Jürgen E Schneider1
1University of Leeds, Leeds, United Kingdom
Synopsis
Cardiovascular Magnetic Resonance (CMR) is a
clinically well-established medical imaging modality that can provide a
comprehensive, multi-parametric assessment of the heart in patients. CMR,
albeit technically challenging, can yield insights at different scales ranging
from the whole heart down to cellular and molecular level in the heart muscle,
thereby spanning several orders of magnitude in resolution. This presentation primarily
focuses on strengths, weaknesses, and opportunities of CMR. Strengths and
weaknesses will be discussed in technical considerations and routine
applications, while opportunities will be exemplified in emerging CMR
techniques.
Introduction:
Cardiovascular Magnetic Resonance (CMR) is a clinically
well-established medical imaging modality that provides a comprehensive,
multi-parametric assessment of the heart in patients. CMR can yield insights at
different scales ranging from the whole heart down to cellular and molecular
level in the heart muscle (“myocardium”), thereby spanning several orders of
magnitude in resolution. The aim of this talk is to provide an overview of both
routine and emerging CMR imaging techniques to illustrate strengths,
weaknesses, and opportunities of CMR.Technical considerations:
CMR scans are
typically reported in a coordinate system, which is defined by the geometry of
the heart rather than the gradients or patient. Scout scans are therefore
required at the beginning of each CMR examination to identify these relevant
views, which are orientated along the short- and long-axes of the heart. To minimise
the influence of motion (caused by the heart pumping blood through the body and
by respiration) on the data acquisition process, fast imaging sequence are
paramount. In addition, CMR sequences are routinely synchronized (i.e.
triggered) to the electrocardiogram (ECG) and combined with patient breath-holding.
CMR images have either bright or black blood contrast: the bright blood
contrast is generated by the inflow effect of unsaturated spins from the
blood pool into the imaging slice. Conversely, dark or black blood contrast can
be achieved through the spin washout effect from the imaging slice in a
spin-echo sequence, and / or by additional black blood magnetisation
preparation schemes (such as double inversion preparation pulses).Routine Applications:
This part
discusses techniques that may be applied during a standard clinical CMR
examination.
- Cardiac function: Bright blood multi-frame imaging techniques using either spoiled
gradient echo or balanced steady state free precession (bSSFP) pulse sequences
are applied at multiple timepoints throughout the cardiac cycle (“cine-MRI”) to
quantify myocardial mass and volumes of the heart’s chambers (“ventricles”).
Equipping the sequences with additional contrast modules such as for example a tagging
module or motion-encoding gradients allows for detailed characterization of the
deformation and the motion pattern as the heart muscle contracts and relaxes.
- T1- and T2-mapping: While diagnostic information historically relied on relative
differences in contrast due to differences in relaxation times between healthy
and diseased myocardium, T1-/T2-weighted imaging techniques have nowadays
largely been superseded by mapping techniques to provide quantitative values of
relaxation times on a voxel-by-voxel basis reflective of intrinsic tissue
properties.
- Contrast-enhanced MRI: Although endogenous contrast mechanisms are a key strength of CMR,
paramagnetic (i.e. Gd-based) contrast agents are typically administered to
assess myocardial blood flow in the myocardium (“first-pass perfusion MRI”),
diffuse and focal fibrosis, respectively. Specifically, native and
post-contrast T1 maps can be combined to derive extracellular volume (ECV)
fraction, while Late Gadolinium Enhancement (LGE) is the reference standard for
non-invasive imaging of myocardial scar (i.e. focal fibrosis).
Emerging Techniques:
This part discusses
CMR techniques that have shown promises but are not yet applied routinely in
the clinics.
- Metabolic Imaging: While CMR primarly utilises the signal of the water protons to
characterise anatomy or function of the heart, the detection and quantification
of signals other than those from water may provide complimentary insights into
the (patho-) physiological status of the myocardium. Metabolism can be
interrogated either directly using Magnetic Resonance Spectroscopy (single
voxel MRS or spectroscopic MR imaging) or, for certain metabolites, indirectly
by means of Chemical Exchange Saturation Transfer (“CEST”) imaging. Both
techniques hold significant potential for identifying novel biomarkers of
cardiovascular disease.
- Elastography: Myocardial stiffness, which has a major influence on cardiac function, is
increased in many cardiac diseases. Cardiac magnetic resonance elastography
(MRE) is a phase‐contrast MRI technique which measures the time-varying cardiac shear
modulus, reflective of cardiac stiffness, by application of low-frequency
acoustic waves during the imaging process. Non-invasive imaging techniques to
assess myocardial stiffness are lacking in the clinic, but cardiac MRE has the potential
to close this gap.
- Diffusion MRI: Cardiac microarchitecture, which is a key determinant of (patho-)
physiologically relevant functions of the heart, can be assessed using
diffusion MRI. Water molecules act as a sensitive marker of tissue integrity
and cellular orientation. The data are commonly modelled using the Diffusion
Tensor, which requires at least six measurements with non-collinear Diffusion
weighting (DW) plus one measurement with no / low DW. Signal-to-noise
constraints, scan-time requirements, motion of the beating heart, strain and
perfusion are major confounder and make the application of this technique very
challenging.
- Multi-parametric
“One-Stop-Shop” Sequences: While the benefits of
quantitative parameter mapping are well established, their scan time
requirements and sensitivity to physiological motion pose major constraints.
Recently, two alternative approaches, namely MR Fingerprinting (1,2) and MR
Multi-tasking (3) have been introduced to overcome these limitations: in both cases multiple
parameter maps are obtained simultaneously during one acquisition.
Conclusion:
CMR is a powerful, albeit technically challenging imaging
modality that allows for a detailed assessment of the heart. While major
advances in hard- and software development over the past two decades resulted
in widespread availability of CMR in the clinic, more research is required to improve
robustness and to push the boundaries of this versatile technique.Acknowledgements
JES acknowledges funding from the British Heart Foundation and the Wellcome Trust.References
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