Investigative Radiology

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Real-Time Magnetic Resonance Imaging: Radial Gradient-Echo Sequences With Nonlinear Inverse Reconstruction

imageObjective
The aim of this study is to evaluate a real-time magnetic resonance imaging (MRI) method that not only promises high spatiotemporal resolution but also practical robustness in a wide range of scientific and clinical applications.
Materials and Methods
The proposed method relies on highly undersampled gradient-echo sequences with radial encoding schemes. The serial image reconstruction process solves the true mathematical task that emerges as a nonlinear inverse problem with the complex image and all coil sensitivity maps as unknowns. Extensions to model-based reconstructions for quantitative parametric mapping further increase the number of unknowns, for example, by adding parameters for phase-contrast flow or T1 relaxation. In all cases, an iterative numerical solution that minimizes a respective cost function is achieved with use of the iteratively regularized Gauss-Newton method. Convergence is supported by regularization, for example, to the preceding frame, whereas temporal fidelity is ensured by downsizing the regularization strength in comparison to the data consistency term in each iterative step. Practical implementations of highly parallelized algorithms are realized on a computer with multiple graphical processing units. It is “invisibly” integrated into a commercial 3-T MRI system to allow for conventional usage and to provide online reconstruction, display, and storage of regular DICOM image series.
Results
Depending on the application, the proposed method offers serial imaging, that is, the recording of MRI movies, with variable spatial resolution and up to 100 frames per second (fps)—corresponding to 10 milliseconds image acquisition times. For example, movements of the temporomandibular joint during opening and closing of the mouth are visualized with use of simultaneous dual-slice movies of both joints at 2 × 10 fps (50 milliseconds per frame). Cardiac function may be studied at 30 to 50 fps (33.3 to 20 milliseconds), whereas articulation processes typically require 50 fps (20 milliseconds) or orthogonal dual-slice acquisitions at 2 × 25 fps (20 milliseconds). Methodological extensions to model-based reconstructions achieve improved quantitative mapping of flow velocities and T1 relaxation times in a variety of clinical scenarios.
Conclusions
Real-time gradient-echo MRI with extreme radial undersampling and nonlinear inverse reconstruction allows for direct monitoring of arbitrary physiological processes and body functions. In many cases, pertinent applications offer hitherto impossible clinical studies (eg, of high-resolution swallowing dynamics) or bear the potential to replace existing MRI procedures (eg, electrocardiogram-gated cardiac examinations). As a consequence, many novel opportunities will require a change of paradigm in MRI-based radiology. At this stage, extended clinical trials are needed.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Real_Time_Magnetic_Resonance_Imaging__Radial.4.aspx

Assessment of Hepatic Perfusion Using GRASP MRI: Bringing Liver MRI on a New Level

imagePurpose
The aim of this study was to demonstrate the feasibility of hepatic perfusion imaging using dynamic contrast-enhanced (DCE) golden-angle radial sparse parallel (GRASP) magnetic resonance imaging (MRI) for characterizing liver parenchyma and hepatocellular carcinoma (HCC) before and after transarterial chemoembolization (TACE) as a potential alternative to volume perfusion computed tomography (VPCT).
Methods and Materials
Between November 2017 and September 2018, 10 patients (male = 8; mean age, 66.5 ± 8.6 years) with HCC were included in this prospective, institutional review board–approved study. All patients underwent DCE GRASP MRI with high spatiotemporal resolution after injection of liver-specific MR contrast agent before and after TACE. In addition, VPCT was acquired before TACE serving as standard of reference. From the dynamic imaging data of DCE MRI and VPCT, perfusion maps (arterial liver perfusion [mL/100 mL/min], portal liver perfusion [mL/100 mL/min], hepatic perfusion index [%]) were calculated using a dual-input maximum slope model and compared with assess perfusion measures, lesion characteristics, and treatment response using Wilcoxon signed-rank test. To evaluate interreader agreement for measurement repeatability, the interclass correlation coefficient (ICC) was calculated.
Results
Perfusion maps could be successfully generated from all DCE MRI and VPCT data. The ICC was excellent for all perfusion maps (ICC ≥ 0.88; P ≤ 0.001). Image analyses revealed perfusion parameters for DCE MRI and VPCT within the same absolute range for tumor and liver tissue. Dynamic contrast-enhanced MRI further enabled quantitative assessment of treatment response showing a significant decrease (P ≤ 0.01) of arterial liver perfusion and hepatic perfusion index in the target lesion after TACE.
Conclusions
Dynamic contrast-enhanced GRASP MRI allows for a reliable and robust assessment of hepatic perfusion parameters providing quantitative results comparable to VPCT and enables characterization of HCC before and after TACE, thus posing the potential to serve as an alternative to VPCT.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Assessment_of_Hepatic_Perfusion_Using_GRASP_MRI_.1.aspx

Image Artifact Management for Clinical Magnetic Resonance Imaging on a 7 T Scanner Using Single-Channel Radiofrequency Transmit Mode

imageObjectives
The aim of this work was to devise mitigation strategies for addressing a range of image artifacts on a clinical 7 T magnetic resonance imaging scanner using the regulatory-approved single-channel radiofrequency transmit mode and vendor-supplied radiofrequency coils to facilitate clinical scanning within reasonable scan times.
Materials and Methods
Optimized imaging sequence protocols were developed for routine musculoskeletal knee and neurological imaging. Sources of severe image nonuniformities were identified, and mitigation strategies were devised. A range of custom-made high permittivity dielectric pads were used to compensate for B1+ and B1− inhomogeneities, and also for magnetic susceptibility-induced signal dropouts particularly in the basal regions of the temporal lobes and in the cerebellum.
Results
Significant improvements in image uniformity were obtained using dielectric pads in the knee and brain. A combination of small voxels, reduced field of view B0 shimming, and high in-plane parallel imaging factors helped to minimize signal loss in areas of high susceptibility-induced field distortions. The high inherent signal-to-noise ratio at 7 T allowed for high receiver bandwidths and thin slices to minimize chemical shift artifacts. Intermittent artifacts due to radiofrequency inversion pulse limitations (power, bandwidth) were minimized with dielectric pads. A patient with 2 implanted metallic cranial fixation devices located within the radiofrequency transmit field was successfully imaged, with minimal image geometric distortions.
Conclusions
Challenges relating to severe image artifacts at 7 T using single-channel radiofrequency transmit functionality in the knee and brain were overcome using the approaches described in this article. The resultant high diagnostic image quality paves the way for incorporation of this technology into the routine clinical workflow. Further developmental efforts are required to expand the range of applications to other anatomical areas, and to expand the evidence- and knowledge-base relating to the safety of scanning patients with implanted metallic devices.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Image_Artifact_Management_for_Clinical_Magnetic.7.aspx

Improved Liver Diffusion-Weighted Imaging at 3 T Using Respiratory Triggering in Combination With Simultaneous Multislice Acceleration

imageObjectives
The aim of this study was to retrospectively compare optimized respiratory-triggered diffusion-weighted imaging with simultaneous multislice acceleration (SMS-RT-DWI) of the liver with a standard free-breathing echo-planar DWI (s-DWI) protocol at 3 T with respect to the imaging artifacts inherent to DWI.
Materials and Methods
Fifty-two patients who underwent a magnetic resonance imaging study of the liver were included in this retrospective study. Examinations were performed on a 3 T whole-body magnetic resonance system (MAGNETOM Skyra; Siemens Healthcare, Erlangen, Germany). In all patients, both s-DWI and SMS-RT-DWI of the liver were obtained. Images were qualitatively evaluated by 2 independent radiologists with regard to overall image quality, liver edge sharpness, sequence-related artifacts, and overall scan preference. For quantitative evaluation, signal-to-noise ratio was measured from signal-to-noise ratio maps. The mean apparent diffusion coefficient (ADC) was measured in each liver quadrant. The Wilcoxon rank-sum test was used for analysis of the qualitative parameters and the paired Student t test for quantitative parameters.
Results
Overall image quality, liver edge sharpness, and sequence-related artifacts of SMS-RT-DWI received significantly better ratings compared with s-DWI (P < 0.05 for all). For 90.4% of the examinations, both readers overall preferred SMS-RT-DWI to s-DWI. Acquisition time for SMS-RT-DWI was 34% faster than s-DWI. Signal-to-noise ratio values were significantly higher for s-DWI at b50 but did not statistically differ at b800, and they were more homogenous for SMS-RT-DWI, with a significantly lower standard deviation at b50. Mean ADC values decreased from the left to right hepatic lobe as well as from cranial to caudal for s-DWI. With SMS-RT-DWI, mean ADC values were homogeneous throughout the liver.
Conclusions
Optimized, multislice, respiratory-triggered DWI of the liver at 3 T substantially improves image quality with a reduced scan acquisition time compared with s-DWI.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Improved_Liver_Diffusion_Weighted_Imaging_at_3_T.2.aspx

Free-Breathing Fast Low-Angle Shot Quiescent-Interval Slice-Selective Magnetic Resonance Angiography for Improved Detection of Vascular Stenoses in the Pelvis and Abdomen: Technical Development

imageObjectives
Balanced steady-state free precession-based quiescent-interval slice-selective (bSSFP QISS) magnetic resonance angiography (MRA) is accurate for the noncontrast evaluation of peripheral arterial disease (PAD); however, drawbacks include the need for breath-holding when imaging the abdomen and pelvis, and sensitivity to off-resonance artifacts. The purpose of this study was to evaluate the image quality and diagnostic accuracy in the pelvis and abdomen of free-breathing fast low-angle shot-based QISS (FLASH QISS) techniques in comparison to bSSFP QISS in patients with PAD, using computed tomographic angiography as the reference.
Materials and Methods
Twenty-seven patients (69 ± 10 years, 17 men) with PAD were enrolled in this institutional review board–approved, Health Insurance Portability and Accountability Act–compliant prospective study between April and December 2018. Patients underwent noncontrast MRA using standard bSSFP QISS and prototype free-breathing radial-FLASH and Cartesian-FLASH QISS at 3 T. A subset of patients (n = 22) also underwent computed tomographic angiography as the reference standard. Nine arterial segments per patient were evaluated spanning the abdomen, pelvis, and upper thigh regions. Objective (signal intensity ratio and relative standard deviation) and subjective image quality (4-point scale) and stenosis (>50%) were evaluated by 2 readers and compared using one-way analysis of variance, Wilcoxon, and McNemar tests, respectively.
Results
A total of 179 vascular segments were available for analysis by all QISS techniques. No significant difference was observed among bSSFP, radial-FLASH, and Cartesian-FLASH QISS techniques in signal intensity ratio (P = 0.428) and relative standard deviation (P = 0.220). Radial-FLASH QISS demonstrated the best image quality (P < 0.0001) and the highest interreader agreement (κ = 0.721). The sensitivity values of bSSFP, radial-FLASH, and Cartesian-FLASH QISS for the detection of greater than 50% stenosis were 76.0%, 84.0%, and 80.0%, respectively, whereas specificity values were 97.6%, 94.0%, and 92.8%, respectively. Moreover, FLASH QISS consistently reduced off-resonance artifacts compared with bSSFP QISS.
Conclusions
Free-breathing FLASH QISS MRA techniques provide improved image quality and sensitivity, high specificity, and reduced off-resonance artifacts for vascular stenosis detection in the abdomen and pelvis.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Free_Breathing_Fast_Low_Angle_Shot.3.aspx

Breast Magnetic Resonance Spectroscopy at 3 T in Biopsy-Proven Breast Cancers: Does Choline Peak Correlate With Prognostic Factors?

imageObjectives
The role of functional techniques, such as magnetic resonance spectroscopy (1H-MRS), as noninvasive tools to increase breast MR imaging reliability has been widely investigated during the last 2 decades. Considering the growing interest in tumor biology and its influence on functional parameters, the aim of this study was to investigate the relationship between 1H-MRS parameters and breast cancer biomarkers and to evaluate whether the results of 1H-MRS at 3 T can correlate with established breast cancer prognostic factors in our clinical experience.
Materials and Methods
One hundred two patients with biopsy-proven breast cancer underwent 3 T breast MR imaging. Single-voxel 1H-MRS was performed after the T1-weighted sequence, using a PRESS water-suppressed sequence (BREASE). Data were collected from a single rectangular volume of interest that encompassed the lesion. Magnetic resonance images and spectra of 102 Breast Imaging Reporting and Data System 6 lesions were prospectively evaluated by 2 radiologists in consensus. 1H-MRS results were considered positive if the choline peak signal-to-noise ratio was 2 or higher. 1H-MRS findings were then compared with morphological features and to histological findings, such as lesion size, nuclear grade, Ki-67, hormone receptor status, and Her2 expression.
Results
Elevated levels of total choline were detectable in 68/102 cases (66.67%) and undetectable in 34/102 (33.33%). A statistically significant association between the presence of choline peak and higher tumor grading (P < 0.0001), greater Ki-67 value (P < 0.0001), and larger lesion size (P < 0.0001) was found. No statistically significant associations were observed between choline peak and the luminal subgroups, even if higher levels of choline were more frequent in nonluminal A lesions.
Conclusions
Our study confirms that 3 T breast 1H-MRS can be a valid additional tool to obtain further information about breast cancer biology and to predict tumor aggressiveness, because the detection of elevated levels of total choline in the spectrum is associated with a biologically aggressive breast cancer phenotype (large dimensions, grade 3, high values of Ki-67). Our results need to be validated in standardized larger-scale studies.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Breast_Magnetic_Resonance_Spectroscopy_at_3_T_in.5.aspx

Low-Radiation-Dose Stress Myocardial Perfusion Measurement Using First-Pass Analysis Dynamic Computed Tomography: A Preliminary Investigation in a Swine Model

imageObjectives
The aim of this study was to assess the feasibility of a prospective first-pass analysis (FPA) dynamic computed tomography (CT) perfusion technique for accurate low-radiation-dose global stress perfusion measurement.
Materials and Methods
The prospective FPA technique was evaluated in 10 swine (42 ± 12 kg) by direct comparison to a previously validated retrospective FPA technique. Of the 10 swine, 3 had intermediate stenoses with fractional flow reserve severities of 0.70 to 0.90. In each swine, contrast and saline were injected peripherally followed by dynamic volume scanning with a 320-slice CT scanner. Specifically, for the reference standard retrospective FPA technique, volume scans were acquired continuously at 100 kVp and 200 mA over 15 to 20 seconds, followed by systematic selection of only 2 volume scans for global perfusion measurement. For the prospective FPA technique, only 2 volume scans were acquired at 100 kVp and 50 mA for global perfusion measurement. All prospective global stress perfusion measurements were then compared with the corresponding reference standard retrospective global stress perfusion measurements through regression analysis. The CTDIvol32 and size-specific dose estimate of the prospective FPA technique were also determined.
Results
All prospective global stress perfusion measurements (PPRO) at 50 mA were in good agreement with the reference standard retrospective global stress perfusion measurements (PREF) at 200 mA (PPRO = 1.07 PREF −0.09, r = 0.94; root-mean-square error = 0.30 mL/min per gram). The CTDIvol32 and size-specific dose estimate of the prospective FPA technique were 2.3 and 3.7 mGy, respectively.
Conclusions
Accurate low-radiation-dose global stress perfusion measurement is feasible using a prospective FPA dynamic CT perfusion technique.

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https://journals.lww.com/investigativeradiology/Fulltext/2019/12000/Low_Radiation_Dose_Stress_Myocardial_Perfusion.6.aspx