Increasing spatial resolution of 3T MRI scanning improves reproducibility of carotid arterial wall dimension measurements

Molecular imaging of cellular immunotherapies in experimental and therapeutic settings

Cell-based cancer immunotherapies are becoming a routine part of the armamentarium against cancer. While remarkable successes have been seen, including durable remissions, not all patients will benefit from these therapies and many can suffer from life-threatening side effects. These differences in efficacy and safety across patients and across tumor types (e.g., blood vs. solid), are thought to be due to differences in how well the immune cells traffic to their target tissue (e.g., tumor, lymph nodes, etc.) whilst avoiding non-target tissues. Across patient variability can also stem from whether the cells interact with (i.e., communicate with) their intended target cells (e.g., cancer cells), as well as if they proliferate and survive long enough to yield potent and long-lasting therapeutic effects. However, many cell-based therapies are monitored by relatively simple blood tests that lack any spatial information and do not reflect how many immune cells have ended up at particular tissues. The ex vivo labeling and imaging of infused therapeutic immune cells can provide a more precise and dynamic understanding of whole-body immune cell biodistribution, expansion, viability, and activation status in individual patients. In recent years numerous cellular imaging technologies have been developed that may provide this much-needed information on immune cell fate. For this review, we summarize various ex vivo labeling and imaging approaches that allow for tracking of cellular immunotherapies for cancer. Our focus is on clinical imaging modalities and summarize the progression from experimental to therapeutic settings. The imaging information provided by these technologies can potentially be used for many purposes including improved real-time understanding of therapeutic efficacy and potential side effects in individual patients after cell infusion; the ability to more readily compare new therapeutic cell designs to current designs for various parameters such as improved trafficking to target tissues and avoidance of non-target tissues; and the long-term ability to identify patient populations that are likely to be positive responders and at low-risk of side effects.

Quantitative assessment of carotid artery atherosclerosis by three-dimensional magnetic resonance and two-dimensional ultrasound imaging: a comparison study

Background

It has been proven that magnetic resonance (MR) and ultrasound imaging are useful tools in the quantification of carotid atherosclerotic plaques. However, there are only a few pieces of evidence to illustrate the links of quantitative measurements of carotid plaques between MR and ultrasound imaging. This study looked to compare the quantitative measurements of carotid plaques and investigate their relationship between three-dimensional (3D) MR vessel wall imaging and two-dimensional (2D) ultrasound imaging.

Methods

Seventy-five asymptomatic elderly subjects (mean age: 73.3±5.7 years; 45 males) with carotid atherosclerotic plaques diagnosed by both ultrasound and MR imaging were included in this study. The plaque size, including the maximum wall thickness (Max WT), plaque length, and plaque area, was measured by 3D MR and ultrasound imaging on longitudinal and cross-sectional views. The quantitative assessments of carotid plaque size were compared and correlated between 3D MR and 2D ultrasound imaging.

Results

In total, the quantitative measurements of 101 plaques on longitudinal views or 44 plaques on cross-sectional views of both MR and ultrasound imaging were compared. The Max WT of the plaques (longitudinal: 2.9±0.8 vs. 2.4±0.9 mm; cross-sectional: 3.2±1.1 vs. 2.6±0.7 mm) and plaque areas (longitudinal: 24.3±13.4 vs. 17.0±12.7 mm²; cross-sectional: 24.9±24.6 vs. 16.8±13.3 mm²) measured by MR imaging were found to be significantly higher than those measured by ultrasound imaging (all P<0.001). Moderate to strong correlations were found in Max WT, plaque area, plaque length between 3D MR and ultrasound imaging.

Conclusions

The quantitative measurements of carotid plaques using 3D MR and 2D ultrasound are significantly correlated. The plaque area and Max WT measured by 3D MR imaging are more significant than these parameters measured by 2D ultrasound imaging, which might be explained by the resolution of MR imaging and the workflow of measurements.

Next-generation imaging of the skeletal system and its blood supply

Bone is organized in a hierarchical 3D architecture. Traditionally, analysis of the skeletal system was based on bone mass assessment by radiographic methods or on the examination of bone structure by 2D histological sections. Advanced imaging technologies and big data analysis now enable the unprecedented examination of bone and provide new insights into its 3D macrostructure and microstructure. These technologies comprise ex vivo and in vivo methods including high-resolution computed tomography (CT), synchrotron-based imaging, X-ray microscopy, ultra-high-field magnetic resonance imaging (MRI), light-sheet fluorescence microscopy, confocal and intravital two-photon imaging. In concert, these techniques have been used to detect and quantify a novel vascular system of trans-cortical vessels in bone. Furthermore, structures such as the lacunar network, which harbours and connects osteocytes, become accessible for 3D imaging and quantification using these methods. Next-generation imaging of the skeletal system and its blood supply are anticipated to contribute to an entirely new understanding of bone tissue composition and function, from macroscale to nanoscale, in health and disease. These insights could provide the basis for early detection and precision-type intervention of bone disorders in the future.

Comparison of four MR carotid surface coils at 3T

Background

The quality of carotid wall MRI can benefit substantially from a dedicated RF coil that is tailored towards the human neck geometry and optimized for image signal-to-noise ratio (SNR), parallel imaging performance and RF penetration depth and coverage. In last decades, several of such dedicated carotid coils were introduced. However, a comparison of the more successful designs is still lacking.

Objective

To perform a head-to-head comparison over four dedicated MR carotid surface coils with 4, 6, 8 and 30 coil elements, respectively.

Material and methods

Ten volunteers were scanned on a 3T scanner. For each subject, multiple black-blood carotid vessel wall images were measured using the four coils with different parallel imaging settings. The performance of the coils was evaluated and compared in terms of image coverage, penetration depth and noise correlations between elements. Vessel wall of a common carotid section was delineated manually. Subsequently, images were assessed based on vessel wall morphology and image quality parameters. The morphological parameters consisted of the vessel wall area, thickness, and normalized wall index (wall area/total vessel area). Image quality parameters consisted of vessel wall SNR, wall-lumen contrast-to-noise ratio (CNR), the vessel g-factor, and CNR_index ((wall–lumen signal) / (wall+lumen signal)). Repeated measures analysis of variance (rmANOVA) was applied for each parameter for the averaged 10 slices for all volunteers to assess effect of coil and SENSE factor. If the rmANOVA was significant, post-hoc comparisons were conducted.

Results

No significant coil effect were found for vessel wall morphological parameters. SENSE acceleration affected some morphological parameters for 6- and 8-channel coils, but had no effect on the 30-channel coil. The 30-channel coil achieved high acceleration factors (10x) with significantly lower vessel g-factor values (ps ≤ 0.01), but lower vessel wall SNR and CNR values (ps ≤ 0.01).

Conclusion

All four coils were capable of high-quality carotid MRI. The 30-channel coil is recommended when rapid image acquisition acceleration is required for 3D measurements, whereas 6- and 8-channel coils demonstrated the highest SNR performance.

Quantitative 3D dynamic contrast-enhanced (DCE) MR imaging of carotid vessel wall by fast T1 mapping using Multitasking

PURPOSE

To develop a dynamic contrast-enhanced (DCE) MRI method capable of high spatiotemporal resolution, 3D carotid coverage, and T1-based quantification of contrast agent concentration for the assessment of carotid atherosclerosis using a newly developed Multitasking technique.

METHODS

5D imaging with 3 spatial dimensions, 1 T1 recovery dimension, and 1 DCE time dimension was performed using MR Multitasking based on low-rank tensor modeling, which allows direct T1 quantification with high spatiotemporal resolution (0.7 mm isotropic and 595 ms, respectively). Saturation recovery preparations followed by 3D segmented fast low angle shot readouts were implemented with Gaussian-density random 3D Cartesian sampling. A bulk motion removal scheme was developed to improve image quality. The proposed protocol was tested in phantom and human studies. In vivo scans were performed on 14 healthy subjects and 7 patients with carotid atherosclerosis. Kinetic parameters including area under the concentration versus time curve (AUC), v_p , K^trans , and v_e were evaluated for each case.

RESULTS

Phantom experiments showed that T1 measurements using the proposed protocol were in good agreement with reference value ( R 2 = 0.96 ). In vivo studies demonstrated that AUC, v_p , and K^trans in the patient group were significantly higher than in the control group (0.63 ± 0.13 versus 0.42 ± 0.12, P < 0.001; 0.14 ± 0.05 versus 0.11 ± 0.03, P = 0.034; and 0.13 ± 0.04 versus 0.08 ± 0.02, P < 0.001, respectively). Results from repeated subjects showed good interscan reproducibility (intraclass correlation coefficient: v_p , 0.83; K^trans , 0.87; v_e , 0.92; AUC, 0.94).

CONCLUSION

Multitasking DCE is a promising approach for quantitatively assessing the vascularity properties of the carotid vessel wall.

Ex vivo vessel wall thickness measurements of the human circle of Willis using 7T MRI

BACKGROUND AND AIMS

MRI can detect intracranial vessel wall thickening before any luminal stenosis is present. Apart from representing a vessel wall lesion, wall thickening could also reflect normal (age-related) variations in vessel wall thickness present throughout the intracranial arterial vasculature. The aim of this study was to perform vessel wall thickness measurements of the major intracranial arteries in ex vivo circle of Willis (CoW) specimens using 7T MRI, to obtain more detailed information about wall thickness variations of the intracranial arteries.

METHODS

Fifteen human CoW specimens were scanned at 7T MRI with an ultrahigh-resolution T₁-weighted sequence. Five specimens were used for validation of MRI measurements with histology and evaluation of inter-rater reliability and agreement. The other 10 specimens from patients with (n = 5) and without (n = 5) cerebrovascular disease were used for vessel wall thickness measurements over the entire length of the major arterial segments of the CoW using MRI only.

RESULTS

MRI measurements showed excellent agreement with histology. Mean wall thickness varied from 0.45 to 0.66 mm, minimum wall thickness from 0.31 to 0.42 mm, maximum wall thickness from 0.52 to 0.86 mm, and normalized wall index from 0.64 to 0.75. On average, vessel walls were thicker for symptomatic patients compared to asymptomatic patients.

CONCLUSIONS

High-resolution MRI enables accurate measurement of vessel wall thickness in ex vivo CoW specimens. Vessel wall thickness measurements over the entire length of segments showed considerable variation both within and between arterial segments of patients.

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment.

Texture analysis of the developing human brain using customization of a knowledge-based system

Background: Pattern recognition software originally designed for geospatial and other technical applications could be trained by physicians and used as texture-analysis tools for evidence-based practice, in order to improve diagnostic imaging examination during pregnancy.Methods: Various machine-learning techniques and customized datasets were assessed for training of an integrable knowledge-based system (KBS), to determine a hypothetical methodology for texture classification of closely-related anatomical structures in fetal brain magnetic resonance (MR) images. Samples were manually categorized according to the magnetic field of the MRI scanner (i.e. 1.5-tesla (1.5T), 3-tesla (3T)), rotational planes (i.e. coronal, sagittal and axial), and signal weighting (i.e. spin-lattice, spin-spin, relaxation, proton density). In the machine-learning sessions, the operator manually selected relevant regions of interest (ROI) in 1.5/3T MR images. Semi-automatic procedures in MaZda/B11 were performed to determine optimal parameter sets for ROI classification. Four classes were defined: ventricles, thalamus, grey matter, and white matter. Various textures analysis methods were tested. The KBS performed automatic data pre-processing and semi-automatic classification of ROIs.Results: After testing 3456 ROIs, statistical binary classification revealed that combination of reduction techniques with linear discriminant algorithms (LDA) or nonlinear discriminant algorithms (NDA) yielded the best scoring in terms of sensitivity (both 100%, 95% CI: 99.79-100), specificity (both 100%, 95% CI: 99.79-100) and Fisher coefficient (≈E+4, ≈E+5, respectively). Conclusions: LDA and NDA in MaZda can be useful data mining tools for screening a population of interest subjected to a clinical test.

Manual versus Automated Carotid Artery Plaque Component Segmentation in High and Lower Quality 3.0 Tesla MRI Scans

PURPOSE

To study the interscan reproducibility of manual versus automated segmentation of carotid artery plaque components, and the agreement between both methods, in high and lower quality MRI scans.

METHODS

24 patients with 30-70% carotid artery stenosis were planned for 3T carotid MRI, followed by a rescan within 1 month. A multicontrast protocol (T1w,T2w, PDw and TOF sequences) was used. After co-registration and delineation of the lumen and outer wall, segmentation of plaque components (lipid-rich necrotic cores (LRNC) and calcifications) was performed both manually and automated. Scan quality was assessed using a visual quality scale.

RESULTS

Agreement for the detection of LRNC (Cohen's kappa (k) is 0.04) and calcification (k = 0.41) between both manual and automated segmentation methods was poor. In the high-quality scans (visual quality score ≥ 3), the agreement between manual and automated segmentation increased to k = 0.55 and k = 0.58 for, respectively, the detection of LRNC and calcification larger than 1 mm2. Both manual and automated analysis showed good interscan reproducibility for the quantification of LRNC (intraclass correlation coefficient (ICC) of 0.94 and 0.80 respectively) and calcified plaque area (ICC of 0.95 and 0.77, respectively).

CONCLUSION

Agreement between manual and automated segmentation of LRNC and calcifications was poor, despite a good interscan reproducibility of both methods. The agreement between both methods increased to moderate in high quality scans. These findings indicate that image quality is a critical determinant of the performance of both manual and automated segmentation of carotid artery plaque components.

Review: Mechanical Characterization of Carotid Arteries and Atherosclerotic Plaques

Cardiovascular disease (CVD) is a leading cause of death and is in the majority of cases due to the formation of atherosclerotic plaques in arteries. Initially, thickening of the inner layer of the arterial wall occurs. Continuation of this process leads to plaque formation. The risk of a plaque to rupture and thus to induce an ischemic event is directly related to its composition. Consequently, characterization of the plaque composition and its proneness to rupture are of crucial importance for risk assessment and treatment strategies. The carotid is an excellent artery to be imaged with ultrasound because of its superficial position. In this review, ultrasound-based methods for characterizing the mechanical properties of the carotid wall and atherosclerotic plaque are discussed. Using conventional echography, the intima media thickness (IMT) can be quantified. There is a wealth of studies describing the relation between IMT and the risk for myocardial infarction and stroke. Also the carotid distensibility can be quantified with ultrasound, providing a surrogate marker for the cross-sectional mechanical properties. Although all these parameters are associated with CVD, they do not easily translate to individual patient risk. Another technique is pulse wave velocity (PWV) assessment, which measures the propagation of the pressure pulse over the arterial bed. PWV has proven to be a marker for global arterial stiffness. Recently, an ultrasound-based method to estimate the local PWV has been introduced, but the clinical effectiveness still needs to be established. Other techniques focus on characterization of plaques. With ultrasound elastography, the strain in the plaque due to the pulsatile pressure can be quantified. This technique was initially developed using intravascular catheters to image coronaries, but recently noninvasive methods were successfully developed. A high correlation between the measured strain and the risk for rupture was established. Acoustic radiation force impulse (ARFI) imaging also provides characterization of local plaque components based on mechanical properties. However, both elastography and ARFI provide an indirect measure of the elastic modulus of tissue. With shear wave imaging, the elastic modulus can be quantified, although the carotid artery is one of the most challenging tissues for this technique due to its size and geometry. Prospective studies still have to establish the predictive value of these techniques for the individual patient. Validation of ultrasound-based mechanical characterization of arteries and plaques remains challenging. Magnetic resonance imaging is often used as the "gold" standard for plaque characterization, but its limited resolution renders only global characterization of the plaque. CT provides information on the vascular tree, the degree of stenosis, and the presence of calcified plaque, while soft plaque characterization remains limited. Histology still is the gold standard, but is available only if tissue is excised. In conclusion, elastographic ultrasound techniques are well suited to characterize the different stages of vascular disease.

Atherosclerosis is Evident in Treated HIV-Infected Subjects With Low Cardiovascular Risk by Carotid Cardiovascular Magnetic Resonance

OBJECTIVE

Premature atherosclerosis has been observed among HIV-infected individuals with high cardiovascular risk using one-dimensional ultrasound carotid intima-media thickness. We evaluated the assessment of HIV-infected individuals with low traditional cardiovascular disease risk using cardiovascular magnetic resonance, which allows three-dimensional assessment of the carotid artery wall.

METHODS

Carotid cardiovascular magnetic resonance was performed in 33 HIV-infected individuals (cases) (19 male, 14 female), and 35 HIV-negative controls (20 male, 15 female). Exclusion criteria included smoking, hypertension, hyperlipidemia (total cholesterol/HDL ratio > 5) or family history of premature atherosclerosis. Cases were stable on combination antiretroviral therapy with plasma HIV-1 RNA <50 copies per milliliter. Using computer modeling, the arterial wall, lumen, and total vessel volumes were calculated for a 4-cm length of each carotid artery centered on the bifurcation. The wall/outer-wall ratio (W/OW), an index of vascular thickening, was compared between the groups.

RESULTS

Cases had a median CD4 cell count of 690 cells per microliter. Mean (±SD) age and 10-year Framingham coronary risk scores were similar for cases and controls (45.2 ± 9.7 years versus 46.9 ± 11.6 years and 3.97% ± 3.9% versus 3.72% ± 3.5%, respectively). W/OW was significantly increased in cases compared with controls (36.7% versus 32.5%, P < 0.0001); this was more marked in HIV-infected females. HIV status was significantly associated with increased W/OW after adjusting for age (P < 0.0001). No significant association between antiretroviral type and W/OW was found-W/OW lowered comparing abacavir to zidovudine (P = 0.038), but statistical model fits poorly.

CONCLUSIONS

In a cohort of treated HIV-infected individuals with low measurable cardiovascular risk, we have observed evidence of premature subclinical atherosclerosis.

Demonstration of synchrotron x-ray phase contrast imaging computed tomography of infiltrative transitional cell carcinoma of the prostatic urethra in a dog

Prostatic urethral transitional cell carcinoma with prostatic invasion in a dog was imaged with abdominal radiography and abdominal ultrasonography antemortem. Synchrotron in-line x-ray phase contrast imaging computed tomography (XPCI-CT) was performed on the prostate ex vivo at the Canadian Light Source Synchrotron and compared to histology. XPCI-CT imaging provides greater soft tissue contrast than conventional absorption-based x-ray imaging modalities, permitting visualization of regions of inflammatory cell infiltration, differentiation of invasive versus noninvasive tumor regions, and areas of necrosis and mineralization. This represents the first report of XPCI-CT images of an invasive prostatic urothelial neoplasm in a dog.

Three-dimensional dynamic contrast-enhanced MRI for the accurate, extensive quantification of microvascular permeability in atherosclerotic plaques

Atherosclerotic plaques that cause stroke and myocardial infarction are characterized by increased microvascular permeability and inflammation. Dynamic contrast-enhanced MRI (DCE-MRI) has been proposed as a method to quantify vessel wall microvascular permeability in vivo. Until now, most DCE-MRI studies of atherosclerosis have been limited to two-dimensional (2D) multi-slice imaging. Although providing the high spatial resolution required to image the arterial vessel wall, these approaches do not allow the quantification of plaque permeability with extensive anatomical coverage, an essential feature when imaging heterogeneous diseases, such as atherosclerosis. To our knowledge, we present the first systematic evaluation of three-dimensional (3D), high-resolution, DCE-MRI for the extensive quantification of plaque permeability along an entire vascular bed, with validation in atherosclerotic rabbits. We compare two acquisitions: 3D turbo field echo (TFE) with motion-sensitized-driven equilibrium (MSDE) preparation and 3D turbo spin echo (TSE). We find 3D TFE DCE-MRI to be superior to 3D TSE DCE-MRI in terms of temporal stability metrics. Both sequences show good intra- and inter-observer reliability, and significant correlation with ex vivo permeability measurements by Evans Blue near-infrared fluorescence (NIRF). In addition, we explore the feasibility of using compressed sensing to accelerate 3D DCE-MRI of atherosclerosis, to improve its temporal resolution and therefore the accuracy of permeability quantification. Using retrospective under-sampling and reconstructions, we show that compressed sensing alone may allow the acceleration of 3D DCE-MRI by up to four-fold. We anticipate that the development of high-spatial-resolution 3D DCE-MRI with prospective compressed sensing acceleration may allow for the more accurate and extensive quantification of atherosclerotic plaque permeability along an entire vascular bed. We foresee that this approach may allow for the comprehensive and accurate evaluation of plaque permeability in patients, and may be a useful tool to assess the therapeutic response to approved and novel drugs for cardiovascular disease.

Repeatability of in vivo quantification of atherosclerotic carotid artery plaque components by supervised multispectral classification

Objective

To evaluate the agreement and scan–rescan repeatability of automated and manual plaque segmentation for the quantification of in vivo carotid artery plaque components from multi-contrast MRI.

Materials and methods

Twenty-three patients with 30–70 % stenosis underwent two 3T MR carotid vessel wall exams within a 1 month interval. T1w, T2w, PDw and TOF images were acquired around the region of maximum vessel narrowing. Manual delineation of the vessel wall and plaque components (lipid, calcification, loose matrix) by an experienced observer provided the reference standard for training and evaluation of an automated plaque classifier. Areas of different plaque components and fibrous tissue were quantified and compared between segmentation methods and scan sessions.

Results

In total, 304 slices from 23 patients were included in the segmentation experiment, in which 144 aligned slice pairs were available for repeatability analysis. The correlation between manual and automated segmented areas was 0.35 for lipid, 0.66 for calcification, 0.50 for loose matrix and 0.82 for fibrous tissue. For the comparison between scan sessions, the coefficient of repeatability of area measurement obtained by automated segmentation was lower than by manual delineation for lipid (9.9 vs. 17.1 mm²), loose matrix (13.8 vs. 21.2 mm²) and fibrous tissue (24.6 vs. 35.0 mm²), and was similar for calcification (20.0 vs. 17.6 mm²).

Conclusion

Application of an automated classifier for segmentation of carotid vessel wall plaque components from in vivo MRI results in improved scan–rescan repeatability compared to manual analysis.

Increasing the Spatial Resolution of 3T Carotid MRI Has No Beneficial Effect for Plaque Component Measurement Reproducibility

Purpose

Different in-plane resolutions have been used for carotid 3T MRI. We compared the reproducibility, as well as the within- and between reader variability of high and routinely used spatial resolution in scans of patients with atherosclerotic carotid artery disease. Since no consensus exists about the optimal segmentation method, we analysed all imaging data using two different segmentation methods.

Materials and Methods

In 31 patient with carotid atherosclerosis a high (0.25 × 0.25 mm²; HR) and routinely used (0.50 × 0.50 mm²; LR) spatial resolution carotid MRI scan were performed within one month. A fully blinded closed and a simultaneously open segmentation were used to quantify the lipid rich necrotic core (LRNC), calcified and loose matrix (LM) plaque area and the fibrous cap (FC) thickness.

Results

No significant differences were observed between scan-rescan reproducibility for HR versus LR measurements, nor did we find any significant difference between the within-reader and between-reader reproducibility. The same applies for differences between the open and closed reads. All intraclass correlation coefficients between scans and rescans for the LRNC, calcified and LM plaque area, as well as the FC thickness measurements with the open segmentation method were excellent (all above 0.75).

Conclusions

Increasing the spatial resolution at the expense of the contrast-to-noise ratio does not improve carotid plaque component scan-rescan reproducibility in patients with atherosclerotic carotid disease, nor does using a different segmentation method.

A computer-simulation study on the effects of MRI voxel dimensions on carotid plaque lipid-core and fibrous cap segmentation and stress modeling

Background

The benefits of a decreased slice thickness and/or in-plane voxel size in carotid MRI for atherosclerotic plaque component quantification accuracy and biomechanical peak cap stress analysis have not yet been investigated in detail because of practical limitations.

Methods

In order to provide a methodology that allows such an investigation in detail, numerical simulations of a T1-weighted, contrast-enhanced, 2D MRI sequence were employed. Both the slice thickness (2 mm, 1 mm, and 0.5 mm) and the in plane acquired voxel size (0.62x0.62 mm² and 0.31x0.31 mm²) were varied. This virtual MRI approach was applied to 8 histology-based 3D patient carotid atherosclerotic plaque models.

Results

A decreased slice thickness did not result in major improvements in lumen, vessel wall, and lipid-rich necrotic core size measurements. At 0.62x0.62 mm² in-plane, only a 0.5 mm slice thickness resulted in improved minimum fibrous cap thickness measurements (a 2–3 fold reduction in measurement error) and only marginally improved peak cap stress computations. Acquiring voxels of 0.31x0.31 mm² in-plane, however, led to either similar or significantly larger improvements in plaque component quantification and computed peak cap stress.

Conclusions

This study provides evidence that for currently-used 2D carotid MRI protocols, a decreased slice thickness might not be more beneficial for plaque measurement accuracy than a decreased in-plane voxel size. The MRI simulations performed indicate that not a reduced slice thickness (i.e. more isotropic imaging), but the acquisition of anisotropic voxels with a relatively smaller in-plane voxel size could improve carotid plaque quantification and computed peak cap stress accuracy.