Brain volume measurements in patients with human T-cell lymphotropic virus-1-associated tropical spastic paraparesis.

Human T-cell lymphotropic virus (HTLV)-1 is associated with a chronic progressive neurologic disease known as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) that affects 0.2% to 3% of HTLV-1-infected people. The authors aimed at exploring, in vivo, whether brain volume reduction occurs in patients with HAM/TSP through the use of magnetic resonance imaging (MRI). T1 pre/postcontrast spin echo-weighted images (WIs) and T2WIs of the brain were obtained in 19 HAM/TSP patients and 14 age-and sex-matched healthy volunteers. Both patients and healthy individuals were imaged at a 1.5-Tesla magnet by employing a conventional head coil. Focal T1 and T2 abnormalities were calculated and two measurements of brain parenchyma fraction (BPF) were obtained by using SIENAx (Structural Image Evaluation,using Normalisation, of Atrophy; University of Oxford, Oxford, UK) and MIPAV (Medical Image Processing, Analysis, and Visualization; National Institutes of Health, Bethesda, USA) from T1WIs. No significant differences in BPF were found between patients and healthy subjects when using either SIENAx or MIPAV. Analysis of individual patients detected that BPF was lower by 1 standard deviation (SD) relative to patients' average BPF in one patient. The authors conclude that reductions in BPF do not occur frequently in patients with HAM/TSP. However, the authors believe that one individual case of significant brain atrophy raises the question as to whether atrophy selectively targets the spinal cord of HAM/TSP patients or may involve the brain as well. A larger patient population analyzing regional brain volume changes could be helpful in determining whether brain atrophy is a marker of disease in patients with HAM/TSP.

Interferon-beta-1b effects on re-enhancing lesions in patients with multiple sclerosis.

Interferon-beta (IFNbeta) reduces the number and load of new contrast-enhancing lesions (CELs) in patients with multiple sclerosis (MS). However, the ability of IFNbeta to reduce lesion sizes and re-enhancements of pre-existing CELs has not been examined extensively. Activity of contrast re-enhancing lesions (Re-CELs) and contrast single-enhancing lesions (S-CELs) were monitored in ten patients with relapsing-remitting (RR) MS. These patients underwent monthly post-contrast magnetic resonance imaging (MRIs) for an 18-month natural history phase and an 18-month therapy phase with subcutaneous IFNbeta-1b, totaling 37 images per patient. The activity was analysed using the first image as a baseline and registering subsequent active monthly images to the baseline. There was a 76.4% reduction in the number of CELs with IFNbeta therapy. The decrease was greater (P = 0.003) for S-CELs (82.3%) than for Re-CELs (57.4%). S-CELs showed no changes in durations of enhancement and maximal lesion sizes with treatment. Exclusively for Re-CELs, IFNbeta-1b significantly decreased maximal lesion sizes, total number of enhancement periods and total months of enhancement. Thus, IFNbeta appears to be effective in reducing the degree of severity of inflammation among Re-CELs, as reflected by their reduced maximal lesion sizes and durations of enhancement.

Principles of functional magnetic resonance imaging: application to auditory neuroscience.

Functional imaging based on magnetic resonance methods is a new research frontier for exploring a wide range of central nervous system (CNS) functions, including information processing in sensory, motor, cognitive, and linguistic systems. Being able to localize and study human brain function in vivo, in relatively high resolution and in a noninvasive manner, makes this a technique of unparalleled importance. In order to appreciate and fully understand this area of investigation, a tutorial covering basic aspects of this methodology is presented. We introduce functional magnetic resonance imaging (fMRI) by providing an overview of the studies of different sensory systems in response to modality-specific stimuli, followed by an outline of other areas that have potential clinical relevance to the medical, cognitive, and communicative sciences. The discussion then focuses on the basic principles of magnetic resonance methods including magnetic resonance imaging, MR spectroscopy, fMRI, and the potential role that MR technology may play in understanding a wide range of auditory functions within the CNS, including tinnitus-related activity. Because the content of the material found herein might be unfamiliar to some, we provide a broad range of background and review articles to serve as a technical resource.

Pulsation artifact in short TR MR imaging and angiography: exacerbation with signal averaging.

Averaging the signals from more than one excitation per phase-encoding view increases the signal-to-noise ratio and, in conventional spin-echo magnetic resonance imaging, reduces most motion artifacts. To determine the effects of signal averaging on two-dimensional gradient-echo images, acquisitions with different TRs and with no averaging versus multiple-signal averaging were compared in a pulsatile flow phantom and the human abdominal aorta. Intraview (each view repeated before changing the phase-encoding value) and interview (obtaining all views sequentially and then repeating the entire set) averaging methods were used. Pulsation artifacts were present on all images of the flow phantom and the aorta. Intraview signal averaging, the method most commonly used, exacerbated rather than ameliorated pulsation artifacts with short TR sequences. Pulsation artifacts on two-dimensional images obtained with a short TR can be minimized by completing the acquisition as rapidly as possible, avoiding signal averaging. If signal averaging is used for short TR images, it should be interview averaging.

Pulsatile flow artifacts in fast magnetization-prepared sequences.

Fast magnetization-prepared magnetic resonance imaging sequences allow clinical acquisitions in about 1 second, with the preparation phase providing the desired contrast. Pulsatile flow artifacts, although reduced by rapid acquisition, can degrade image quality. The authors explore the causes of aortic pulsatile flow artifacts in inversion-recovery-prepared acquisitions of the abdomen, taking into consideration various parameters. The flow signal within an 8-mm-thick section was simulated and subsequently Fourier transformed to determine the location and extent of flow artifacts. Results of simulations were validated with abdominal images of human subjects. Recording all encodings within one cardiac cycle reduced pulsatile flow artifacts in nonsegmented acquisitions with sequential phase-encoding order, regardless of the location of magnetization preparation within the cardiac cycle. In segmented acquisitions, however, the sequential order always increased flow artifacts. To reduce the artifacts in short TI acquisitions, the magnetization should be prepared during diastole. In clinical acquisitions, flow artifacts were further reduced by modifying the phase-encoding scheme.

Two-dimensional pulsatile hemodynamic analysis in the magnetic resonance angiography interpretation of a stenosed carotid arterial bifurcation.

A two-dimensional pulsatile hemodynamic analysis based on the finite-element technique was performed on a minimally stenosed carotid artery to identify the possible explanation for the differences in the x-ray and magnetic resonance carotid angiograms of a patient. The magnetic resonance angiogram was obtained by applying the maximum intensity projection algorithm to axial slices, acquired using the time-of-flight principle. The differences in the x-ray and magnetic resonance depictions were interpreted based on velocity profile, wall shear stress, and streamline data provided by the hemodynamic analysis. The specific contribution of the stenosis was further isolated from that of the bifurcation by comparing the flow patterns within the stenotic artery with those of its normal counterpart. The Doppler spectral velocity wave form of the patient constituted the basis for the pulsatile flow velocity specification. The analysis took into consideration the non-Newtonian viscosity of blood. The numerical procedure was validated through different convergence criteria and through shear stress comparisons. The importance of hemodynamic analyses in relation to magnetic resonance angiography was further discussed along with possible shortcomings of the technique.

Chemical shift artifact along the section-select axis.

Chemical shift artifact (CSA), familiar to radiologists along the frequency-encoding axis, also occurs along the section-select axis. The authors observed a case in which CSA mimicked a renal mass. Subsequent retrospective analysis of 50 abdominal magnetic resonance (MR) imaging studies was performed to assess occurrence of CSA adjacent to the upper and lower renal poles. CSA along the section-select axis was observed in 76% of cases and adjacent to 39% of all renal poles imaged. CSA along the section-select axis is common in abdominal MR imaging and may occasionally mimic disease.

Partial angle inversion recovery (PAIR) MR imaging: spin-echo and snapshot implementation.

The effects of varying the inversion or excitation RF pulse flip angles on image contrast and imaging time have been investigated in IR imaging theoretically, with phantoms and with normal volunteers. Signal intensity in an IR pulse sequence as a function of excitation, inversion and refocusing pulse flip angles was calculated from the solution to the Bloch equations and was utilized to determine the contrast behavior of a lesion/liver model. Theoretical and experimental results were consistent with each other. With the TI chosen to suppress the fat signal, optimization of the excitation pulse flip angle results in an increase in lesion/liver contrast or allows reduction in imaging time which, in turn, can be traded for an increased number of averages. This, in normal volunteers, improved spleen/liver contrast-to-noise ratio (9.0 vs. 5.7, n = 8, p less than 0.01) and suppressed respiratory ghosts by 33% (p less than 0.01). Reducing or increasing the inversion pulse from 180 degrees results in shorter TI needed to null the signal from the tissue of interest. Although this decreases the contrast-to-noise ratio, it can substantially increase the number of sections which can be imaged per given TR in conventional IR imaging or during breathold in the snapshot IR (turboFLASH) technique. Thus, the optimization of RF pulses is useful in obtaining faster IR images, increasing the contrast and/or increasing the number of imaging planes.

Color flow-encoded MR imaging.

Magnetic resonance phase images can enable identification of any type of motion, including the velocity and direction of flow, and thus provide valuable supplements to magnitude images, which depict stationary tissue most effectively. A method is described for the simultaneous display of phase and magnitude by color encoding the phase data and superimposing it on the magnitude image to facilitate clinical interpretation. Color-encoded data not only depict the location and direction of flow along specific axes but can also provide relative velocity information through shades of color. Implementation of the technique is described, and the factors to be considered during interpretation of color flow-encoded images are discussed.

Liver and pancreas: improved spin-echo T1 contrast by shorter echo time and fat suppression at 1.5 T.

T1-weighted spin-echo magnetic resonance (MR) images have had limited soft-tissue contrast at 1.5 T. The authors investigated the effects of echo-time (TE) minimization and fat suppression on MR images of the liver and pancreas. Two sets of MR images were obtained with identical repetition times and other parameters. In 10 subjects with seven liver lesions, images with TEs of 20 and 12 msec were compared. In 18 additional subjects with seven liver lesions and five pancreatic carcinomas, images with identical TEs but with and without fat suppression were compared. Contrast-to-noise ratios (CNRs) were greater with a TE of 12 msec than with a TE of 20 msec for liver versus spleen (7.6 vs 4.9, P = .014) and liver versus lesion (6.9 vs 3.9, P = .031). In patients without fatty liver, CNR for six lesions versus liver was greater (9.5 vs 6.0, P = .014) with fat suppression. CNR between glandular pancreas and cancer was most conspicuous with fat suppression, but fat planes were less distinct. Minimization of TE improves T1-weighted images significantly. Fat suppression also improves CNR, but the disadvantages of fat suppression do not allow elimination of conventional T1-weighted images.

MR fluoroscopy: initial clinical studies.

Magnetic resonance (MR) fluoroscopy is a method for high-speed MR image acquisition with the goals of short acquisition time per image (500 msec or less), high image rate (10 images or more per second), and high-speed image reconstruction (150 msec or less from data acquisition to image display). The authors present their results with the first two goals in volunteers. MR fluoroscopic image data were acquired with a limited flip angle pulse sequence with reduced repetition times (TRs) and fewer phase encodings used per image. The sequence was applied continuously, and images were formed by updating one set of data with data from the most recently taken measurements. Sample head images were generated with TR/echo times as small as 11/5.5 msec and 48 phase encodings for a total acquisition time of about 500 msec. Images were acquired while the volunteer flexed his head. Artifacts from the motion became less evident on images as progressively shorter acquisition times were used.

Fast limited flip angle MR subtraction angiography.

A fast MR angiography method is introduced that is capable of generating difference images of blood vessels in scan times of 10-20 s. This is an order of magnitude faster than many previous methods. The fundamental concept of this approach is to use cardiac gating and acquire several phase encodings at least twice during each cardiac cycle using limited flip angles (LFAs) and repetition times in the 20 to 50 ms range. The encodings acquired during diastole are subtracted from those acquired during systole to generate the difference image. The contrast in the difference image is due both to the influx of unsaturated spins and to the loss of phase coherence of systolic blood moving at high velocity along a magnetic gradient. The systolic peak of the cardiac cycle is determined during reconstruction by shifting the systolic and diastolic "windows" until the difference signal is maximized. Ghost artifacts due to pulsatile flow are eliminated by a phase reordering technique similar in concept to those developed for suppression of breathing artifacts. Arteries in thick slices are successfully imaged and initial in vivo results are presented.

MR fluoroscopy: technical feasibility.

A method of magnetic resonance image acquisition and reconstruction is described in which high imaging rates and fast reconstruction times are allowed. The acquisition is a modification of the basic FLASH sequence but with a restricted number N of phase encodings. The encodings are applied sequentially, periodically, and continuously. Images are formed by sliding a window of width N encodings along the acquired data and reconstructing an image for each position of the window. In general the acquisition time per image exceeds the time between successive images, and the method thus has a temporal lag. Experimental studies were performed with a dynamic phantom using 48 phase encodings and a TR of 20 ms, for an image acquisition time of about 1 s. The image display rate in the reconstructed sequence was 12.5 images/s, and the image sequence portrayed the motion of the phantom. Additional studies were done with 24 encodings. It is shown how the sliding window technique lends itself to high-speed reconstruction, with each newly acquired echo used to quickly update the image on display. The combination of the acquisition technique described and a hardware implementation of the reconstruction algorithm can result in realtime MR image acquisition and reconstruction.

MR subtraction angiography with a matched filter.

The technique of matched filtering (MF) has been used in the past with X-ray digital subtraction angiography as a method of improving signal-to-noise ratio (SNR) in subtraction angiographic images. In this work we describe how MF can be applied to a series of images produced by cinematographic magnetic resonance (cine MR) to produce angiographic images. Likewise, a simple subtraction image can be formed by subtracting an image in which flow is not well visualized from an image at the same location but with flow visualization. Theory predicts that a subtraction image resulting from the MF technique will yield typical SNR improvements of 60% over results from simple subtraction. Twenty-one studies of the human popliteal, canine aorta, and canine carotid artery were undertaken in which MF was compared with simple subtraction. It was determined that cine MR can be used to produce subtraction angiographic images and that MF can produce a modest improvement in SNR over simple subtraction.

Pulse sequence extrapolation with MR image synthesis.

Previous reports have presented validation studies of magnetic resonance (MR) image synthesis in which multiple spin-echo (MSE) source data were used to generate spin-echo images for various echo times and repetition times (TRs). A new method-"pulse sequence extrapolation" -synthesizes images for pulse sequences different from that of the acquisition. MSE data acquired in a time equivalent to a TR of 2,000 msec can be used to generate inversion-recovery (IR) images for arbitrarily chosen TI inversion times. Other combinations of pulse sequences were also studied, and synthetic images were compared visually and quantitatively to directly acquired images with corresponding parameters. Synthetic IR signals of the brain parenchyma consistently matched directly acquired signals to within 6%, with respect to the full magnetization signal. The noise level of synthetic signals was generally no more than twice that of direct acquisition signals, as predicted. This method can achieve selective fat suppression and enhancement in IR imaging.