3D interleaved water and fat image acquisition with chemical-shift correction

A Comparative Study of High-Resolution Chemical-Shift-Eliminated Magnetic Resonance Imaging of Finger Specimens with Microcomputed Tomography

Objective:

High-resolution images of finger joints with chemical-shift elimination can be obtained using an interleaved water-fat (IWF) sequence. This study assessed IWF imaging of finger joints in the delineation of bone structures by comparing images of cadaver fingers with those of microcomputed tomography (CT) that served as a standard reference.

Materials and Methods:

IWF images with spatial resolution of 176 µ × 176 µ × 300 µ were obtained from the distal and proximal interphalangeal joints of two cadaver finger specimens using a custom-built radiofrequency receive coil at 1.5T. Regular three-dimensional gradient-echo (GRE) images were also acquired with similar parameters and compared with the IWF images to evaluate the effects of chemical shift. Micro-CT scans were obtained and served as the standard reference. The image data were reviewed by two experienced musculoskeletal radiologists in consensus. The delineation of normal joint structures and abnormalities in the finger specimens as revealed by the magnetic resonance imaging (MRI) and micro-CT images were compared. The IWF and regular GRE images were assigned scores 0–3 for the depiction of apparent marginal bone defects, with zero being the same in appearance to the micro-CT image and three as having minimal resemblance to it. Statistical analysis of the scoring results was conducted to compare the two MRI techniques.

Results:

The high-resolution IWF images provided accurate delineation of bone and calcified structures as seen in micro-CT. The thickness of subchondral bone was depicted similarly on the IWF water + fat and the micro-CT images but not on the regular GRE images. The regular GRE sequence showed false marginal bone defects not observed with IWF and micro-CT. In addition, the IWF water-only images facilitated the identification of bone cyst by revealing its water content.

Conclusion:

High-resolution IWF imaging should be useful for the early diagnosis and treatment assessment of arthritis and should also benefit basic research in the pathophysiology of the disease.

Water-fat separation from a single spatiotemporally encoded echo based on nominal k-space peaking and joint regularized estimation

PURPOSE

To present a new high-resolution single-point water-fat separation algorithm based on the spatiotemporally encoded chemical shift imaging technique.

THEORY

Identifying water and fat peaks on the ensemble of the nominal k-space profiles of all spatiotemporally encoded lines enables evaluation of the mean off-resonance frequencies of the two components. With utilization of the spatial smoothness and filtering regularizations, the water/fat profiles can be discriminated with twice joint linear least squares estimations line-by-line.

METHODS

The effectiveness of the proposed algorithm was assessed by experiments on oil-water phantoms and in vivo in rats at 7T using a spatiotemporally encoded variant of the multishot spin-echo sequence. The results were compared with those obtained from previously proposed 1-point Dixon, 2-point Dixon, and 3-point IDEAL methods.

RESULTS

The results demonstrate that the new technique can achieve high-quality water-fat separations, comparable in signal-to-noise ratio and contrast to the multipoint methods and is more robust in cases when large areas of low signals or motion artifacts jeopardize the results from the 1-point Dixon method.

CONCLUSIONS

The proposed technique is potentially a new viable alternative for single-point water-fat separation.

High-resolution interleaved water-fat MR imaging of finger joints with chemical-shift elimination

PURPOSE

To study the use of an interleaved water-fat (IWF) sequence with a custom-made radiofrequency (RF) coil for high-resolution imaging of arthritic finger joints.

MATERIALS AND METHODS

High-resolution finger magnetic resonance imaging (MRI) was performed using a custom-made dedicated RF receiver coil and an IWF sequence. A phantom, a cadaver finger specimen, and the fingers of two normal controls and six arthritic subjects were imaged with a resolution of 156 × 156 × 600 μm. The appearance of anatomic structures on the IWF images were compared with images acquired with a regular sequence. The images were reviewed by two musculoskeletal radiologists for the depiction of anatomical structures and for the presence of abnormalities.

RESULTS

The high-resolution images revealed detailed structures of the finger joints not detectable using typical clinical resolution. The IWF sequence gave more realistic depiction of subchondral bone thickness, and avoided false bone erosions displayed in the regular sequence. It also allowed better visualization of ligaments and tendons.

CONCLUSION

This pilot study shows the feasibility and the potential usefulness of high-resolution IWF imaging for finger joint evaluation. This technique may be useful for the diagnosis and treatment assessment of arthritis, and for the study of joint disease pathogenesis.

In vivo lactate signal enhancement using binomial spectral-selective pulses in selective MQ coherence (SS-SelMQC) spectroscopy

Tumor vasculature and tissue oxygen pressure can influence tumor growth, metastases, and patient survival. Elevated levels of lactate may be observed during the process of aggressive tumor development accompanied by angiogenesis (the evolution of the microenvironment). The noninvasive MR detection of lactate in tumor tissues as a potential biomarker is difficult due to the presence of co-resonating lipids that are present at high concentrations. Methods were previously reported for lactate editing using the SELective Multiple Quantum Coherence (SelMQC) method. Here we report a sequence "SS-SelMQC," Spectral-Selective SelMQC, which is a modified version of SelMQC using binomial pulses. Binomial pulses were employed in this editing sequence for frequency excitation or inversion of selective lactate resonances. Lactate detection has been demonstrated using SS-SelMQC, both in vitro (30 mM lactate/H(2)O doped with 25 microM Gd-DTPA) and in vivo (Dunning R3337-AT prostate tumors), and compared to similar measurements made with SelMQC. Lactate areas were measured from nonlocalized spectra, one-dimensional (1D) localized spectra, and two-dimensional chemical shift images (CSI) of the localized slice. In data from whole phantoms, the modified pulse sequence yielded enhancement of the lactate signal of 2.4 +/- 0.40 times compared to SelMQC. Similar in vivo lactate signal enhancement of 2.3 +/- 0.24 times was observed in 1D slice-localized experiment.

Interleaved water and fat imaging and applications to lipid quantitation using the gradient reversal technique

PURPOSE

To implement and evaluate the gradient reversal-based chemical shift imaging technique to obtain qualitative and quantitative spatially-registered fat and water images with high imaging efficiency at very high field.

MATERIALS AND METHODS

A multiecho gradient reversal-based sequence allowing interleaved water-fat imaging during a single acquisition and quantitation of fat/water content is presented. The sequence was optimized and implemented at 11.7T. The quantitation was verified with water-fat phantoms and applied to lipid measurement in an in vivo mouse model.

RESULTS

Results from phantoms, in vivo lipid measurement in mouse liver and hind limb muscle, and ex vivo rat knee imaging experiments demonstrated the robustness and high selectivity of this technique for interleaved and quantitative water and fat imaging at very high field.

CONCLUSION

The proposed MRI technique permits interleaved water and fat imaging, with which spectrally well-separated water and fat images at the identical slice locations could be obtained in a single acquisition without increasing scan time. The technique could be used for in vivo quantitative mapping of lipid content and applied to investigations using small animal experiment models.

Fat-suppressed three-dimensional dual echo dixon technique for contrast agent enhanced MRI

PURPOSE

To develop a fast T1-weighted, fat-suppressed three-dimensional dual echo Dixon technique and to demonstrate its use in contrast agent enhanced MRI.

MATERIALS AND METHODS

A product fast three-dimensional gradient echo pulse sequence was modified to acquire dual echoes after each RF excitation with water and fat signals in-phase (IP) and opposed-phase (OP), respectively. An on-line reconstruction algorithm was implemented to automatically generate separate water and fat images. The signal to noise ratio (SNR) of the new technique was compared to that of the product technique in phantom. In vivo abdomen and breast images of cancer patients were acquired at 1.5 Tesla using both techniques before and after intravenous administration of gadolinium contrast agent.

RESULTS

In phantom, the new technique yields a close to the theoretically predicted 41% increase in SNR in comparison to the product technique without fat suppression (FS). In vivo images of the new technique show noticeably improved FS and image quality in comparison to the images acquired of the same patients using the product technique with FS.

CONCLUSION

The three-dimensional dual echo Dixon technique provides excellent image quality and can be used for T1-weighted, fat-suppressed imaging with contrast agent injection.

Accuracy of cartilage and subchondral bone spatial thickness distribution from MRI

PURPOSE

To assess three-dimensional measurement accuracy of articular cartilage (AC) and subchondral bone (SB) thickness from MRI.

MATERIALS AND METHODS

A computer program was used to calculate AC and SB thickness from MRI (three-dimensional spoiled gradient echo (SPGR),.31-mm resolution, 1-mm slice thickness) of six adult femoral heads. Specimens were imaged in five anatomical planes ranging between +30 degrees to -30 degrees from neutral and cut into 2-mm thick sections along the five anatomical planes. Faxitron x-ray was used to produce microradiographic (.05-mm resolution) images of the sections.

RESULTS

In-plane measurement accuracy was.165 +/-.108 mm for AC thickness and.387 +/-.174 mm for SB thickness. Taking into account chemical-shift misregistration in SB thickness, accuracy of measurements improved to.213 +/- 128 mm. Out-of-plane (three-dimensional) thickness accuracy of the model, assessed by numerical simulation, was.015 mm. However, three-dimensional thickness errors in specimens were.319 +/-.256 mm for AC and.253 +/-.183 mm for SB thickness.

CONCLUSION

Errors in three-dimensional AC thickness were attributed to volume-averaging effects caused by oblique intersection of the image plane with the joint surface. Errors in three-dimensional SB thickness were attributed to chemical-shift artifact. We conclude that accuracy of AC thickness is within clinically acceptable standards but that more sophisticated pulse sequences are needed to improve the measurement of SB thickness.

Subchondral bone and cartilage thickness from MRI: effects of chemical-shift artifact

Magnetic resonance imaging (MRI) is the modality of choice for visualizing and quantifying articular cartilage thickness. However, difficulties persist in MRI of subchondral bone using spoiled gradient-echo (SPGR) and other gradient-echo sequences, primarily due to the effects of chemical-shift artifact. Fat suppression techniques are often used to reduce these artifacts, but they prevent measurement of bone thickness. In this report, we assess the magnitude of chemical-shift effects (phase-cancellation and misregistration artifacts) on subchondral bone and cartilage thickness measurements in human femoral heads using a variety of pulse sequence parameters. Phase-cancellation effects were quantified by comparing measurements from in-phase images (TE=13.5 ms) to out-of-phase images (TE=15.8 ms). We also tested the assumption of the optimal in-phase TE by comparing thickness measures at small variations on TE (13.0, 13.5 and 14.0 ms). Misregistration effects were quantified by comparing measurements from water+fat images (water-only+fat-only images) to the measurements from in-phase (TE=13.5) images. A correction algorithm was developed and applied to the in-phase measurements and then compared to measurements from water+fat images. We also compared thickness measurements at different image resolutions. Results showed that both phase-cancellation artifact and misregistration artifact were significant for bone thickness measurement, but not for cartilage thickness measurement. Using an in-phase TE and correction algorithm for misregistration artifact, the errors in bone thickness relative to water+fat images were non-significant. This information may be useful for developing pulse sequences for optimal imaging of both cartilage and subchondral bone.

Multicoil Dixon chemical species separation with an iterative least-squares estimation method

This work describes a new approach to multipoint Dixon fat-water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single- and multicoil fat-water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fat-silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed.

Interleaved water and fat dual-echo spin echo imaging with intrinsic chemical-shift elimination

A new technique was developed to simultaneously acquire water and fat dual-echo spin echo images in a single acquisition period. Chemical shifts between water and fat images are intrinsically eliminated, and the images are combined to form water-plus-fat image. In vivo water-only images show fat suppression superior to that of conventional spin echo images. This technique may be clinically useful for musculoskeletal imaging.