herniated disks who had not previously undergone surgery. This technique produced the greatest degree of fat suppression in the phantom study. In six of the patients who had previously undergone surgery, epidural enhancement was more obvious on the fat-suppressed images than on conventional spin-echo images, while in four patients, enhancement was equivalent. The herniated disks in two patients not previously operated on were not enhanced with either technique. Contrast enhancement was universally distinguishable from fat signal and from nonenhancing water-containing tissue on the fat-suppressed images obtained after contrast material administration. This technique may reduce the need for precontrast imaging. Furthermore, postoperative enhancement of nerve roots was more obvious on fat-suppressed images in seven of eight patients. This finding might represent previously undiagnosed degrees of arachnoidal inflammation, which may be a factor in the failed back syndrome.

Popliteal artery hemodynamics: MR imaging-US correlation

Temporally resolved velocity measurements in the popliteal arteries of 11 healthy subjects were obtained by means of magnetic resonance (MR) imaging with use of the Fourier flow-encoding technique. Excellent agreement with corresponding Doppler ultrasonography (US) data (r = .97, slope = 0.99, intercept = -1.5 cm/sec) was demonstrated over the entire velocity range from 50 to -20 cm/sec. The method was rapid and its implementation straightforward. Further, MR imaging was shown to provide the intraluminal velocity distribution relevant for the determination of true flow rates, not obtainable with Doppler US.

First principles of magnetic resonance angiography

For several years, magnetic resonance imaging (MRI) has shown promise as a noninvasive tool for the study of the vascular system. One interesting format, so-called MR angiography, produces results resembling conventional x-ray angiographic images. This article lays a solid, rigorous foundation for an intuitive understanding of these effects without the use of advanced mathematical concepts. The current state of the art in data acquisition and postprocessing is illustrated. Finally, relevant hemodynamic concepts are introduced, in order to characterize the physiology of complex blood flow at bifurcations. MR angiography is particularly sensitive to artifacts associated with complex flow. The article ends with a call to investigate these phenomena, because they will directly affect the success of MR angiography as a technique.

Hydrogen ultrathin phase-encoded spectroscopy (HUPSPEC)

This paper describes a new clinical spectroscopy pulse program, hydrogen ultrathin phase-encoded spectroscopy. This sequence combines high spatial resolution with magnitude hydrogen spectroscopy. A linear volume is spatially frequency encoded with a conventional readout gradient and phase encoded spectrally by incrementing the timing of acquisition. The pulse sequence is implemented on a whole-body MRI scanner and supports several standard scanner options, including autoprescanning, offset of the center of field of view in the frequency-encoding direction, and oblique imaging. Some preliminary experimental experience is reported, demonstrating the possibility of observing the spectral linewidths of fat and water in marrow, and of observing the spectral linewidths of fat and water in marrow, and of observing multiline spectra. Combination of the technique with water suppression methods in a sequence called WASHUP is also discussed.

Magnetic resonance imaging and magnetic resonance spectroscopy of bone tumors and bone marrow disease

The authors have made use of an integrated magnetic resonance imaging/spectroscopy (MRI/MRS) examination to study seven patients with a variety of bone tumors. The spatial localization method used in the 31P portion of the examination was surface coil localization and a one-dimensional chemical shift imaging method (3 cases). The authors found that the precision of spatial localization was critical in many of these cases, since most of these bone tumors were surrounded by muscle tissue that contained high concentrations of phosphocreatine (PCr). For this reason, they suggest that the metabolite ratios should be referenced to the adenosine triphosphate (beta-NTP) resonance rather than PCr. The phosphate monoester (PME) to beta-NTP ratio was elevated as compared with normal muscle in all of the bone tumors studied. The authors found that all of these tumors exhibited pHs between 7.0 and 7.2, which are similar to the values found for normal muscle. They also show the feasibility of using a line-selective proton chemical shift imaging sequence with high spatial resolution for investigating changes in the fatty composition of bone marrow. This method is illustrated in an example of a patient with advanced avascular necrosis in the femoral heads.

The correction of nonuniform signal intensity profiles in magnetic resonance imaging

The increasing use of digital image data in Radiology has opened the door to the routine use of numerical image-enhancement techniques. Of course, numerical image processing cannot put information into the image which is not already there. However, if some means can be found to separate diagnostic image information from noise or artifact, the diagnostic information can be extracted with post-processing. The diagnostic quality of an exam may be enhanced by such numerical manipulations, even though technically, the information content of the digitized image is reduced.

AUR memorial Award. Induced alignment of flowing sickle erythrocytes in a magnetic field. A preliminary report

Deoxygenated sickle erythrocytes in static suspension align perpendicular to a magnetic field. To assess the importance of this observation to MRI of sickle-cell disease, an in vitro flow apparatus was devised and the orientation of sickle erythrocytes flowing through a 0.38 T magnetic field was investigated. We showed a significant perpendicular alignment of fully deoxygenated sickle erythrocytes flowing at 3 to 4 mm/minute (P less than .001). These results suggest that deoxygenated erythrocytes in a sickle-cell patient could orient perpendicular to a magnetic field, and therefore that MRI of such patients could possibly result in worsening of vaso-occlusive complications. Further studies are needed to assess the possible hazards of MRI of sickle-cell disease, especially at high field strengths.