Rapid MR imaging of the pediatric brain using the fast spin-echo technique.

PURPOSE: To evaluate diagnostic reliability and to establish optimal scanning techniques of a recently developed Fast Spin-echo MR pulse sequence that allows rapid proton density-weighted and T2-weighted imaging. METHODS: We compared lesion conspicuity and signal intensity measurements on Fast Spin-echo and conventional spin-echo sequences in 81 patients ranging from 1 week to 25 years in age on a 1.5-T MR unit. A total of 28 Fast Spin-echo dual-echo images (14 slice locations) were obtained in 2:08 minutes with a 256 x 128 matrix or in 3:12 minutes with a 256 x 192 matrix at a TR of 2000 msec and two excitations. RESULTS: Lesion conspicuity and characterization on Fast Spin-echo images compared favorably with conventional spin-echo images in our series when pseudo-TEs of 15 and 90 msec were employed for proton density-weighted and T2-weighted images, respectively. Fast Spin-echo images yielded diagnostic information in four nonsedated patients whose conventional spin-echo images were either degraded by motion or unobtainable. Fat signal remained bright on T2-weighted Fast Spin-echo images. Magnetic-susceptibility effects were slightly reduced with Fast Spin-echo but did not pose any diagnostic problem in our series. CONCLUSION: Diagnostically reliable rapid dual-echo brain images can be obtained with Fast Spin-echo sequences.

Phase-encode order and its effect on contrast and artifact in single-shot RARE sequences.

Substantial manipulation of tissue contrast can be achieved by varying the order in which phase-encode values are applied to individual echoes within a 128-echo single-shot rapid acquisition relaxation enhanced (RARE) sequence. Appropriate ordering can then permit imaging of short T2 species like muscle and white matter with single-shot RARE. For sequential phase encoding with an arbitrary initial phase-encode value, the timing of the zero phase (ZP) encoded echo is found to be analogous to the echo time (TE) of standard spin-echo sequences. This is demonstrated qualitatively with human brain images and is verified quantitatively with NiCl2 phantoms by correlating the time constant for signal decay with ZP echo time, with transverse relaxation times T2, as obtained with a 128-echo Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence. Banding artifacts accompanying the discontinuous traverse through K space are experimentally demonstrated in a rectangular phantom and expressions are developed for determining the dependence of this artifact on the phase-encode gradient increments and durations, the ZP echo number, echo spacing, and T2. Simulations based on the expressions are shown to be useful for characterizing the observed "banding" artifacts perpendicular to the phase-encode direction and for predicting the extent of tissue-tissue overlap to be expected with the use of this ultrafast rf echo planar imaging method.

1D spectroscopic imaging with rf echo planar (SIRFEN) methods.

A recently developed rf echo planar imaging method has been modified to rapidly generate spectroscopic information along one in-plane axis and spatial information along the other. The method allows the production of one-dimensional chemical shift images (1D CSIs) in acquisition times of 18 sec or less. A specific phase-encode-reordering algorithm provides convenient manipulation of T2 weighting, yielding partial suppression of short T2 species like muscle water. The method is demonstrated in phantoms and in vivo with 1D CSIs of human brain and limbs. Abnormal fat distribution is demonstrated in the calf of a patient with aggressive fibromatosis. The advantages of short acquisition times obtainable with SIRFEN are offset by limited spectral resolution, suggesting that primary applications will be confined to rapid spatial mapping of major spectral components.

Magnetization transfer effects in multislice RARE sequences.

Magnetization transfer effects are demonstrated to be significant in determining the signal intensity from brain tissues on images acquired with multislice rapid acquisition relaxation enhanced (RARE) sequences. We report studies designed to determine how the signal intensities vary with slice number or, equivalently, off-resonance power deposition. The results obtained in fat, gray matter, and white matter are similar in form to those reported in kidney tissues during classic magnetization transfer experiments (J. Eng, T. L. Ceckler, and R. S. Balaban, Magn. Reson. Med. 17, 304 (1991)). Of clinical significance to RARE practitioners is the increase of contrast-to-noise ratios between gray and white matter on proton density-weighted images with increasing slice number.

Partial RF echo planar imaging with the FAISE method. I. Experimental and theoretical assessment of artifact.

The fast acquisition interleaved spin-echo (FAISE) method is a partial RF echo-planar technique which utilizes a specific phase-encode reordering algorithm to manipulate image contrast (Melki et al., J. Magn. Reson. Imaging 1:319, 1991). The technique can generate "spin-echo" like images up to 16 times faster than conventional spin-echo methods. However, the presence of T2 decay throughout the variable k-space trajectories used to manipulate T2 contrast ensures the presence of image artifacts, especially along the phase-encode direction. In this work, we experimentally and theoretically examine the type and extent of artifacts associated with the FAISE technique. We demonstrate the existence of well-defined minima of phase-encode ghost noise for selected k-space trajectories, examine the extent of blurring and edge enhancement artifacts, demonstrate the influence of matrix size and number of echoes per train on phase-encode artifact, and show how proper choice of FAISE sequence parameters can lead to proton density brain images which are practically indistinguishable from conventional spin-echo proton density images. A comparison of contrast between FAISE and standard spin-echo methods is presented in a companion article referred to as II.

Partial RF echo-planar imaging with the FAISE method. II. Contrast equivalence with spin-echo sequences.

The fast acquisition interleaved spin-echo (FAISE) sequence and its dual-echo version (DEFAISE) are partial RF echo-planar methods which utilize a specific phase-encode reordering algorithm to manipulate T2 contrast via an operator-controlled pseudo-echo time, pTE. The repetition time, TR, between successive applications of the Carr-Purcell-Meiboom-Gill (CPMG) echo trains used in FAISE may be reduced to introduce T1 weighting. To quantitatively determine the extent to which FAISE T1 and T2 contrast characteristics agree with spin-echo methods, signal intensities from FAISE acquisitions were compared with signal intensities from equivalent CPMG acquisitions. In phantoms and in human heads, the contrast characteristics of FAISE are found to be highly correlated with that obtained with equivalent CPMG sequences. However, conventional SE sequences generally utilize longer echo spacings than employed with FAISE/CPMG. Thus, echo spacing-dependent mechanisms such as spin-spin coupling and magnetic susceptibility lead to some differences in contrast between conventional SE and FAISE. Finally, FAISE appears to be more sensitive to magnetization transfer effects than conventional SE sequences since more off-resonance irradiation is applied to individual slices during multislice acquisitions.

T2-weighted thin-section imaging with the multislab three-dimensional RARE technique.

A novel three-dimensional (3D) RARE (rapid acquisition with relaxation enhancement) sequence was implemented on a clinical imager. In this technique, multiple slabs are excited in the same way as in the multisection spin-echo sequence, and each slab is further phase encoded into eight sections along the section-slab direction. With a 16-echo RARE sequence, 128 excitations cover the 256 X 256 X 8 3D k space. With a TR of 2,500 msec, 10 slabs can be excited sequentially at each TR, yielding 80 sections in 5 minutes. Slabs were overlapped to give contiguous sections after discarding of the aliased sections at slab edges. This relatively fast sequence makes contiguous thin-section T2-weighted imaging possible, an impractical achievement with the much longer spin-echo method. Compared with 3D Fourier transform gradient-echo imaging, the sensitivity of 3D RARE sequences to magnetic susceptibility is reduced. The clinical potential of T2-weighted 3D imaging is illustrated with high-resolution brain, spine, and temporomandibular joint images.

Comparing the FAISE method with conventional dual-echo sequences.

The FAISE (fast-acquisition interleaved spin-echo) technique consists of a hybrid rapid-acquisition relaxation-enhanced (RARE) sequence combined with a specific phase-encode reordering method. Implemented on a 1.5-T unit, this multisection, high-resolution technique permits convenient contrast manipulation similar to that of spin-echo imaging, with selection of a pseudo-echo-time parameter and a TR interval. With a TR of 2 seconds, eight 256 x 256 images are obtained in 34 seconds with either T2 or proton-density weighting. A direct comparison between FAISE and spin echo for obtaining T2-weighted head images in healthy subjects indicates that FAISE and spin-echo images are qualitatively and quantitatively similar. Image artifacts are more pronounced on "proton-density" FAISE images than on the T2-weighted FAISE images. T1 contrast can be obtained with inversion recovery and short TR FAISE images. Preliminary temperature measurements in saline phantoms do not indicate excessive temperature increases with extended FAISE acquisitions. However, extensive studies of radio-frequency power deposition effects should be performed if the FAISE technique is to be fully exploited.

MRI in chronic progressive radiation myelopathy.

OBJECTIVE: Our goal was to assess medullary lesions in patients suffering from chronic progressive radiation myelopathy (CPRM) using MRI. MATERIALS AND METHODS: In a group of 10 patients suffering from CPRM, MRI findings (11 examinations), radiation protocols, and patient prognoses were reviewed. RESULTS: A cord enlargement was demonstrated in five cases, whereas four cases presented with medullary atrophy. As demonstrated by MRI, radiation-induced medullary lesions progressed toward cord atrophy in one patient. When MRI and/or comparison myelogram were performed within 8 months following the onset of the myelopathy, a cord enlargement was usually encountered. When the patient was evaluated > 8 months after the first neurological symptoms, a cord atrophy was always demonstrated. Medullary lesions extended beyond the boundaries of the radiation field in 67% of the cases. However, with the exception of one case, the main focus of the cord damage was included within an irradiated cord segment. An enlarged cord was often associated with a neurologic deterioration and a fatal outcome. In patients with cord atrophy, the neurologic deficit was often static and survival rates were better. CONCLUSION: These results suggest a revision of classic criteria used for the diagnosis of CPRM. By demonstrating cord lesions, MRI helps to establish disease prognosis.

Vascular rupture complicating transluminal angioplasty applied on a failed dialysis vascular access in a patient under chronic steroid therapy.

We report a case of venous rupture complicating percutaneous transluminal angioplasty (PTA) applied on a failed dialysis vascular access (VA) in a patient on chronic steroid therapy. This complication resulted in a rapidly growing hematoma which was successfully controlled by a prolonged reinflation of the balloon catheter at the angioplasty site. The absence of oversizing of the balloon catheter and the low inflation pressure at which the perforation occurred suggest a vessel fragility which was probably induced by a long-standing steroid therapy. In dialysis patients in whom steroid therapy does not represent an infrequent therapeutic modality, this potential risk of vascular rupture should be carefully weighted while treating VA stenoses with the use of PTA.

Head and neck: initial clinical experience with fast spin-echo MR imaging.

Fast spin-echo (FSE) is a new magnetic resonance (MR) imaging pulse sequence that employs echo trains of 180 degrees radio-frequency pulses to generate multiple refocused echoes during a single repetition-time interval. Phase encoding is reordered with the lowest spatial frequency views obtained in the echoes nearest the desired effective echo time. These techniques were used to examine 30 patients with pathologic conditions of the head and neck. The images were compared with closely matched conventional T2-weighted spin-echo (SE) images obtained during the same examination. Three unblinded readers evaluated 15 sets of both images for lesion conspicuity, motion artifact, diagnostic information, number of lesions seen, image quality, and sharpness of lesion borders. In all categories, FSE images were judged better than or equal to SE images, and in most cases FSE images were obtained in one-fourth to one-half the imaging time. In no case were lesions depicted on conventional T2-weighted SE images missed on FSE images; in fact, more lesions were seen on FSE images than on conventional T2-weighted SE images.

Comparison of single-breath-hold fast spin-echo sequences with routine non-breath-hold techniques: application to MRI of renal masses.

In 24 patients presenting with 55 renal lesions (mean size, 20.8 mm), single-breath-hold (SBH) fast spin-echo (FSE) techniques allowing T1 and T2 images to be produced within 20 and 23 sec, respectively, were compared with routine non-breath-hold (NBH) spin-echo (SE) T1 and NBH-FSE T2 sequences. Contrast-to-noise ratios (CNRs) measured from SBH-FSE T1 images were an average of 97% higher than their NBH counterparts (P = .0001) and allowed an improved lesion conspicuity in 80% of the cases (P = 0.0001). For T2 imaging, SBH-FSE and NBH-FSE sequences were not statistically different with respect to lesion conspicuity (P = .55) and CNR values (P = .19). This was observed despite a 35% average decrease in CNR of SBH-FSE compared to NBH-FSE images. By reducing respiratory motion artifacts while preserving SE-like image contrast, SBH-FSE techniques have the potential to replace routine NBH sequences for an optimal diagnosis of renal masses.

Brain hemorrhage: evaluation with fast spin-echo and conventional dual spin-echo images.

Signal intensity of blood products on proton-density- and T2-weighted images obtained with spin-echo (SE) and fast SE (FSE) sequences was evaluated in 15 patients with central nervous system hemorrhage to determine the extent of differences between the two techniques when signal loss from magnetic susceptibility effects in hemorrhagic lesions is considered. Within operator-defined regions of interest, signal intensity of hemorrhage, iron-containing nuclei, white matter, scalp fat, and noise was measured along the phase-encoding direction. Hemosiderin, deoxyhemoglobin, and iron-containing nuclei had slightly higher signal intensity on FSE images than on SE images, but the differences were not statistically significant. Signal intensity of methemoglobin was similar with both sequences, whereas that of scalp fat was higher on FSE images. Signal intensity measurements for most tissues studied were comparable, but the signal-to-noise ratios with FSE imaging were less than those with SE imaging. Although paramagnetic blood products may show slightly higher signal intensity with FSE imaging, contrast with the two sequences was comparable and lesion conspicuity was nearly identical.