CARDIAC MRI

By: Ali Shaikh

HISTORY OF MRI

Magnetic Resonance Imaging (MRI)

MRI is a non-invasive imaging technique that came into clinical use in the early 1980s.

It is based on the principles of nuclear magnetic resonance (NMR) that were developed in the 1930s.

Significant advances were necessary to go from the basic principles of NMR to generating images of the human body.

Techniques were developed to localize the small amount of radio frequency (RF) energy generated from spinning hydrogen protons when a patient is placed in a strong magnetic field.

Image production today relies upon magnetic fields created by superconducting magnets and sophisticated electronics which manipulate and process the RF energy.

Courtesy: http://www.magnet.fsu.edu/educa&on/tutorials/magnetacademy/mri/images/mri-scanner.jpg

MRI & the Heart

MRI has revolutionized medical imaging for many organ systems.

However, due to the motion of the heart, the development of cardiac MRI has been slow as compared to MRI for other organs due to the requirement for faster acquisition techniques.

With advancements in technology, these obstacles have been overcome and cardiac MRI has become a validated tool for imaging the heart.

WHAT IS CARDIAC MRI?

Cardiac MRI Cardiac MRI creates

pictures of the heart as it's beating, producing both still and moving pictures of the heart and major blood vessels

Doctors use cardiac MRI to get pictures of the beating heart and to look at its structure and function

These pictures can help them decide how to treat people who have heart problems

Courtesy: https://jobs.stmarys.org/centers/radiology/mri/images/mri_cardiac.gif

Cardiac MRI

Unlike computed tomography (CT) scans and standard X-rays, MRI doesn't use ionizing radiation or carry any risk of causing cancerCardiac MRI test is used to diagnose and evaluate a number of diseases and conditions, including: * Coronary heart disease * Damage caused by a heart attack * Heart failure * Heart valve problems * Congenital heart defects * Pericarditis (a condition in which the membrane, or sac, around the heart is inflamed) * Cardiac tumors

PROGRESS IN CARDIAC MRI

Pulse Sequences

Pulse sequences are a pattern of radiofrequency pulses and magnetic gradients that are used to produce an image

There are a variety of different pulse sequences that are used in cardiac imaging that can be broadly divided into either black-blood techniques or bright-blood techniques

Spin echo (SE) cardiac sequences are typically black-blood techniques, while gradient echo sequences are typically bright-blood techniques

Bla ck-Blo o d Te chniq ue s

Spin-echo (SE) was the first sequence used for evaluating cardiac morphology

The development of ECG-gating made SE techniques especially useful by substantially reducing motion artifacts

SE sequences generally provide good contrast between the myocardium and blood

These are called “black-blood” images because of the signal void created by flowing blood

Blood signal may appear brighter in slower flowing areas, such as immediately adjacent to the chamber wall

Bla ck-Blo o d Te chniq ue s

Presaturation with radiofrequency (RF) and reduction of the echo time (TE) minimizes blood signal and increases contrast on gated SE images

SE imaging has limited temporal resolution and is degraded by respiratory and other motion-related artifacts.

Shorter acquisition times are achieved with fast SE (FSE) pulse sequences, also known as rapid acquisition relaxation enhancement (RARE)

Soft-tissue contrast may be less optimal than with SE techniques because of the wide range of acquired TEs inherent in FSE methods

Single-shot FSE (SSFSE) sequences use a very long echo train in tandem with half-Fourier reconstruction

Bla ck-Blo o d Te chniq ue s

In cardiac imaging, the basic SSFSE technique has not proven to be useful because the long echo trains required coupled with the relatively short T2 leads to poor image contrast and blurring

However, the SSFSE sequence can be modified for better cardiacresults by reducing the echo train length, lowering the effective TE, and using a blood-suppressed preparation method

T2-weighted inversion recovery (IR) imaging is now used as the frontline sequence for depiction of cardiac morphology

Bla ck-Blo o d Te chniq ue s

This technique uses a selective and a non-selective 180° inversion pulse followed by a long inversion time to null blood magnetization

A second selective 180° inversion pulse can also be applied tonull fat. This is referred to as double (DIR) or triple (TIR)inversion recovery.

The sequence is acquired with either a breath-hold or a non-breath-hold technique and provides excellent delineation of myocardial–blood interfaces

Figure 1. Comparison of short-axis views acquired with ECG-gated SE (left) and T2-weighted DIR imaging.Note that the ventricular blood signal is minimized and that the blood–myocardial interface is more clearly depicted on the DIR.

Brig ht-Blo o d Te chniq ue s

Bright-blood imaging yields both morphologic and functional data.

Blood generates bright signal intensity (SI), and multiple consecutive images are acquired that can be viewed dynamically to depict cardiac motion

Sequences include gradient-recalled echo (GRE), fast GRE (fGRE), segmented k-space fGRE, and steady state free precession (SSFP)

GRE imaging is well suited for cardiac imaging because of its short TEs and TRs

Blood appears bright compared to adjacent myocardium due to time-of-flight effects as well as the relatively long T2

Brig ht-Blo o d Te chniq ue s

A segmented k-space approach provides high-resolution dynamic images of the heart that are acquired much more rapidly than prior techniques

Using short TEs (2 msec) and TRs (10 msec), multiplelines (segments) of k-space are acquired during each cardiac cycle.

The technique is limited by the need to maintain adequate enhancement of inflowing blood

The inability to further reduce TR effectively limits achievable spatial and temporal resolution

A new approach to improve cine imaging involves a technique known as SSFP

Brig ht-Blo o d Te chniq ue s

Image contrast in SSFP depends on the T1/T2 ratio of tissue, and is less dependent on flow compared to the GRE techniques

SSFP uses the available blood signal very efficiently and accuratelydepicts blood, myocardium, and epicardial fat

SSFP sequences result in improved contrast between myocardium and ventricular cavities, with a clearer delineation of trabeculation and papillary muscles as compared to segmented k-space fGRE techniques

The other advantage of SSFP is improved temporal resolution

Figure 2. Comparison of mid-diastolic short-axis views acquired with segmented k-space GE imaging (left) and SSFP (right). Substantial blood pool heterogeneities are present in the segmented k-space GE image (left) as compared with the homogeneous blood pool on the SSFP image (right). The SSFP technique has improved endocardial border definition throughout the cardiac cycle as compared with the older technique.

Figure 3. Set of end-diastolic images obtained in a healthy volunteer with a cine 3D SSFP (FIESTA) sequence within a single breath-hold. The acquisition was acquired with a variable temporal k-space sampling scheme (VAST), and a 256x192 matrix in a 34-cm FOV with 4-mm partitions.

Myocardial Perfusion

Myocardial regional blood flow is assessed using dynamic MRI during the first pass of a contrast agent

The myocardial territory affected by a coronary artery lesion may or may not exhibit a perfusion deficit during firstpass imaging under resting conditions

However, under pharmacological stress the stenotic vessel is unable to respond like a healthy vessel because of its higher vascular resistance, which results in a “vascular steal” phenomenon with increased blood flow to the territories supplied by the nonstenotic vessels

A perfusion deficit appears in the perfused myocardial territory served by the stenotic vessel

Acq uis itio n Te chniq ue s

Conventional fGRE or fast low-angle single-shot (FLASH) techniques have been used for the assessment of myocardial perfusion

These strategies consist of a data acquisition segment that is preceded by an IR (180° flip angle) or preparatory radiofrequency (RF) pulse

This preparatory pulse generates T1 contrast between the enhancing normal myocardial tissue and non-enhancing regions of perfusion deficit

However, these approaches were limited by acquisition times of 500–700 msec per image, resulting in only one or two scan locations every one or two heartbeats

The long acquisition window degrades image quality, as cardiac motion results in artifacts and edge blurring.

Figure 4. Selected myocardial strain (circumferential shortening, Ecc; lower row) maps obtained with the HARP technique on conventional tagged MR images (upper row) of a canine heart with a left ventricular pacing. Images are shown in late diastole (left), early systole (middle), and late systole (right). Blue indicates contraction in the activated pacing site during early systole (solid arrows) and in the whole myocardium in late systole. Red indicates stretching opposite the pacing site in early systole (arrowheads).

Myocardial Viability

The ability to differentiate between viable and nonviable myocardium plays a critical role in the prognosis of patients with coronary artery disease

Until recently, thallium SPECT and PET were the primary tools for this evaluation, with dobutamine stress echocardiography playing an ancillary but growing role

In the last few years, however, MRI has made a dramatic appearance in this arena with the introduction and rapid acceptance of the delayed enhancement (DE) MRI technique

This imaging sequence identifies irreversible myocardial damage in both the acute and chronic settings, and combined with cine imaging can identify reversibly injured tissue that may benefit from revascularization.

Figure 7. 2D DE-MRI in a patient with chronic ischemic disease of the LAD distribution. A two-chamber long-axis image (left) reveals thinned tissue with transmural hyperenhancement involving the majority of the anterior wall and the entire apex, indicating a chronic transmural myocardial infarction. The left ventricular cavity is dilated and there is apical thrombus (black arrow). Cine images (right) at end-diastole (upper) and end-systole (lower) demonstrate akinesis of the anterior wall and apex (arrowhead).

Figure 8. 3D DE-MRI performed in a patient with a chronic infarction of the LAD distribution. This series of short-axis images from apex to base was acquired in a single breath-hold and has image quality nearly comparable to that of 2D images.

Coronary MRA (CMRA)

The coronary arteries have long been known to be one of the most difficult arterial circulations to image using MRI

The challenges for CMRA are the inherent complex geometry and tortuosity of the coronary arteries, their small caliber (2–4 mm), and their continual displacement by respiratory and cardiac motion

The wide variety of 2D and 3D CMRA techniques investigated over the past decade has been testimony to these difficulties

Bre a th-Ho ld Te chniq ue s

In the early 1990s, Edelman et al and Pennell et al reported successful coronary illustration using a breath-hold fat-suppressed fast 2D GRE pulse sequence with a segmented k-space scheme

A single image was obtained during each breath-hold and imaging was targeted for diastole, when the heart is less mobile and coronary flow more brisk

Subsequent clinical assessments, however, revealed mixed success with sensitivities for the detection of hemodynamically significantstenoses

Figure 9. DE-MRI in acute myocardial infarction. The lower images demonstrate DE-MRI (left) and cine images in end-diastole (middle) and end-systole (right). DE-MRI reveals transmural irreversible damage of the distal septum and apex, while the cine images reveal corresponding dysfunctional myocardium (arrow). The DE-MRI image in the upper row was performed in the two-chamber long-axis projection and reveals transmural hyperenhancement of acute irreversible damage in the LAD distribution. Note the microvascular obstruction (‘noreflow’ or focal nonhyperenhancing regions) in the subendocardium of the anterior wall (arrowheads), which is associated with greater post-infarction complications and poorer prognosis.

Na vig a to r-Echo Te chniq ue s

A novel technique for tracking of “view-to-view” tissue position using a “navigator” echo

This technique can be used for prospective or retrospective gating of free-breathing CMRA

Navigator-gating CMRA is typically performed using a fat-suppressed 3D GRE technique

Figure 11. Comparison between (a and b) free-breathing navigator-echo gated 2D spiral imaging and (c and d) breath-held multislice 2D spiral imaging. Image acquisition parameters for all images were: 20-cm FOV, 3.0-mm section thickness, 2048 20 acquisition matrix, and 70° flip angle. This provided a spatial resolution of 0.96x0.96x3.0 mm for all images. Note that the quality of the free-breathing images (3-minute scan time) was comparable to that of the breath-held images (19-second scan time).

FUTURE OF CARDIAC MRI

Predictions

The acquisition of cardiac MR will become easier ECG and respiratory gated sequences will allow high

resolution imaging during free breathing Three-dimensional time-resolved images of the heart will

provide images in any desired plane ECG gating without leads might become a reality, based on

the automatic detection of cardiac motion Perfusion imaging will replace nuclear methods as the gold

standard MRI-guided interventions will become available that can

visualise morphology and monitor changes in flow and limb perfusion during therapeutic interventions

Summary of General Trends

Time Frame General Trends

Short term(Present to 3 Years)

Advances in fast scan techniques (pulse sequences, RF coils, gradients) will have a major impact in improving the efficiency and performance of current exams

lntermediate term(3 to 5 Years)

- Groups of sequences will be better integrated into more comprehensive exams- MR data acquisition will become more intelligent

Long term(Beyond 5 Years)

- The clinical role of MRI will expand well beyond that of diagnosis- The flexibility of contrast and quantitative nature of MRI will be further exploited- The scientific role of MRI will expand, making it the gold standard for many applications

Conclusions

Cardiac MRI continues to develop and advance The advances include substantial overall improvements in

temporal resolution, spatial resolution, motion and other artifact reduction, and improved depiction of contrast enhancement for perfusion and viability analyses

Its clinical use has been limited, but is increasing because of its proven clinical efficacy, the proliferation of cardiac-capable MRI systems, and the development of improved pulse sequences

Resources

Bremerich, Jens, et al. “MRI: Now and in Future.” 01 Mar 2006. http://www.hospitalmanagement.net/features/feature645/

“Cardiac MRI: The Basics.” 2006. http://www.med-ed.virginia.edu/Courses/rad/cardiacmr/index.html

Earls, James, et al. “Cardiac MRI: Recent Progress and Continued Challenges.” Jo urna l o f Mag ne tic Re s o nanc e Im a g ing . 16:111–127 (2002).

Riederer, Stephen. “The Future Technical Development of MRI.” Jo urna l o f Mag ne tic Re s o nanc e Im a g ing . 1:52-56 (1996).

“What is Cardiac MRI.” Jul 2009. http://www.nhlbi.nih.gov/health/dci/Diseases/mri/mri_whatis.html

QUESTIONS