High-field MR microscopy using fast spin-echoes

Comparison of three-dimensional isotropic T1-weighted fast spin-echo MR arthrography with two-dimensional MR arthrography of the shoulder

PURPOSE

To determine the accuracy of a three-dimensional (3D) isotropic T1-weighted fast spin-echo (FSE) magnetic resonance (MR) sequence as compared with a conventional two-dimensional (2D) sequence in the diagnosis of rotator cuff tears and labral lesions.

MATERIALS AND METHODS

Institutional review board approval was obtained, and the informed consent requirement was waived. Forty-nine patients who had undergone direct or indirect shoulder MR arthrography with the 2D T1-weighted FSE sequence and the 3D isotropic T1-weighted FSE sequence and subsequent arthroscopy were included. Each MR imaging sequence was independently scored by two readers retrospectively for the presence of full- or partial-thickness tears of the supraspinatus (SST) and infraspinatus (IST) tendons and the subscapularis tendon (SCT) and labral lesions. Diagnostic performance based on each sequence type was compared by using the area under the receiver operating characteristic curve (AUC).

RESULTS

Arthroscopic findings enabled confirmation of the presence of 17 full-thickness SST-IST tears, 18 partial-thickness SST-IST tears, four full-thickness SCT tears, 17 partial-thickness SCT tears, and 17 labral lesions. The AUCs for the readers using the 3D T1-weighted FSE sequence versus those obtained with the 2D sequence were 0.771-0.989 versus 0.837-0.998 for reader A and 0.771-0.989 versus 0.797-0.989 for reader B in the detection of rotator cuff tears and 0.885 versus 0.897 for reader A and 0.895 versus 0.895 for reader B in the detection of labral lesions. The mean AUCs between the 2D and 3D sequences were not significantly different, with the exception of partial-thickness SCT tears for one reader.

CONCLUSION

The accuracy of 3D isotropic FSE MR arthrography may be comparable with that of conventional 2D MR arthrography in the diagnosis of rotator cuff tears and labral lesions with a shorter imaging time.

Isotropic 3D fast spin-echo with proton-density-like contrast: a comprehensive approach to musculoskeletal MRI

OBJECTIVE

Scanning time considerations have restricted routine use of 3D Fourier transform (3DFT)-encoded MRI to gradient-recalled echo sequences. We sought to combine isotropic 3DFT acquisition with fast spin-echo at a practical scan duration. This strategy offers versatile image contrast for musculoskeletal evaluation and facilitates image reformation tailored to the depiction of small anatomic features.

CONCLUSION

Isotropic 3DFT fast spin-echo is feasible on current MRI scanners and has the potential to improve musculoskeletal evaluation.

MR microscopy of the articular cartilage with a 1.0T permanent magnet portable MR system: preliminary results

The present study was designed to evaluate the use of a 1.0T portable permanent magnet MR system in obtaining microscopic MR images of the hyaline cartilage in vitro. A clear laminar appearance was demonstrated with this system. In addition, it was possible to demonstrate cartilage surface irregularity, a decrease in cartilage thickness, and T2 alteration by proteoglycan depletion following up to 12 hours of trypsin treatment. In summary, the portable MR system is useful for investigating cartilage in vitro.

Diffusivity- and T2 imaging at 3 Tesla for the detection of degenerative changes in human-excised tissue with high resolution: atherosclerotic arteries

RATIONALE AND OBJECTIVES

We investigated whether it is possible to investigate degenerative changes in human tissue on a sub-100-microm resolution scale not only on special high-field small-bore MR-microscopy systems but also on a 3T whole-body MR-scanner.

METHODS

Spin-spin relaxation, proton density, and diffusion microimaging were investigated in studying human atherosclerotic arteries. Strong diffusion weighting and high spatial resolution was achieved by means of a strong dedicated gradient system and a small birdcage radiofrequency resonator.

RESULTS

Quantitative parameter maps were obtained at voxel sizes down to 73 x 73 x 600 microm3. The morphologic structure and pathology connected to lipid deposits, plaques, small thrombi, and bifurcations were well visualized.

CONCLUSION

High-resolution parameter-weighted and parameter-imaging at sub-100-microm pixel resolution can be achieved for excised tissue on a 3.0 T whole body MR system. Perspectives for the characterization of atherosclerotic plaques imply not only cost advantages but also equivalence of contrast, especially as to T(2), for in vivo and high-resolution ex vivo investigations on the same MR scanner.

Ultrafast pulse sequence techniques for cardiac magnetic resonance imaging

Cardiac magnetic resonance imaging is a rapidly emerging field that has seen tremendous advances in the past decade. Central to the development of effective imaging strategies has been the advent of high-performance gradient hardware and the exploitation of their speed characteristics through specialized pulse sequences well suited for cardiac imaging. These advances have facilitated unprecedented acquisition times that now approach echocardiographic frame rates, while maintaining excellent image quality. This article provides a detailed overview of advanced pulse sequence technology and approaches currently taken to maximize speed performance and image quality. In particular, segmented K-space techniques that include single-echo and multiecho spoiled gradient-echo imaging as well as steady-state free precession imaging are discussed. Finally, spiral and fast spin-echo techniques are explored. Examples of common applications of these pulse sequences are presented.

Noninvasive In vivo high-resolution magnetic resonance imaging of atherosclerotic lesions in genetically engineered mice

BACKGROUND

The pathogenesis of atherosclerosis is currently being investigated in genetically engineered small animals. Methods to follow the time course of the developing pathology and/or the responses to therapy in vivo are limited.

METHODS AND RESULTS

To address this problem, we developed a noninvasive MR microscopy technique to study in vivo atherosclerotic lesions (without a priori knowledge of the lesion location or lesion type) in live apolipoprotein E knockout (apoE-KO) mice. The spatial resolution was 0.0012 to 0.005 mm3. The lumen and wall of the abdominal aorta and iliac arteries were identified on all images in apoE-KO (n=8) and wild-type (n=5) mice on chow diet. Images obtained with MR were compared with corresponding cross-sectional histopathology (n=58). MR accurately determined wall area in comparison to histopathology (slope=1.0, r=0.86). In addition, atherosclerotic lesions were characterized in terms of lesion shape and type. Lesion type was graded by MR according to morphological appearance/severity and by histopathology according to the AHA classification. There was excellent agreement between MR and histopathology in grading of lesion shape and type (slope=0.97, r=0.91 for lesion shape; slope=0. 64, r=0.90 for lesion type).

CONCLUSIONS

The combination of high-resolution MR microscopy and genetically engineered animals is a powerful tool to investigate serially and noninvasively the progression and regression of atherosclerotic lesions in an intact animal model and should greatly enhance basic studies of atherosclerotic disease.

Imaging birds in a bird cage: in-vivo FSE 3D MRI of bird brain

An in-vivo magnetic resonance imaging (MRI) procedure is described that allows one to obtain three-dimensional high quality images of the entire brain of small birds such as the canary (20 g) and the starling (75 g) with an image resolution of 0.1 mm (58-113 microm, dependent on the size of the imaged bird). The entire imaging procedure took about 2 h after which the birds recovered from anaesthesia uneventfully and could be reused for subsequent additional imaging. This non invasive MRI technique enables to correlate brain measures with behavioural or physiological data that are dynamic in nature and could permit significant progress for bird neurological research.

Magnetic resonance microscopy and histopathology: comparative approach of bromobenzene-induced hepatotoxicity in the rat

The development of magnetic resonance (MR) microscopy has provided new approaches to histology and histopathology. Recent work has shown the promise of increased sensitivity in animal models of chemically induced hepatotoxicity. However, the field is so new that there is little experience to relate changes seen in MR micrographs to the more traditional optical images stained with hematoxylin and eosin. This work compares the sensitivity and reproducibility of MR microscopy with conventional histopathology in detecting bromobenzene-induced hepatotoxicity in the rat. A time-course study was undertaken to provide a range of histopathologies. Specimens were studied at 24, 48, 72, and 96 hours after exposure to 10% of the median lethal dose of bromobenzene. Using 4 animals per group (a total of 32 rats) added statistical significance to the study and defined a range of interanimal variability over 96 hours. This work shows that MR microscopy, besides being nondestructive and three-dimensional, is at least as sensitive as conventional hematoxylin-eosin staining in detecting bromobenzene-induced centrilobular lesions and recovery of the hepatocellular architecture in the rat. This study further suggests that, as we begin to understand the underlying mechanisms of contrast in MR histology, MR may, in fact, supply even higher specificity than more traditional studies: variations were observed in MR images of treated livers at a given time point that could be not be differentiated based on the grading of necrosis and inflammation on hematoxylin-eosin-stained sections.

A fast spin echo technique with circular sampling

This paper presents a fast spin echo (FSE) imaging method that employs circular sampling of k-space. The technique has been implemented on a 2 Tesla imaging system and validated on both phantoms and living animals. Experimental studies have shown that circular sampling can produce artifact-free FSE images without the need of phase correction. Although not fully explored, preliminary results also show that circular sampling may have advantages over the conventional rectilinear FSE in signal-to-noise ratio and imaging efficiency. A major disadvantage is the increased sensitivity to off-resonance effects. The authors expect that the FSE technique with circular sampling will find its applications in magnetic resonance microscopy, neuro-functional imaging, and real-time dynamic studies.

Fast 3D large-angle spin-echo imaging (3D FLASE)

A rapid steady-state 3D spin-echo imaging pulse sequence, based on the principle of nutating the spins by an angle greater than 90 degrees, has been designed and implemented on a clinical 1.5-T whole-body MR scanner. The pulse sequence, denoted fast large-angle spin-echo (FLASE), has been optimized for high-resolution imaging of tissues with short T2 and T2*. Features of FLASE include a minimum-phase Shinnar-Le Roux excitation pulse and distribution of phase- and slice-encoding gradients before and after the 180 degrees refocusing pulse to minimize the critical time delay between inversion and restoration of the residual longitudinal magnetization and for minimizing echo time. A Bloch equation analysis, corroborated by experimental data, shows FLASE signal-to-noise to be superior to its closest analog, 3D rapid spin-echo excitation (RASEE) (Jara et al., Magn Reson Medicine 29, 528 (1993)), and 3D gradient-recalled acquisition in steady state (GRASS). It is demonstrated that with judicious RF phase-cycling and steady state operation, FLASE can produce high-quality microimages free of intravoxel phase dispersion from susceptibility-induced background gradients. The performance of the method is exemplified with ultra high-resolution images of trabecular bone in vitro and in vivo in the human calcaneus and wrist at voxel sizes as low as 98 x 98 x 200 microns3. Finally, the contrast behavior of refocused FLASE can be altered by disrupting the steady state analogous to gradient echo imaging.

Detection of bromobenzene-induced hepatocellular necrosis using magnetic resonance microscopy

The authors used magnetic resonance (MR) microscopy to assess hepatic tissue damage induced by bromobenzene both in living rats and in fixed rat liver tissues. Experiments were conducted at 7 Tesla on three groups of Fisher rats treated with bromobenzene at a single dose of 68, 135, and 269 mg/kg, respectively. Optical microscopy of hematoxylin and eosin stained sections showed liver damage only at the highest dose, whereas with MR microscopy, tissue alterations were detected at all three doses both in vivo and ex vivo. The contrast mechanism of the superior sensitivity of MR microscopy is believed to be related to the changes in local diffusion coefficients that accompany cellular degeneration and death, although other contrast mechanisms may also be involved. The superior sensitivity of MR microscopy, as demonstrated in this study, has many implications for potential use of MR techniques to perform in vivo histology.

Three-dimensional nuclear magnetic resonance microimaging of trabecular bone

The conventional approach to measuring structural parameters in trabecular bone rests on stereology from optical images, derived from sections of embedded bone. In order to provide data that are statistically representative of a sufficiently large volume, multiple sections need to be analyzed in each of the three orthogonal planes. In this work, an alternative technique is presented which is based on three-dimensional (3D) volumetric proton nuclear magnetic resonance (NMR) microimaging. The method presented provides from 9 x 9 x 4 mm3 volumes of defatted bone specimens in 15-20 minutes scan time at isotropic resolution corresponding to (78 microm)3 voxel size. Surface-rendered images of bovine and human trabecular bone are shown and an algorithm was developed and implemented for determining the orientation and magnitude of the principle axes of the mean intercept length tensor.

Three dimensional magnetic resonance microangiography of rat neurovasculature

Techniques are described to perform three dimensional (3D) MR microangiography. We have combined the use of a blood pool agent (Gd-DTPA-complexed with bovine serum albumin), three dimensional Fourier encoding, careful animal stabilization, and volume rendering to permit imaging with voxels of 60 x 60 x 60 microns. 3DFT encoding has been performed at 7.1 T with very large arrays (256 x 512 x 512). Interactive volume rendering allows a number of unique display opportunities that effectively exploit these isotropic 3D arrays.

Imaging the cochlea by magnetic resonance microscopy

The isolated, fixed cochlea of the mustached bat was studied with three dimensional magnetic resonance (MR) microscopy. The cochlea of this animal is about 4 mm in diameter and its entire volume was imaged. With the field of view and matrix size used, the volume elements (voxels) making up the volume data set were isotropic 25 x 25 x 25 micron cubes. Three dimensional (3D) MR microscopy based on isotropic voxels has many advantages over commonly used light microscopy: 1) it is non destructive; 2) it is much less time consuming; 3) no dehydration is required and shrinkage is minimized; 4) the data set can be used to create sections in any desired plane; 5) the proper alignment of sections is inherent in the 3D acquisition so that no reference points are required; 6) the entire data set can be viewed from any point of view in a volume rendered image; 7) the data is digital and features can be enhanced by computer image processing; and 8) the isotropic dimensions of the voxels make the data well-suited for structural reconstructions and measurements. Good images of the osseous spiral lamina, spiral ligament, scala tympani, scala vestibuli, and nerve bundles were obtained. The vestibular (Reissner's) membrane was easily identified in the mustached bat and it appears to bulge into the scala vestibuli. The visibility of this structure suggests that MR microscopy would be well-suited for studies of endolymphatic hydrops.

Reduction of ringing and blurring artifacts in fast spin-echo imaging

A simple method was devised to reduce ringing and blurring artifacts caused by discontinuous T2 weighting of k-space data in fast spin-echo magnetic resonance (MR) imaging. The method demodulates the weighting function along the phase-encoding direction by using multiple T2 values derived from a set of non-phase-encoded echoes obtained from an extra excitation. The performance of this method was evaluated by computer simulations and experiments, which confirmed its capability of effectively reducing or, in some cases, even completely removing the ringing and blurring artifacts. The results also show that the proposed method produces better results than other artifact reduction methods. The method is particularly useful at high magnetic field strengths (7.1-9.4 T) and with strong gradients (> 20 G/cm) used in MR microscopy, in which the apparent T2 values are short for most tissues. The authors expect that the proposed method will find useful applications in various fast spin-echo pulse sequences.

Diffusion-weighted MR microscopy with fast spin-echo

A diffusion-weighted fast spin-echo (FSE) imaging sequence for high-field MR microscopy was developed and experimentally validated in a phantom and in a live rat. Pulsed diffusion gradients were executed before and after the initial 180 degrees pulse in the FSE pulse train. This produced diffusion-related reductions in image signal intensity corresponding to gradient ("b") factors between 1.80 and 1352 s/mm2. The degree of diffusion weighting was demonstrated to be independent of echo train length for experiments using trains up to 16 echoes long. Quantitative measurements on a phantom and on a live rat produced diffusion coefficients consistent with literature values. Importantly, the eight- to 16-fold increase in imaging efficiency with FSE was not accompanied by a significant loss of spatial resolution or contrast. This permits acquisition of in vivo three-dimensional data in time periods that are appropriate for evolving biological processes. The combination of accurate diffusion weighting and high spatial resolution provided by FSE makes the technique particularly useful for MR microscopy.