THE USE OF SMALL FIELD-OF-VIEW 3 TESLA MAGNETIC RESONANCE IMAGING FOR IDENTIFICATION OF ARTICULAR CARTILAGE DEFECTS IN THE CANINE STIFLE: AN EX VIVO CADAVERIC STUDY.

Noninvasive identification of canine articular cartilage injuries is challenging. The objective of this prospective, cadaveric, diagnostic accuracy study was to determine if small field-of-view, three tesla magnetic resonance imaging (MRI) was an accurate method for identifying experimentally induced cartilage defects in canine stifle joints. Forty-two canine cadaveric stifles (n = 6/group) were treated with sham control, 0.5, 1.0, or 3.0 mm deep defects in the medial or lateral femoral condyle. Proton density-weighted, T1-weighted, fast-low angle shot, and T2 maps were generated in dorsal and sagittal planes. Defect location and size were independently determined by two evaluators and compared to histologic measurements. Accuracy of MRI was determined using concordance correlation coefficients. Defects were identified correctly in 98.8% (Evaluator 1) and 98.2% (Evaluator 2) of joints. Concordance correlation coefficients between MRI and histopathology were greater for defect depth (Evaluator 1: 0.68-0.84; Evaluator 2: 0.76-0.83) compared to width (Evaluator 1: 0.30-0.54; Evaluator 2: 0.48-0.68). However, MRI overestimated defect depth (histopathology: 1.65 ± 0.94 mm; Evaluator 1, range of means: 2.07-2.38 mm; Evaluator 2, range of means: 2-2.2 mm) and width (histopathology: 6.98 ± 1.32 mm; Evaluator 1, range of means: 8.33-8.8 mm; Evaluator 2, range of means: 6.64-7.16 mm). Using the paired t-test, the mean T2 relaxation time of cartilage defects was significantly greater than the mean T2 relaxation time of adjacent normal cartilage for both evaluators (P < 0.0001). Findings indicated that MRI is an accurate method for identifying cartilage defects in the cadaveric canine stifle. Additional studies are needed to determine the in vivo accuracy of this method.

Evaluation of the WARP-turbo spin echo sequence for 3 Tesla magnetic resonance imaging of stifle joints in dogs with stainless steel tibial plateau leveling osteotomy implants.

Susceptibility artifacts caused by ferromagnetic implants compromise magnetic resonance imaging (MRI) of the canine stifle after tibial plateau leveling osteotomy (TPLO) procedures. The WARP-turbo spin echo sequence is being developed to mitigate artifacts and utilizes slice encoding for metal artifact reduction. The aim of the current study was to evaluate the WARP-turbo spin echo sequence for imaging post TPLO canine stifle joints. Proton density weighted images of 19 canine cadaver limbs were made post TPLO using a 3 Tesla MRI scanner. Susceptibility artifact sizes were recorded and compared for WARP vs. conventional turbo spin echo sequences. Three evaluators graded depiction quality for the tibial tuberosity, medial and lateral menisci, tibial osteotomy, and caudal cruciate ligament as sufficient or insufficient to make a diagnosis. Artifacts were subjectively smaller and local structures were better depicted in WARP-turbo spin echo images. Signal void area was also reduced by 75% (sagittal) and 49% (dorsal) in WARP vs. conventional turbo spin echo images. Evaluators were significantly more likely to grade local anatomy depiction as adequate for making a diagnosis in WARP-turbo spin echo images in the sagittal but not dorsal plane. The proportion of image sets with anatomic structure depiction graded adequate to make a diagnosis ranged from 28 to 68% in sagittal WARP-turbo spin echo images compared to 0-19% in turbo spin echo images. Findings indicated that the WARP-turbo spin echo sequence reduces the severity of susceptibility artifacts in canine stifle joints post TPLO. However, variable depiction of local anatomy warrants further refinement of the technique.

Patellofemoral friction syndrome: magnetic resonance imaging correlation of morphologic and T2 cartilage imaging.

OBJECTIVE: This study aimed to investigate whether patellofemoral T2 cartilage changes are associated with lateral patellofemoral friction syndrome (PFS), as indicated by an edema-like signal within the superolateral infrapatellar (Hoffa) fat pad. METHODS: In this institutional review board-approved retrospective study of 510 consecutive patients, 49 patients with 50 knee magnetic resonance imaging examinations demonstrating normal or low-grade patellofemoral cartilage abnormalities (whole-organ magnetic resonance imaging score [WORMS] score, ≤2) were included. Twenty-two examinations with PFS (cases) were compared with an age- and sex-matched cohort of 28 examinations without PFS (controls). A 3-T magnetic resonance imaging was performed with multi-echo, spin-echo T2 mapping. Two readers measured in consensus malalignment parameters, including patellar height index, tibial tuberosity to trochlear groove distance, and sulcus angle. Bulk T2 cartilage values in the lateral and medial patellofemoral compartment, central weight-bearing medial and lateral femoral condyles were measured independently. Interobserver agreement was quantified using concordance correlation coefficients. Demographics, anatomic measurements, whole-organ magnetic resonance imaging scores, and cartilage T2 values were compared between cases and controls using Fisher exact test, Wilcoxon rank sum test, and mixed-effects models. RESULTS: Cases demonstrated higher patellar height index (P = 0.002) and tibial tuberosity to trochlear groove distance (P = 0.02). Interobserver agreement for T2 values was good overall (concordance correlation coefficient range, 0.65-0.93). Cases demonstrated higher medial facet patellar bulk T2 (38.1 [7.5] ms) versus controls (33.6 [7.3] ms) (P = 0.02); otherwise, there were no significant differences in regional T2 values. CONCLUSIONS: T2 mapping in patients with PFS demonstrates increased cartilage T2 in the medial patellar facet, possibly reflecting collagen alteration from early chondromalacia (softening) or increased water content related to altered contact pressures.

Magnetic resonance neurography: technical considerations.

Proper performance of magnetic resonance neurography (MRN) is essential not only to make the examination easier to interpret but also for its accurate evaluation. This article outlines the technical considerations of MRN, various imaging pulse sequences available on current scanners, as well as their relative advantages and disadvantages. In addition, a guide to the optimal use of high-resolution and high-contrast MRN technique is provided, which will aid clinicians in attaining a good-quality examination.

Magnetic resonance imaging in cadaver dogs with metallic vertebral implants at 3 Tesla: evaluation of the WARP-turbo spin echo sequence.

STUDY DESIGN: Laboratory investigation, ex vivo. OBJECTIVE: Postoperative complications are common after spinal implantation procedures, and magnetic resonance imaging (MRI) would be the ideal modality to image these patients. Unfortunately, the implants cause artifacts that can render MRI nondiagnostic. The WARP-turbo spin echo (TSE) sequence has been developed to mitigate artifacts caused by metal. The objective of this investigation was to evaluate the performance of the WARP-TSE sequence in canine cadaver specimens after implantation with metallic vertebral implants. SUMMARY OF BACKGROUND DATA: Magnetic field strength, implant type, and MRI acquisition technique all play a role in the severity of susceptibility artifacts. The WARP-TSE sequence uses increased bandwidth, view angle tilting, and SEMAC (slice-encoding metal artifact correction) to correct for susceptibility artifact. The WARP-TSE technique has outperformed conventional techniques in patients, after total hip arthroplasty. However, published reports of its application in subjects with vertebral column implants are lacking. METHODS: Ex vivo anterior stabilization of the atlantoaxial joint was performed on 6 adult small breed (<8 kg) cadaver dogs using stainless steel screws and polymethylmethacrylate. Axial and sagittal T2-weighted and short tau inversion recovery MRI was performed using conventional pulse sequences and WARP-TSE sequences at 3 T. Images were assessed qualitatively and quantitatively. RESULTS: Images made with the WARP-TSE sequence had smaller susceptibility artifacts and superior spinal cord margin depiction. WARP-TSE sequences reduced the length over which susceptibility artifacts caused spinal cord margin depiction interference by 24.9% to 71.5% with scan times of approximately 12 to 16 minutes. CONCLUSION: The WARP-TSE sequence is a viable option for evaluating the vertebral column after implantation with stainless steel implants. LEVEL OF EVIDENCE: N/A.

Clinical evaluation of single-shot and readout-segmented diffusion-weighted imaging in stroke patients at 3 T.

BACKGROUND: Diffusion-weighted imaging (DWI) magnetic resonance imaging (MRI) is most commonly performed utilizing a single-shot echo-planar imaging technique (ss-EPI). Susceptibility artifact and image blur are severe when this sequence is utilized at 3 T. PURPOSE: To evaluate a readout-segmented approach to DWI MR in comparison with single-shot echo planar imaging for brain MRI. MATERIAL AND METHODS: Eleven healthy volunteers and 14 patients with acute and early subacute infarctions underwent DWI MR examinations at 1.5 and 3T with ss-EPI and readout-segmented echo-planar (rs-EPI) DWI at equal nominal spatial resolutions. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) calculations were made, and two blinded readers ranked the scans in terms of high signal intensity bulk susceptibility artifact, spatial distortions, image blur, overall preference, and motion artifact. RESULTS: SNR and CNR were greatest with rs-EPI (8.1 ± 0.2 SNR vs. 6.0 ± 0.2; P <10(-4) at 3T). Spatial distortions were greater with single-shot (0.23 ± 0.03 at 3T; P <0.001) than with rs-EPI (0.12 ± 0.02 at 3T). Combined with blur and artifact reduction, this resulted in a qualitative preference for the readout-segmented scans overall. CONCLUSION: Substantial image quality improvements are possible with readout-segmented vs. single-shot EPI - the current clinical standard for DWI - regardless of field strength (1.5 or 3 T). This results in improved image quality secondary to greater real spatial resolution and reduced artifacts from susceptibility in MR imaging of the brain.

Thoracic and lumbar spinal cord diffusion tensor imaging in dogs.

PURPOSE: To analyze four clinically applicable diffusion tensor imaging (DTI) protocols (two each in the transverse and sagittal planes) in the normal dog. MATERIALS AND METHODS: Seven healthy Dachshund dogs were scanned with four DTI protocols. Within each plane, identical spatial resolution was used while the number of diffusion-encoding directions and averages varied. Agreement of measured fractional anisotropy (FA) and apparent diffusion coefficient (ADC) was analyzed with Bland-Altman methods, subjective image quality within each plane was compared, and FA and ADC were explored as a function of anatomic location. RESULTS: There was good agreement in FA and ADC values within each plane. FA had the smallest bias and most precision. No difference was detected in subjective image quality within each plane. FA and ADC were slightly higher cranial to the lumbar intumescence compared to within it. CONCLUSION: DTI is a promising tool in the assessment of spinal cord injury (SCI) in the study of dogs with intervertebral disk herniation as a preclinical model of human SCI.

Contrast-enhanced 3-dimensional SPACE versus MP-RAGE for the detection of brain metastases: considerations with a 32-channel head coil.

PURPOSE: The aim of this study was to compare the detection of brain metastases at 3 T using a 32-channel head coil with 2 different 3-dimensional (3D) contrast-enhanced sequences, a T1-weighted fast spin-echo-based (SPACE; sampling perfection with application-optimized contrasts using different flip angle evolutions) sequence and a conventional magnetization-prepared rapid gradient-echo (MP-RAGE) sequence. MATERIALS AND METHODS: Seventeen patients with 161 brain metastases were examined prospectively using both SPACE and MP-RAGE sequences on a 3-T magnetic resonance system. Eight healthy volunteers were similarly examined for determination of signal-to-noise ratio (SNR) values. Parameters were adjusted to equalize acquisition times between the sequences (3 minutes and 30 seconds). The order in which sequences were performed was randomized. Two blinded board-certified neuroradiologists evaluated the number of detectable metastatic lesions with each sequence relative to a criterion standard reading conducted at the Gamma Knife facility by a neuroradiologist with access to all clinical and imaging data. RESULTS: In the volunteer assessment with SPACE and MP-RAGE, SNR (10.3 ± 0.8 vs 7.7 ± 0.7) and contrast-to-noise ratio (0.8 ± 0.2 vs 0.5 ± 0.1) were statistically significantly greater with the SPACE sequence (P < 0.05). Overall, lesion detection was markedly improved with the SPACE sequence (99.1% of lesions for reader 1 and 96.3% of lesions for reader 2) compared with the MP-RAGE sequence (73.6% of lesions for reader 1 and 68.5% of lesions for reader 2; P < 0.01). CONCLUSIONS: A 3D T1-weighted fast spin echo sequence (SPACE) improves detection of metastatic lesions relative to 3D T1-weighted gradient-echo-based scan (MP-RAGE) imaging when implemented with a 32-channel head coil at identical scan acquisition times (3 minutes and 30 seconds).

SEMAC-VAT and MSVAT-SPACE sequence strategies for metal artifact reduction in 1.5T magnetic resonance imaging.

OBJECTIVES: To evaluate the ability of four magnetic resonance imaging (MRI) techniques to correct for metallic artifacts. These techniques consisted of 3 2D techniques and one 3D technique. In 2D imaging the techniques View Angle Tilting (VAT), Slice Encoding for Metal Artifact Correction (SEMAC) and a technique that employed a combination of the first two (SEMAC-VAT) were evaluated. In 3D imaging the technique Multiple Slab acquisition with VAT based on a SPACE sequence was evaluated (MSVAT-SPACE). MATERIALS AND METHODS: Agarose phantoms and tissue phantoms with two commonly used metal implants (stainless steel and titanium) as well as two volunteers with metal implants were imaged at 1.5T. All phantoms and volunteers were imaged using VAT, SEMAC, SEMAC-VAT and MSVAT-SPACE techniques, as well as 2D and 3D conventional imaging techniques. Each technique was optimized for different image contrast mechanisms. Artifact reduction was quantitatively assessed in the agarose phantoms by volumetric measurement. Image quality was qualitatively assessed by blinded reads employing two readers. Each reader independently viewed the tissue phantom images and in vivo human images. Statistical analysis was performed using a Friedman test, Wilcoxon test and weighted Cohen's kappa test. RESULTS: T1-weighted, T2-weighted, PD-weighted and STIR image contrasts were successfully implemented with the evaluated artifact reduction sequences in both the phantom experiments and in vivo images. For all evaluated image contrasts and both metal implants, a reduction in the volume of metal artifacts was seen when compared with 2D conventional acquisitions. The 2D metal artifact volumes on average were reduced by 49% ± 16%, 56% ± 15% and 63% ± 15% for VAT, SEMAC and SEMAC-VAT acquisitions respectively. When Friedman and Wilcoxon tests were applied the difference in metal artifact volume was found to be statistically significant when VAT, SEMAC and SEMAC-VAT were compared with the 2D conventional techniques. In 3D imaging on average MSVAT-SPACE reduced metal artifact volume compared with the 3D conventional imaging technique by 72% ± 23% for all evaluated image contrasts and both metal implants. The metal artifact volume differences were statistically significant when MSVAT-SPACE was compared with the 3D conventional technique. The blinded reads demonstrated that SEMAC-VAT and MSVAT-SPACE had distinctly superior quality compared with conventional acquisitions. Quality was measured in terms of artifact size, distortions, image quality and visualization of bone marrow and soft tissues adjacent to metal implants. This was the case for both tissue phantom images and human images with good interobserver agreement. CONCLUSIONS: SEMAC-VAT (2D) and MSVAT-SPACE (3D) demonstrated a consistent, marked reduction of metal artifacts for different metal implants and offered flexible image contrasts (T1, T2, PD and STIR) with high image quality. These techniques likely will improve the evaluation of postoperative patients with metal implants.

The application of three-dimensional diffusion-weighted PSIF technique in peripheral nerve imaging of the distal extremities.

PURPOSE: To evaluate whether the addition of the three-dimensional diffusion-weighted reversed fast imaging with steady state free precession (3D DW-PSIF) sequence improves the identification of peripheral nerves in the distal extremities. MATERIALS AND METHODS: Twelve MR neurography (MRN) studies of the distal upper extremity and 12 MRN studies of distal lower extremity were evaluated. From the 24 subjects who were enrolled, 10 had clinically suspected peripheral neuropathy, whereas 14 suffered from various orthopedic diseases and had no clinical signs of neuropathy. In each examination, the ability to identify each peripheral nerve on T2-weighted and 3D DW-PSIF sequences was evaluated using a semi-quantitative (0-2) scale. Thereafter, a total certainty score was registered for each sequence. RESULTS: Combining the results of all studies, the mean certainty score was 1.92 ± 0.28 on the 3D DW-PSIF images and 1.50 ± 0.72 on the T2-weighted images (P < 0.001). In the upper extremity studies, the corresponding certainty scores were 2.0 and 1.70 ± 0.55, respectively (P = 0.008), and in the lower extremity studies, 1.86 ± 0.35 and 1.36 ± 0.79, respectively (P < 0.001). CONCLUSION: The 3D DW-PSIF images provide improved identification of the nerves compared with the T2-weighted images, and should be incorporated in the MRN protocol, whenever accurate nerve localization and/or presurgical evaluation are required.

Rapid 3D-T(1) mapping of cartilage with variable flip angle and parallel imaging at 3.0T.

PURPOSE: To rapidly acquire T(1)-weighted images using a three-dimensional fast low angle shot (3D FLASH) sequence in combination with generalized autocalibrating partially parallel acquisitions (GRAPPA) and variable flip angle (VFA) method at 3.0T. MATERIALS AND METHODS: 3D T(1) maps of model systems (gadolinium [Gd] and agarose phantoms), bovine cartilage, and human subjects were constructed on a 3.0T clinical whole-body MR scanner. The T(1) values of model systems measured using the 2D inversion-recovery fast-spin-echo (IR-FSE) sequence were considered as a reference method to validate the rapid 3D method for comparison. RESULTS: The root mean square coefficient of variation percentage (RMS-CV%) of the median T(1) of agarose phantom across different acquisition methods was approximately 6.2%. The RMS-CV% of the median T(1) of bovine cartilage across different acquisition methods was approximately 4.1%. The RMS-CV% of median T(1) of the cartilages among the subjects was between approximately 7.3% to 11.1%. In our study, rapid 3D-T(1) mapping with VFA and parallel imaging with different acceleration factors (AFs) (AF = 1, 2, 3, and 4) seems to have no obvious influence on the T(1) mapping (before and after contrast agent administration). CONCLUSION: The preliminary results demonstrate that it is possible to quantify 3D-T(1) mapping of the whole knee joint (with 0.7 mm(3) isotropic resolution) under approximately five minutes with excellent in vivo reproducibility at 3.0T.

Comparing real-world advantages for the clinical neuroradiologist between a high field (3 T), a phased array (1.5 T) vs. a single-channel 1.5-T MR system.

PURPOSE: To evaluate signal-to-noise ratio (SNR) and neuroradiologists' subjective assessments of image quality in 3-Tesla (3-T) or phased-array MR systems that are now available for clinical neuroimaging. MATERIALS AND METHODS: Brain MR images of six normal volunteers were obtained on each of three scanners: a 1.5-T single-channel system, a 12-channel, phased-array system, and a 3-T single-channel system. Additionally, clinically optimized images acquired from 28 patients who underwent imaging in more than one of these systems were analyzed. SNRs were measured and image quality and artifact conspicuity were graded by two blinded readers. RESULTS: The phased-array system produced higher SNR than either the 1.5-T or the 3-T single-channel systems, and in no instance was it outperformed. Both blinded readers judged the phased-array images to be of higher quality than those produced by the single-channel systems, with significantly less artifact. The 3-T magnet produced images with high SNR, but with increased artifact conspicuity. The phased-array system markedly decreased acquisition times without introduction of artifacts. CONCLUSION: Both quantitatively and qualitatively, the phased-array system provided image quality superior to that of the 1.5-T and 3-T single-channel systems.