Myocardial perfusion quantification using the T 1 -based FAIR-ASL method: The influence of heart anatomy, cardiopulmonary blood flow and look-locker readout

Real-time Triggered RAdial Single-Shot Inversion recovery for arrhythmia-insensitive myocardial T1 mapping: motion phantom validation and in vivo comparison

PURPOSE

Cardiac T₁ mapping has become an increasingly important imaging technique, contributing novel diagnostic options. However, currently utilized methods are often associated with accuracy problems because of heart rate variations and cardiac arrhythmia, limiting their value in clinical routine. This study aimed to introduce an improved arrhythmia-related robust T₁ mapping sequence called RT-TRASSI (real-time Triggered RAdial Single-Shot Inversion recovery).

METHODS

All measurements were performed on a 3.0T whole-body imaging system. A real-time feedback algorithm for arrhythmia detection was implemented into the previously described pulse sequence. A programmable motion phantom was constructed and measurements with different simulated arrhythmias arranged. T₁ mapping accuracy and susceptibility to artifacts were analyzed. In addition, in vivo measurements and comparisons with 3 prevailing T₁ mapping sequences (MOLLI, ShMOLLI, and SASHA) were carried out to investigate the occurrence of artifacts.

RESULTS

In the motion phantom measurements, RT-TRASSI showed excellent agreement with predetermined reference T₁ values. Percentage scattering of the T₁ values ranged from -0.6% to +1.9% in sinus rhythm and -1.0% to +3.1% for high-grade arrhythmias. In vivo, RT-TRASSI showed diagnostic image quality with only 6% of the acquired T₁ maps including image artifacts. In contrast, more than 40% of the T₁ maps acquired with MOLLI, ShMOLLI, or SASHA included motion artifacts.

CONCLUSION

Accuracy issues because of heart rate variability and arrhythmia are a prevailing problem in current cardiac T₁ mapping techniques. With RT-TRASSI, artifacts can be minimized because of the short acquisition time and effective real-time feedback, avoiding potential data acquisition during systolic heart phase.

Evaluation of Modified Look-Locker Inversion Recovery and Arrhythmia-Insensitive Rapid Cardiac T1 Mapping Pulse Sequences in Cardiomyopathy Patients

OBJECTIVE

The aim of this study was to compare the performance of arrhythmia-insensitive rapid (AIR) and modified Look-Locker inversion recovery (MOLLI) T1 mapping in patients with cardiomyopathies.

METHODS

In 58 patients referred for clinical cardiac magnetic resonance imaging at 1.5 T, we compared MOLLI and AIR native and postcontrast T1 measurements. Two readers independently analyzed myocardial and blood T1 values. Agreement between techniques, interreader agreement per technique, and intrascan agreement per technique were evaluated.

RESULTS

The MOLLI and AIR T1 values were strongly correlated (r = 0.98); however, statistically significantly different T1 values were derived (bias 80 milliseconds, pooled data, P < 0.01). Both techniques demonstrated high repeatability (MOLLI, r = 1.00 and coefficient of repeatability [CR] = 72 milliseconds; AIR, r = 0.99 and CR = 184.2 milliseconds) and produced high interreader agreement (MOLLI, r = 1.00 and CR = 51.7 milliseconds; AIR, r = 0.99 and CR = 183.5 milliseconds).

CONCLUSIONS

Arrhythmia-insensitive rapid and MOLLI sequences produced significantly different T1 values in a diverse patient cohort.

MRI-based quantification of renal perfusion in mice: Improving sensitivity and stability in FAIR ASL

PURPOSE

The importance of the orientation of the selective inversion slice in relation to the anatomy in flow-sensitive alternating inversion recovery arterial spin labeling (FAIR ASL) kidney perfusion measurements is demonstrated by comparing the standard FAIR scheme to a scheme with an improved slice selective control experiment.

METHODS

A FAIR ASL method is used. The selective inversion preparation slice is set perpendicular to the measurement slice to decrease the unintended labeling of arterial spins in the control experiment. A T₁^*-based quantification method compensates for the effects of the imperfect inversion on the edge of the selective inversion slice. The quantified perfusion values are compared to the standard experiment with parallel orientation of imaging and selective inversion slice.

RESULTS

Perfusion maps acquired with the perpendicular inversion slice orientation show higher sensitivity compared to the parallel orientation. The T₁^*-based quantification method removes artifacts arising from imperfect inversion slice profiles. The stability is improved.

CONCLUSION

Adjusting the labeling technique to the anatomy is of high importance. Improved sensitivity and reproducibility could be demonstrated. The proposed method provides a solution to the problem of FAIR ASL measurements of renal perfusion in coronal view.

A look-locker acquisition scheme for quantitative myocardial perfusion imaging with FAIR arterial spin labeling in humans at 3 tesla

PURPOSE

A novel method for quantitative measurement of myocardial blood flow (MBF) using arterial spin labeling (ASL) in a single breath-hold is presented, evaluated by simulations, phantom studies and in vivo studies and tested for reproducibility and variability.

METHODS

A flow-sensitive alternating inversion recovery (FAIR) ASL method with Look-Locker readout (LL-FAIR-ASL) was implemented at 3 tesla. Scans were performed on 10 healthy volunteers and MBF measured in three slices. The method was investigated for reproducibility by Bland-Altman analysis and statistical measures, the coefficients of reproducibility (CR) and variation (CV) are reported.

RESULTS

The MBF values for the basal, mid, and apical slices were 1.04 ± 0.40, 1.06 ± 0.46, and 1.06 ± 0.38 ml/g/min, respectively (mean ± SD), which compare well with literature values. The CV across all scans, 43%, was greater than the between-session and within-session values, at 16 and 13%, respectively, for the mid-ventricular slice. The change in MBF required for detection, from the CR, was 61% between-session and 53% within-session for the mid-ventricle.

CONCLUSION

This study shows the feasibility of the LL-FAIR-ASL method for the quantification of MBF. The statistical measures reported will allow the planning of future clinical research studies involving rest and stress measurements. Magn Reson Med 78:541-549, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

High field magnetic resonance imaging of rodents in cardiovascular research

Transgenic and gene knockout rodent models are primordial to study pathophysiological processes in cardiovascular research. Over time, cardiac MRI has become a gold standard for in vivo evaluation of such models. Technical advances have led to the development of magnets with increasingly high field strength, allowing specific investigation of cardiac anatomy, global and regional function, viability, perfusion or vascular parameters. The aim of this report is to provide a review of the various sequences and techniques available to image mice on 7-11.7 T magnets and relevant to the clinical setting in humans. Specific technical aspects due to the rise of the magnetic field are also discussed.

Myocardial arterial spin labeling

Arterial spin labeling (ASL) is a cardiovascular magnetic resonance (CMR) technique for mapping regional myocardial blood flow. It does not require any contrast agents, is compatible with stress testing, and can be performed repeatedly or even continuously. ASL-CMR has been performed with great success in small-animals, but sensitivity to date has been poor in large animals and humans and remains an active area of research. This review paper summarizes the development of ASL-CMR techniques, current state-of-the-art imaging methods, the latest findings from pre-clinical and clinical studies, and future directions. We also explain how successful developments in brain ASL and small-animal ASL-CMR have helped to inform developments in large animal and human ASL-CMR.

Self-navigation under non-steady-state conditions: Cardiac and respiratory self-gating of inversion recovery snapshot FLASH acquisitions in mice

PURPOSE

An algorithm is presented to enable cardiac and respiratory self-gating in combination with Inversion Recovery Look-Locker read-outs.

METHODS

A radial inversion recovery snapshot FLASH sequence was adapted for retrospective cardiac T₁ measurements in mice. Cardiac and respiratory data were extracted from the k-space center of radial projections and an adapted method for retrospective cardiac synchronization is introduced. Electrocardiogram (ECG) data was acquired concurrently for validation of the proposed self-gating technique. T₁ maps generated by the proposed technique were compared with maps reconstructed with the ECG reference.

RESULTS

Respiratory gating and cardiac trigger points could be obtained for the whole time course of the relaxation dynamic and correlate very well to the ECG signal. T₁ maps reconstructed with the self-gating technique are in very good agreement with maps reconstructed with the external reference.

CONCLUSION

The proposed method extends "wireless" cardiac MRI to non-steady-state inversion recovery measurements. T₁ maps were generated with a quality comparable to ECG based reconstructions. As the method does not rely on an ECG trigger signal it provides easier animal handling. Magn Reson Med 76:1887-1894, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

Quantification of perfusion in murine myocardium: A retrospectively triggered T1 -based ASL method using model-based reconstruction

PURPOSE

A method for the quantification of perfusion in murine myocardium is demonstrated. The method allows for the reconstruction of perfusion maps on arbitrary time points in the heart cycle while addressing problems that arise due to the irregular heart beat of mice.

METHODS

A flow-sensitive alternating inversion recovery arterial spin labeling method using an untriggered FLASH-read out with random sampling is used. Look-Locker conditions are strictly maintained. No dummy pulses or mechanism to reduce deviation from Look-Locker conditions are needed. Electrocardiogram and respiratory data are recorded for retrospective gating and triggering. A model-based technique is used to reconstruct missing k-space data to cope with the undersampling inherent in retrospectively gated methods. Acquisition and reconstruction were validated numerically and in phantom measurements before in vivo experimentation.

RESULTS

Quantitative perfusion maps were acquired within a single slice measurement time of 11 min. Perfusion values are in good accordance to literature values. Myocardial infarction could be clearly visualized and results were confirmed with histological results.

CONCLUSION

The proposed method is capable of producing quantitative perfusion maps on arbitrary positions in the heart cycle within a short measurement time. The method is robust against irregular breathing patterns and heart rate changes and can be implemented on all scanners.

MOLLI and AIR T1 mapping pulse sequences yield different myocardial T1 and ECV measurements

Both post-contrast myocardial T1 and extracellular volume (ECV) have been reported to be associated with diffuse interstitial fibrosis. Recently, the cardiovascular magnetic resonance (CMR) field is recognizing that post-contrast myocardial T1 is sensitive to several confounders and migrating towards ECV as a measure of collagen volume fraction. Several recent studies using widely available Modified Look-Locker Inversion-recovery (MOLLI) have reported ECV cutoff values to distinguish between normal and diseased myocardium. It is unclear if these cutoff values are translatable to different T1 mapping pulse sequences such as arrhythmia-insensitive-rapid (AIR) cardiac T1 mapping, which was recently developed to rapidly image patients with cardiac rhythm disorders. We sought to evaluate, in well-controlled canine and pig experiments, the relative accuracy and precision, as well as intra- and inter-observer variability in data analysis, of ECV measured with AIR as compared with MOLLI. In 16 dogs, as expected, the mean T1 was significantly different (p < 0.001) between MOLLI (891 ± 373 ms) and AIR (1071 ± 503 ms), but, surprisingly, the mean ECV between MOLLI (21.8 ± 2.1%) and AIR (19.6 ± 2.4%) was also significantly different (p < 0.001). Both intra- and inter-observer agreements in T1 calculations were higher for MOLLI than AIR, but intra- and inter-observer agreements in ECV calculations were similar between MOLLI and AIR. In six pigs, the coefficient of repeatability (CR), as defined by the Bland-Altman analysis, in T1 calculation was considerably lower for MOLLI (32.5 ms) than AIR (82.3 ms), and the CR in ECV calculation was also lower for MOLLI (1.8%) than AIR (4.5%). In conclusion, this study shows that MOLLI and AIR yield significantly different T1 and ECV values in large animals and that MOLLI yields higher precision than AIR. Findings from this study suggest that CMR researchers must consider the specific pulse sequence when translating published ECV cutoff values into their own studies.