Quantification of calcifications in endarterectomy samples by means of high-resolution ultra-short echo time imaging

Automatic Classification of Magnetic Resonance Histology of Peripheral Arterial Chronic Total Occlusions Using a Variational Autoencoder: A Feasibility Study

The novel approach of our study consists in adapting and in evaluating a custom-made variational autoencoder (VAE) using two-dimensional (2D) convolutional neural networks (CNNs) on magnetic resonance imaging (MRI) images for differentiate soft vs. hard plaque components in peripheral arterial disease (PAD). Five amputated lower extremities were imaged at a clinical ultra-high field 7 Tesla MRI. Ultrashort echo time (UTE), T1-weighted (T1w) and T2-weighted (T2w) datasets were acquired. Multiplanar reconstruction (MPR) images were obtained from one lesion per limb. Images were aligned to each other and pseudo-color red-green-blue images were created. Four areas in latent space were defined corresponding to the sorted images reconstructed by the VAE. Images were classified from their position in latent space and scored using tissue score (TS) as following: (1) lumen patent, TS:0; (2) partially patent, TS:1; (3) mostly occluded with soft tissue, TS:3; (4) mostly occluded with hard tissue, TS:5. Average and relative percentage of TS was calculated per lesion defined as the sum of the tissue score for each image divided by the total number of images. In total, 2390 MPR reconstructed images were included in the analysis. Relative percentage of average tissue score varied from only patent (lesion #1) to presence of all four classes. Lesions #2, #3 and #5 were classified to contain tissues except mostly occluded with hard tissue while lesion #4 contained all (ranges (I): 0.2-100%, (II): 46.3-75.9%, (III): 18-33.5%, (IV): 20%). Training the VAE was successful as images with soft/hard tissues in PAD lesions were satisfactory separated in latent space. Using VAE may assist in rapid classification of MRI histology images acquired in a clinical setup for facilitating endovascular procedures.

Making the invisible visible-ultrashort echo time magnetic resonance imaging: Technical developments and applications

Magnetic resonance imaging (MRI) uses a large magnetic field and radio waves to generate images of tissues in the body. Conventional MRI techniques have been developed to image and quantify tissues and fluids with long transverse relaxation times (T2s), such as muscle, cartilage, liver, white matter, gray matter, spinal cord, and cerebrospinal fluid. However, the body also contains many tissues and tissue components such as the osteochondral junction, menisci, ligaments, tendons, bone, lung parenchyma, and myelin, which have short or ultrashort T2s. After radio frequency excitation, their transverse magnetizations typically decay to zero or near zero before the receiving mode is enabled for spatial encoding with conventional MR imaging. As a result, these tissues appear dark, and their MR properties are inaccessible. However, when ultrashort echo times (UTEs) are used, signals can be detected from these tissues before they decay to zero. This review summarizes recent technical developments in UTE MRI of tissues with short and ultrashort T2 relaxation times. A series of UTE MRI techniques for high-resolution morphological and quantitative imaging of these short-T2 tissues are discussed. Applications of UTE imaging in the musculoskeletal, nervous, respiratory, gastrointestinal, and cardiovascular systems of the body are included.

Diagnostic accuracy of non-contrast quiescent-interval slice-selective (QISS) MRA combined with MRI-based vascular calcification visualization for the assessment of arterial stenosis in patients with lower extremity peripheral artery disease

OBJECTIVES

The proton density-weighted, in-phase stack-of-stars (PDIP-SOS) MRI technique provides calcification visualization in peripheral artery disease (PAD). This study sought to investigate the diagnostic accuracy of a combined non-contrast quiescent-interval slice-selective (QISS) MRA and PDIP-SOS MRI protocol for the detection of PAD, in comparison with CTA and digital subtraction angiography (DSA).

METHODS

Twenty-six prospectively enrolled PAD patients (70 ± 8 years) underwent lower extremity CTA and 1.5-T or 3-T PDIP-SOS/QISS MRI prior to DSA. Two readers rated image quality and graded stenosis (≥ 50%) on QISS MRA without/with calcification visualization. Sensitivity, specificity, and area under the curve (AUC) were calculated against DSA. Calcification was quantified and compared between MRI and non-contrast CT (NCCT) using paired t test, Pearson's correlation, and Bland-Altman analysis.

RESULTS

Image quality ratings were significantly higher for CTA compared to those for MRA (4.0 [3.0-4.0] and 3.0 [3.0-4.0]; p = 0.0369). The sensitivity and specificity of QISS MRA, QISS MRA with PDIP-SOS, and CTA for ≥ 50% stenosis detection were 85.4%, 92.2%, and 90.2%, and 90.3%, 93.2%, and 94.2%, respectively, while AUCs were 0.879, 0.928, and 0.923, respectively. A significant increase in AUC was observed when PDIP-SOS was added to the MRA protocol (p = 0.0266). Quantification of calcification showed significant differences between PDIP-SOS and NCCT (80.6 ± 31.2 mm³ vs. 88.0 ± 29.8 mm³; p = 0.0002) with high correlation (r = 0.77, p < 0.0001) and moderate mean of differences (- 7.4 mm³).

CONCLUSION

QISS MRA combined with PDIP-SOS MRI provides improved, CTA equivalent, accuracy for the detection of PAD, although its image quality remains inferior to CTA.

KEY POINTS

• Agreement in stenosis detection rate using non-contrast quiescent-interval slice-selective MRA compared to DSA improved when calcification visualization was provided to the readers. • An increase was observed in both sensitivity and specificity for the detection of ≥ 50% stenosis when MRI-based calcification assessment was added to the protocol, resulting in a diagnostic accuracy more comparable to CTA. • Quantification of calcification showed statistical difference between MRI and non-contrast CT; however, a high correlation was observed between the techniques.

Ultrashort Time to Echo Magnetic Resonance Evaluation of Calcium Pyrophosphate Crystal Deposition in Human Menisci

OBJECTIVES

In human menisci, we aimed to investigate whether calcium pyrophosphate crystal deposition (CPPD) affects biomechanical and quantitative MR properties, and their zonal distribution.

MATERIALS AND METHODS

From 9 cadaveric knees, sectioned triangular meniscus pieces were harvested. Samples were classified into "normal" or "CPPD" groups based upon visual inspection. Micro computed tomography scan verified CPPD. Using magnetic resonance imaging, ultrashort echo time (UTE) T2* and spin echo (SE) T2, quantitative values in 3 zones (red, red-white, and white) were determined. Using biomechanical test, indentation forces in the same zones were determined. Effects of CPPD and meniscal zone on indentation force and quantitative MR values were compared.

RESULTS

On UTE MRI scans, CPPD-affected menisci exhibited punctate dark regions, found mostly (92%) in avascular white and red-white zones. Indentation forces were significantly higher for CPPD samples in the red-white (all P < 0.02) and white (all P < 0.004) zones but not in the vascular red zone (all P > 0.2). Similarly, UTE T2* red zone values were similar between both groups (~6.6 milliseconds, P = 0.8), whereas in the red-white and white zones, CPPD samples had significantly lower values (~5.1 milliseconds, P = 0.005 to 0.007). In contrast, SE T2 values showed no difference with CPPD (P = 0.12 to 0.16). UTE T2*, but not SE T2, correlated significantly with indentation force (R = -0.29, P = 0.009).

CONCLUSIONS

Dark CPP deposits were detectable on UTE images featuring high signal intensity from surrounding meniscal tissue. Preliminary results indicate that CPP deposits were almost exclusively found in the avascular zones. Compared with normal, CPPD menisci featured higher indentation stiffness and lower UTE T2* values in the affected zones.

Short-T2 MRI: Principles and recent advances

Among current modalities of biomedical and diagnostic imaging, MRI stands out by virtue of its versatile contrast obtained without ionizing radiation. However, in various cases, e.g., water protons in tissues such as bone, tendon, and lung, MRI performance is limited by the rapid decay of resonance signals associated with short transverse relaxation times T₂ or T₂*. Efforts to address this shortcoming have led to a variety of specialized short-T₂ techniques. Recent progress in this field expands the choice of methods and prompts fresh considerations with regard to instrumentation, data acquisition, and signal processing. In this review, the current status of short-T₂ MRI is surveyed. In an attempt to structure the growing range of techniques, the presentation highlights overarching concepts and basic methodological options. The most frequently used approaches are described in detail, including acquisition strategies, image reconstruction, hardware requirements, means of introducing contrast, sources of artifacts, limitations, and applications.

Quantitative Intracranial Atherosclerotic Plaque Characterization at 7T MRI: An Ex Vivo Study with Histologic Validation

BACKGROUND AND PURPOSE

In recent years, several high-resolution vessel wall MR imaging techniques have emerged for the characterization of intracranial atherosclerotic vessel wall lesions in vivo. However, a thorough validation of MR imaging results of intracranial plaques with histopathology is still lacking. The aim of this study was to characterize atherosclerotic plaque components in a quantitative manner by obtaining the MR signal characteristics (T1, T2, T2*, and proton density) at 7T in ex vivo circle of Willis specimens and using histopathology for validation.

MATERIALS AND METHODS

A multiparametric ultra-high-resolution quantitative MR imaging protocol was performed at 7T to identify the MR signal characteristics of different intracranial atherosclerotic plaque components, and using histopathology for validation. In total, 38 advanced plaques were matched between MR imaging and histology, and ROI analysis was performed on the identified tissue components.

RESULTS

Mean T1, T2, and T2* relaxation times and proton density values were significantly different between different tissue components. The quantitative T1 map showed the most differences among individual tissue components of intracranial plaques with significant differences in T1 values between lipid accumulation (T1 = 838 ± 167 ms), fibrous tissue (T1 = 583 ± 161 ms), fibrous cap (T1 = 481 ± 98 ms), calcifications (T1 = 314 ± 39 ms), and the intracranial arterial vessel wall (T1 = 436 ± 122 ms).

CONCLUSIONS

Different tissue components of advanced intracranial plaques have distinguishable imaging characteristics with ultra-high-resolution quantitative MR imaging at 7T. Based on this study, the most promising method for distinguishing intracranial plaque components is T1-weighted imaging.

Classification of coronary atherosclerotic plaques ex vivo with T1, T2, and ultrashort echo time CMR

OBJECTIVES

This study sought to determine whether the classification of human coronary atherosclerotic plaques with T1, T2, and ultrashort echo time (UTE) cardiac magnetic resonance (CMR) would correlate well with atherosclerotic plaque classification by histology.

BACKGROUND

CMR has been extensively used to classify carotid plaque, but its ability to characterize coronary plaque remains unknown. In addition, the detection of plaque calcification by CMR remains challenging. Here, we used T1, T2, and UTE CMR to evaluate atherosclerotic plaques in fixed post-mortem human coronary arteries. We hypothesized that the combination of T1, T2, and UTE CMR would allow both calcified and lipid-rich coronary plaques to be accurately detected.

METHODS

Twenty-eight plaques from human donor hearts with proven coronary artery disease were imaged at 9.4-T with a T1-weighted 3-dimensional fast low-angle shot (FLASH) sequence (250-μm resolution), a T2-weighted rapid acquisition with refocused echoes (RARE) sequence (in-plane resolution 0.156 mm), and an UTE sequence (300-μm resolution). Plaques showing selective hypointensity on T2-weighted CMR were classified as lipid-rich. Areas of hypointensity on the T1-weighted images, but not the UTE images, were classified as calcified. Hyperintensity on the T1-weighted and UTE images was classified as hemorrhage. Following CMR, histological characterization of the plaques was performed with a pentachrome stain and established American Heart Association criteria.

RESULTS

CMR showed high sensitivity and specificity for the detection of calcification (100% and 90%, respectively) and lipid-rich necrotic cores (90% and 75%, respectively). Only 2 lipid-rich foci were missed by CMR, both of which were extremely small. Overall, CMR-based classification of plaque was in complete agreement with the histological classification in 22 of 28 cases (weighted κ = 0.6945, p < 0.0001).

CONCLUSIONS

The utilization of UTE CMR allows plaque calcification in the coronary arteries to be robustly detected. High-resolution CMR with T1, T2, and UTE contrast enables accurate classification of human coronary atherosclerotic plaque.

Ex vivo study of carotid endarterectomy specimens: quantitative relaxation times within atherosclerotic plaque tissues

PURPOSE

Previous studies reporting relaxation times within atherosclerotic plaque have typically used dedicated small-bore high-field systems and small sample sizes. This study reports quantitative T(1), T(2) and T(2) relaxation times within plaque tissue at 1.5 T using spatially co-matched histology to determine tissue constituents.

METHODS

Ten carotid endarterectomy specimens were removed from patients with advanced atherosclerosis. Imaging was performed on a 1.5-T whole-body scanner using a custom built 10-mm diameter receive-only solenoid coil. A protocol was defined to allow subsequent computation of T(1), T(2) and T(2) relaxation times using multi-flip angle spoiled gradient echo, multi-echo fast spin echo and multi-echo gradient echo sequences, respectively. The specimens were subsequently processed for histology and individually sectioned into 2-mm blocks to allow subsequent co-registration. Each imaging sequence was imported into in-house software and displayed alongside the digitized histology sections. Regions of interest were defined to demarcate fibrous cap, connective tissue and lipid/necrotic core at matched slice-locations. Relaxation times were calculated using Levenberg-Marquardt's least squares curve fitting algorithm. A linear-mixed effect model was applied to account for multiple measurements from the same patient and establish if there was a statistically significant difference between the plaque tissue constituents.

RESULTS

T(2) and T(2) relaxation times were statistically different between all plaque tissues (P=.026 and P=.002 respectively) [T(2): lipid/necrotic core was lower 47 ± 13.7 ms than connective tissue (67 ± 22.5 ms) and fibrous cap (60 ± 13.2 ms); T(2): fibrous cap was higher (48 ± 15.5 ms) than connective tissue (19 ± 10.6 ms) and lipid/necrotic core (24 ± 8.2 ms)]. T(1) relaxation times were not significantly different (P=.287) [T(1): Fibrous cap: 933 ± 271.9 ms; connective tissue (1002 ± 272.9 ms) and lipid/necrotic core (1044 ± 304.0 ms)]. We were unable to demarcate hemorrhage and calcium following histology processing.

CONCLUSIONS

This study demonstrates that there is a significant difference between qT(2) and qT(2) in plaque tissues types. Derivation of quantitative relaxation times shows promise for determining plaque tissue constituents.

Review. The Agfa Mayneord lecture: MRI of short and ultrashort T₂ and T₂* components of tissues, fluids and materials using clinical systems

A variety of techniques are now available to directly or indirectly detect signal from tissues, fluids and materials that have short, ultrashort or supershort T₂ or T₂* components. There are also methods of developing image contrast between tissues and fluids in the short T₂ or T₂* range that can provide visualisation of anatomy, which has not been previously seen with MRI. Magnetisation transfer methods can now be applied to previously invisible tissues, providing indirect access to supershort T₂ components. Particular methods have been developed to target susceptibility effects and quantify them after correcting for anatomical distortion. Specific methods have also been developed to image the effects of magnetic iron oxide particles with positive contrast. Major advances have been made in techniques designed to correct for loss of signal and gross image distortion near metal. These methods are likely to substantially increase the range of application for MRI.

Advanced techniques for MRI of atherosclerotic plaque

This review examines the state of the art in vessel wall imaging by magnetic resonance imaging (MRI) with an emphasis on the biomechanical assessment of atherosclerotic plaque. Three areas of advanced techniques are discussed. First, alternative contrast mechanisms, including susceptibility, magnetization transfer, diffusion, and perfusion, are presented as to how they facilitate accurate determination of plaque constituents underlying biomechanics. Second, imaging technologies including hardware and sequences, are reviewed as to how they provide the resolution and signal-to-noise ratio necessary for determining plaque structure. Finally, techniques for combining MRI data into an overall assessment of plaque biomechanical properties, including wall shear stress and internal plaque strain, are presented. The paper closes with a discussion of the extent to which these techniques have been applied to different arteries commonly targeted by vessel wall MRI.