Grimm, Roger C.

ABSTRACT Background Clinical T 2 ‐weighted (T2W) MRI of prostate cancer (PCa) usually implements a 2D fast‐spin‐echo (FSE) sequence, but understanding the echo‐time‐dependent contrast behavior is not straightforward due to the complicated FSE signal evolution. Consequently, consensus‐based recommendations on the optimal effective echo time (TE eff ) to maximize contrast are lacking. Purpose To investigate the effect of TE eff on PCa lesion contrast in prostate T2W MRI. Study Type Phantom and in vivo prospective study. Subjects A standard MRI phantom and 53 prostate cancer patients (29 for image evaluation and 24 for quantitative lesion analysis). Field Strength and Sequence Multi‐slice T2W FSE sequence at 3 T acquired at multiple TE eff values. Assessment Three readers (13, 25, and 27 years' experience) evaluated TE eff = 100 ms versus 150 ms on diagnostic quality and reader preference. The effective T 2 relaxation rates ( T 2,eff ) were measured using FSE sequences in the phantom and used to predict the optimal TE eff maximizing T2W contrasts. From multi‐TE eff scans of subjects with suspected PCa, T 2,eff values of PCa lesions and healthy tissue were measured and used to calculate the optimal TE eff maximizing the lesion contrast. In vivo contrast metrics were measured as a function of TE eff . Statistical Tests Wilcoxon test of reader scores, with p < 0.05 defining statistical significance. Results No statistically significant difference was found in diagnostic quality between TE eff = 150 ms and 100 ms images, but all three readers significantly favored TE eff = 150 ms. Phantom experiments demonstrated that TE eff ‐dependent FSE signal progression can be characterized by T 2,eff values typically 40% longer than actual T 2 values, allowing accurate prediction of optimal TE eff within 2%–5%. Applied to PCa imaging, in vivo measurements indicated that effective T 2 relaxation times of the prostate are on average 1.5× longer than published T 2 values. Data Conclusion The benefits of TE eff = 150 ms versus 100 ms in prostate T2W FSE were supported by reader preference, phantom experiments, in vivo measurements, and analytic optimizations. Evidence Level 2. Technical Efficacy 2.

PurposeThe purpose is to develop a model‐based image‐reconstruction method using wavelet sparsity regularization for maintaining restoration of through‐plane resolution but with improved retention of SNR versus linear reconstruction using Tikhonov (TK) regularization in high through‐plane resolution (1 mm) T2‐weighted spin‐echo (T2SE) images of the prostate.MethodsA wavelet sparsity (WS)–regularized image reconstruction was developed that takes as input a set of ≈80 overlapped 3‐mm‐thick slices acquired using a T2SE multislice scan and typically 30 coil elements. After testing in contrast and resolution phantoms and calibration in 6 subjects, the WS reconstruction was evaluated in 16 consecutive prostate T2SE MRI exams. Results reconstructed with nominal 1‐mm thickness were compared with those from the TK reconstruction with the same raw data. Results were evaluated radiologically. The ratio of magnitude of prostate signal to periprostatic muscle signal was used to assess the presence of noise reduction. Technical performance was also compared with a commercial 3D‐T2SE sequence.ResultsThe new WS reconstruction was assessed as superior statistically to TK for overall SNR, contrast, and multiple evaluation criteria related to sharpness while retaining the high (1 mm) through‐plane resolution. Wavelet sparsity tended to provide improved overall diagnostic quality versus TK, but not significantly so. In all 16 studies, the prostate‐to‐muscle signal ratio increased.ConclusionsModel‐based WS‐regularized reconstruction consistently provides improved SNR in high (1 mm) through‐plane resolution images of prostate T2SE MRI versus linear reconstruction using TK regularization.

Objectives Quantitative magnetization transfer (qMT) is useful for measurement of murine renal fibrosis at high and ultrahigh field strengths. However, its utility at clinical field strengths and in human-like kidneys remains unknown. We tested the hypothesis that qMT would successfully detect fibrosis in swine kidneys with unilateral renal artery stenosis (RAS) at 3.0 T. Methods The qMT protocol is composed of MT scans with variable flip angles and offset frequencies, and of B0 , B1 , and T1 mapping. Pigs were scanned 10 weeks after RAS or control. A 2-pool model was used to fit the bound pool fraction f of the renal cortex (CO) and outer medulla (OM). Then qMT-derived f in 5 normal and 10 RAS pigs was compared with histological fibrosis determined using Masson's trichrome staining and to renal perfusion assessed with computed tomography. Results The qMT 2-pool model provided accurate fittings of data collected on swine kidneys. Stenotic kidneys showed significantly elevated f in both the CO (9.8% ± 2.7% vs 6.4% ± 0.9%, P = 0.002) and OM (7.6% ± 2.2% vs 4.7% ± 1.1%, P = 0.002), as compared with normal kidneys. Histology-measured renal fibrosis and qMT-derived f correlated directly in both the cortex (Pearson correlation coefficient r = 0.93, P < 0.001) and OM (r = 0.84, P = 0.002), and inversely with stenotic kidney perfusion (r = 0.85, P = 0.002). Conclusions This study demonstrates the feasibility of qMT for measuring fibrosis in human-like swine kidneys, and the association between tissue macromolecule content and renal perfusion. Therefore, qMT may be useful as a tool for noninvasive assessment of renal fibrosis in subjects with RAS at clinical field strengths.

PurposeThe purpose is to develop a retrospective correction for subtle slice‐to‐slice positional inconsistencies that can occur when overlapped slices are acquired for super resolution in T2‐weighted spin‐echo multislice imaging.MethodsSpin‐echo acquisition of overlapped slices is typically done using multiple passes. After the passes are assembled into the final slice set, consecutive slices are correlated due to their overlap. Cross correlation was used to measure slice‐to‐slice displacement. After Z‐dependent filtering to preserve true object shape, the displacements were used to correct slice position. The method was tested in a phantom moved slowly (0.16‐0.63 mm/pass) under computer control and in vivo in 16 patients having prostate MRI.ResultsOver the motion range, the correlation method had an accuracy within 0.03 mm/pass and precision ± 0.20 mm (ie, subpixel). Corrected images visually resemble the true object. Over the patient studies, the mean range of motion in the anterior–posterior direction was 1.63 mm. Motion‐corrected axial images and the sagittal reformats were evaluated as significantly superior over those formed without motion correction.ConclusionThe retrospective correlation‐based motion‐correction method provides significant improvement in the slice‐to‐slice registration necessary for effective super resolution using overlapped slices.

PurposeTo reduce slice‐to‐slice motion effects in multislice ‐weighted fast‐spin‐echo ( FSE) imaging, manifest as “scalloping” in reformats, by modification of the acquisition strategy and to show applicability in prostate MRI.Methods FSE images of contiguous or overlapping slices are typically acquired using multiple passes in which each pass is comprised of multiple slices with slice‐to‐slice gaps. Combination of slices from all passes provides the desired sampling. For enhancement of through‐plane resolution with super resolution or for reformatting into other orientations, subtle ≈1 mm motion between passes can cause objectionable “scalloping” artifact. Here we address this by subdivision of each pass into multiple segments. Interleaving of segments from the multiple passes causes all slices to be acquired over substantially the same time, reducing pass‐to‐pass motion effects. This was implemented in acquiring 78 overlapped FSE axial slices and studied in phantoms and in 14 prostate MRI patients. Super‐resolution axial images and sagittal reformats from the original and new segmented acquisitions were evaluated by 3 uroradiologists.ResultsFor all criteria of sagittal reformats, the segmented acquisition was statistically superior to the original. For all sharpness criteria of axial images, although the trend preferred the original acquisition, the difference was not significant. For artifact in axial images, the segmented acquisition was significantly superior.ConclusionsFor prostate MRI the new segmented acquisition significantly reduces the scalloping motion artifact that can be present in reformats due to long time lags between the acquisition of adjacent or overlapped slices while retaining image sharpness in the acquired axial slices.

Abstract Soft tissue sarcomas are a rare and heterogeneous group of malignancies that present significant diagnostic and therapeutic challenges. Patient stratification based on tumor aggressiveness and early therapeutic response based on quantitative imaging may improve prediction of treatment response and the evaluation of new treatment strategies in clinical trials. The purpose of this pilot study was to determine the technical feasibility of magnetic resonance elastography (MRE) and dynamic contrast-enhanced (DCE) MRI for the evaluation of sarcoma stiffness and perfusion in 9 patients with histologically confirmed sarcoma. Additionally, we assessed the feasibility of utilizing MRE and DCE-MRI for the early evaluation of response to radiation therapy in 4 patients to determine the utility of further evaluation in a larger cohort study. Tumor size, stiffness, and perfusion parameters all decreased from baseline at the time of the pre-surgery or follow-up MRI, and results were compared to pathology or conventional imaging. MRE and DCE-MRI may be useful for the quantitative evaluation of tumor stiffness and perfusion, and therapy response assessment in soft tissue sarcomas.

PurposeThe goal of this work is to demonstrate 1 mm through‐plane resolution in multislice T2SE MRI using k‐space processing of overlapping slices and show applicability in prostate MRI.MethodsMultiple overlapped slices are acquired and Fourier transformed in the slice‐select direction. The slice profile is incorporated into a Tikhonov‐regularized reconstruction. Through‐plane resolution is tested in a resolution phantom. An anthropomorphic prostate phantom is used to study the SNR, and results are compared with theoretical prediction. The proposed method is tested in 16 patients indicated for clinical prostate MRI who gave written informed consent as overseen by our IRB. The “proposed” vs. “reference” multislice images are compared using multiple evaluation criteria for through‐plane resolution.ResultsThe modulation transfer function (MTF) plots of the resolution phantom show good modulation at frequency 0.5 lp/mm, demonstrating 1 mm through‐plane resolution restoration. The SNR measurements experimentally match the theoretically predicted values. The radiological evaluation shows that the proposed method is superior to the reference method for five criteria of sharpness but inferior with respect to artifacts.ConclusionsIn conjunction with overlapped slices a k‐space‐based reconstruction approach can be used to improve through‐plane resolution in multislice T2SE MRI. 1 mm resolution is demonstrated from 3.2 mm thick slices. The in vivo results from prostate MRI show improved sharpness when compared to the standard multislice method.

PurposeTo assess the success rate, image quality, and the ability to stage liver fibrosis of a standard 2D gradient‐recalled echo (GRE) and four different spin‐echo (SE) magnetic resonance elastography (MRE) sequences in patients with different liver iron concentrations.Materials and MethodsA total of 332 patients who underwent 3T MRE examinations that included liver fat and iron quantification were enrolled, including 136 patients with all five MRE techniques. Thirty‐four patients had biopsy results for fibrosis staging. The liver stiffness, region of interest area, image quality, and success rate of the five sequences were compared in 115/136 patients. The area under the receiver operating characteristic curves (AUCs) and the accuracies for diagnosing early‐stage fibrosis and advanced fibrosis were compared. The effect of BMI (body mass index), the R2* relaxation time, and fat fraction on the image quality and liver stiffness measurements were analyzed.ResultsThe success rates were significantly higher in the four SE sequences (99.1–100%) compared with GRE MRE (85.3%) (all P < 0.001). There were significant differences of the mean ROI area between every pair of sequences (all P < 0.0001). There were no significant differences in the AUC of the five MRE sequences for discriminating advanced fibrosis (10 P‐values ranging from 0.2410–0.9171). R2* had a significant effect on the success rate and image quality for the noniron 2D echo‐planar imaging (EPI), 3D EPI and 2D GRE (all P < 0.001) sequences. BMI had a significant effect on the iron 2D EPI (P = 0.0230) and iron 2D SE (P = 0.0040) sequences.ConclusionAll five techniques showed good diagnostic performance in staging liver fibrosis. The SE MRE sequences had higher success rates and better image quality than GRE MRE in 3T clinical hepatic imaging.Level of Evidence: 3Technical Efficacy: Stage 5J. Magn. Reson. Imaging 2018;47:976–987.

In this paper, the feasibility of extending previously described magnetic resonance elastography (MRE) dynamic displacement (and associated elasticity) measurement techniques, currently used successfully in tissue, to solid materials which have much higher shear rigidity and much lower nuclear spin densities, is considered. Based on these considerations, the MRE technique is modified in a straightforward manner and used to directly visualize shear wave displacements within two polymeric materials, one of which is relatively stiff.