Applied Radiology

EDITOR'S NOTE

This issue of Applied Imaging discusses the various applications of MRI for diseases of the heart. While MRI has been useful for many years to characterize masses in the heart, only recently have the new MR systems with stronger, faster "cardiovascular" gradients been able to acquire images quickly enough to avoid motion artifact. MRI can now be used to assess ventricular function, myocardial perfusion and wall motion, valvular function, and myocardial viability (using "delayed hyperenhancement" imaging). With anticipated future improvements in coronary MR angiography, MRI will truly give us the opportunity for "one-stop shopping" for cardiac evaluation.­­William G. Bradley, Jr., MD, PhD, FACR

Cardiac MRI

Cardiac magnetic resonance imaging (CMRI) has been performed for almost 2 decades, since MRI was first introduced into clinical practice. ¹ Early nongated images were rapidly supplanted by those with cardiac gating, using either chest EKG leads or finger plethysmography. Typically, T1-weighted images were acquired gated to every r-wave and T2-weighted images were acquired gated to every second or third r-wave. Multislice, multiphase imaging was time-intensive and the spatial and temporal resolution was fairly limited. The current excitement in CMRI reflects the relatively recent introduction of stronger, faster gradients and novel k-space acquisition methods. As a result of the sophisticated imaging techniques and the complicated anatomy and pathology in the heart, the most successful CMRI programs are based on collaboration between MR radiologists and cardiologists. This issue of Applied Imaging: Applications in MRI describes the current status of MRI of the heart.

Technology

The major technological advancements that have made CMRI a clinical reality include stronger and faster gradients, the ability to more reliably obtain ECG gating, and use of a surface coil. Current CMRI units have gradients of 40 mT/m, rise times of 200 to 250 msec, and slew rates of 150 to 200 T/m/sec. Such speed is necessary to provide images of reasonable spatial resolution of a beating heart. With current technology, it is possible to provide a complete image of one slice through the heart in 96 msec, effectively eliminating motion artifact. A complete multislice, multiphase (4D) data set of the heart can be acquired in a fraction of a minute. CMRI units can provide 25-msec temporal and 1-mm spatial resolution or better. Images are further optimized by using a phased array surface coil.

The images acquired during the multiple phases of the cardiac cycle can be viewed in a cine loop, thereby displaying the contracting heart. Chest ECG leads provide optimal timing for image acquisition, but finger plethysmography can be used if it is not possible to obtain an adequate ECG signal. ECG lead technology has improved with use of carbon fiber. Arrhythmias still pose problems for image acquisition and bradycardia increases study time. Magnetohydrodynamic (MHD) effects on the ECG elevate the T wave, which can result in triggering on both the R and T waves thereby yielding non-diagnostic cine images. MHD effects also reduce ST segment depression, which can affect identification of myocardial ischemia. The greater blood velocity induced by stress studies results in a larger MHD effect, further complicating stress studies.

CMRI techniques

The three orthogonal cardiac imaging planes used in nuclear imaging include the short axis, 4-chamber (horizontal) long axis, and 2-chamber (vertical) long axis images. Using fast gradient echo imaging, the imaging plane can be adjusted in real time similar to ultrasound (MR fluoroscopy).

The following imaging techniques are necessary for optimal CMRI: 1) "bright blood" cine imaging (for ventricular function and stenotic and regurgitant valvular lesions) (Figure 1); 2) "black blood" imaging (for anatomy) (Figure 2); 3) perfusion imaging (for stress and rest perfusion to evaluate ischemia) (Figure 3); 4) delayed hyperenhancement (for precise sizing of myocardial infarction) (Figure 4); 5) real-time" imaging (for dobutamine stress imaging and MR fluoroscopy); 6) phase-contrast imaging (for flow quantification); 7) myocardial "tagging" (for quantification of regional wall motion, stress and strain); and 8) MR angiography (for evaluation of the coronary arteries and great vessels). Each of these will now be discussed in the context of the specific clinical application.

Myocardial function

Ventricular function (ventricular volumes [systolic and diastolic at stress and at rest], ejection fraction, stroke volume, and cardiac output) is best assessed with "bright blood techniques" (Figure 1). ¹ These images are acquired with ECG gating using a fast gradient echo pulse sequence, e.g., TrueFISP or FIESTA, with short TR (e.g., 5 to 10 msec) and TE (e.g., 1 to 2 msec) and a flip angle of 10š to 20š. Imaging starts after the R wave and can be prospectively gated (i.e., traditional EKG triggering that samples end diastole poorly due to respiratory variations) or retrospectively gated (which requires rebinning the data after the acquisition to cover the complete cardiac cycle). A segmented k-space acquisition (aka "multishot echo planar imaging") reduces the time required to acquire images by increasing the number of lines of k-space acquired per cine frame. The cine frame duration equals the TR interval multiplied by the lines of k-space acquired (typically 8) and is ideally <100 msec to prevent ghosting. Images are acquired at a single anatomic location for each breathhold with multiple (e.g., 12 to 20) cine frames (phases) per RR interval. Stationary tissue is saturated hence it is dark. Blood is bright because of flow-related enhancement. ²

Myocardial ischemia imaging

Myocardial ischemia can be evaluated on the basis of perfusion or wall motion, comparing rest and stress. Stress imaging with either an intravenous (IV) infusion of a vasodilator (e.g., adenosine) or an inotropic agent (e.g., dobutamine) requires the presence of medical personnel with expertise in evaluation of ischemia and in advanced cardiac life support.

First-pass myocardial perfusion imaging utilizes a T1-weighted pulse sequence that results in enhancement of normal myocardium. These images are a result of perfusion and do not depend on gadolinium moving into the myocyte. Therefore, on immediate post-injection images, normal myocardium is bright, and nonperfused myocardium is dark (Figure 3).

The pulse sequence used for myocardial perfusion is segmented echo planar imaging in which it is possible to acquire >=7 anatomic slices per RR interval at rest and per 2 RR intervals during stress. Ideally, these acquisitions should be performed during a breathhold. The chances of success are increased if oxygen is administered by mask before starting. Since it may still not be possible for many patients to hold their breath during a 60-second acquisition, they should be instructed to take small, even breaths when they start to breathe so respiratory motion artifacts are minimized.

MR perfusion stress imaging is performed with vasodilator stress (i.e., adenosine or dipyridamole IV). ³ During MR perfusion studies, it is of tantamount importance that signs and symptoms of ischemia be monitored rigorously, since only a rhythm strip and not a full 12-lead ECG is available during imaging. In addition, the current pulse sequence requires the patient to hold her/his breath for approximately 1 minute while machine noise precludes effective verbal communication.

The definition of ischemia on a perfusion stress study is an area of decreased perfusion on the stress, but not the non-stress images. The definition of myocardial infarction on a perfusion stress study is an area of decreased perfusion on both the stress and the non-stress images. Studies of multislice myocardial perfusion imaging have yielded sensitivities of 72% to 91% and specificities of 94% to 98%. Comparable numbers for cine imaging in the same patients were 85% and 94%. ^4-6

Dobutamine stress imaging assesses the induction of ischemia by evaluation of regional wall motion during progressively increased doses of IV dobutamine. Ischemia results in a new or worsening wall motion abnormality compared with the baseline study. Real-time white blood cine imaging and the ability to simultaneously view wall motion for all the doses of dobutamine are essential to identify ischemia-induced wall motion abnormality so the test can be terminated immediately. The temporal course of the ischemic cascade consists of a reduction of blood flow followed by abnormal wall motion, symptoms, ECG ischemic changes, hypotension, and arrhythmias. ⁷ There is a 60-second interval between onset of abnormal wall motion and chest pain and hypotension. (Unfortunately, ECG is not useful while the patient is in the magnet due to magnetohydrodynamic effects). Hundley et al ⁸ studied 153 patients who had poor acoustic windows that prevented adequate second harmonic transthoracic echocardiographic (TTE) imaging. Using 50% coronary artery stenosis, as defined by computer-assisted quantitative coronary angiography (QCA), the sensitivity and specificity were both 83%.

Cardiac wall motion can also be studied with myocardial tagging. Saturation bands or grids are produced by applying a sequence of saturation pulses to evaluate myocardial contraction throughout the cardiac cycle. For each RR interval, the tagging sequence is applied immediately following the ECG trigger and before data acquisition. Longitudinal magnetization is altered so that tissue appears dark in the subsequent images. The tagged images are used to assess regional myocardial contraction and for sophisticated analysis of myocardial stress and strain. ⁹

Another exciting measure of myocardial ischemia is the coronary flow reserve (CFR). It is defined as maximal hyperemic flow divided by resting flow. In normal patients, the coronary blood flow increases by a factor of 3 to 5 following vasodilator stress. CFR has been used to assess the functional significance of coronary artery stenoses involving the left main and left anterior descending (LAD) coronary arteries. Using a phase-contrast technique, Hundley and Link ¹⁰ demonstrated that a CFR <1.7 was 100% sensitive and 83% specific for a >=70% LAD stenosis.

Myocardial infarction imaging

A new technique known as "delayed hyperenhancement" is proving to be extremely sensitive and specific for the diagnosis and quantitation of myocardial infarction. Normal myocardium is dark 20 to 30 minutes following injection of Gd because the Gd has washed out and infarcted myocardium is bright. ^11,12 The pulse sequence is a segmented k-space, inversion recovery (IR), i.e., magnetization-prepared, gated fast gradient-recalled echo (GRE) technique. The T1 relaxation time of the normal myocardium determines the inversion time of approximately 180 msec so normal myocardium is dark. Delayed hyperenhancement is more accurate in estimating infarct volume than enzymes, which tend to have a narrow temporal window and wash out quickly with the increased use of angioplasty and stenting.

Viability imaging

Myocardium that exhibits abnormal wall motion can still be viable. The ability to identify viable myocardium accurately is extremely important. "Stunned" myocardium occurs when the nutrient coronary artery is occluded but reopens. Wall motion is abnormal but normalizes with small doses of dobutamine. "Hibernating" myocardium does not contract normally but is still metabolically active and will improve after revascularizaton.

Right ventricular dysplasia

Right ventricular dysplasia (RVD) is a genetic cardiomyopathy characterized pathologically by fibrous/adipose replacement of the right (and rarely left) ventricular myocardium and is associated with ventricular arrhythmias in young patients. CMRI is able to not only assess the anatomic and functional abnormalities of the RV (e.g., wall thinning and wall motion abnormality) but can also image the fibrous/adipose replacement of myocardium. ¹⁷ This is best accomplished with black blood or double IR techniques that null signal from moving blood, decreasing partial volume averaging between the otherwise bright blood and the bright, fatty myocardium being sought. ¹⁸ [Two 180š pulses applied prior to the fast spin echo (FSE) acquisition optimizes reduction of the blood signal and related artifacts. After the ECG trigger, a non-slice selective 180š pulse inverts all spins including blood. This is followed by a second, slice-selective 180š pulse that re-inverts only the spins in the slice. During the inversion time TI between 180š pulses, inverted spins in the blood that is outside the imaging slice (with negative magnetization) recover to zero magnetization making them black. For a heart rate of 60 beats per minute, the TI is approximately 650 msec. A third 180š pulse can be applied before the FSE acquisition to null fat based on its short T1 (similar to the STIR technique). This is known as "black blood with fat suppression" or "triple IR." At 1.5T the interval before the FSE readout should be 150 msec to null fat.] The diagnosis of fatty replacement is optimally made by finding high signal on the double IR images that is nulled on the triple IR images.

Cardiac valve function

Black blood images also provide excellent anatomic detail of cardiac valves. White blood imaging provides information about turbulence produced by stenotic or regurgitant lesions because of the dephasing of the spins. ¹⁹ Phase-contrast imaging can provide quantification of right ventricular (RV) and left ventricular (LV) stroke volume (SV) by interrogating the flow measurements from the proximal pulmonary artery and aorta, respectively. ²⁰ If there is only one regurgitant valve, then the regurgitant volume can be calculated by LV SV minus RV SV.

Congenital abnormalities

Black and white blood imaging can provide excellent anatomic detail in patients with congenital cardiovascular abnormalities. ²¹

Phase-contrast imaging is helpful in several congenital cardiovascular diseases. ²² It is possible to accurately measure flow in the main pulmonary artery (Qp) and ascending aorta (Qs) ²³ and thereby calculate the Qp/Qs ratio in simple lesions such as atrial and ventricular septal defects, patent dutus arteriosus, and partially anomalous pulmonary venous connection. Follow-up of patients after repair of Tetralogy of Fallot using CMRI provides information on residual anatomic problems, the extent of pulmonary stenosis, the amount of pulmonary regurgitation, and ventricular size and function. ²⁴

Miscellaneous

CMRI is excellent at evaluating the 3 major categories of cardiomyopathy: dilated, hypertrophic, and restrictive. Anatomy, function, and valvular disease of cardiomyopathies can be assessed using black and white blood imaging. ²⁵ In addition, the effect of therapy on ventricular size and function can be monitored. ²⁶ In thalassemia, myocardial iron concentration can be assessed using a T2*- weighted technique. In sarcoidosis, the myocardium appears inhomogeneous with black blood imaging and enhancement occurs with Gd.

CMRI with Gd visualizes the site, activity, and extent of inflammation in acute myocarditis. ²⁷ CMRI can image vegetations, as well as other anatomic complications of endocarditis. ²⁸

CMRI is excellent for imaging the pericardium. ²⁹ Black blood imaging at multiple cardiac levels enables anatomic characterization of the pericardium. Diseases that can be assessed include congenital absence of the pericardium, as well as pericardial effusion, thickening, cysts, diverticula, and tumors. It is possible to differentiate constrictive pericarditis from restrictive pericardial disease, which is an important distinction since constrictive pericarditis is treated by surgical removal of the pericardium.

Cardiac tumors

CVMR is not only able to identify the anatomy of cardiac tumors but can also provide tissue characterization (Figure 5). ³⁰ The most common primary cardiac malignancy is angiosarcoma, which often occurs in the right atrium and involves the pericardium. If hemorrhage is present, T1-weighted images can demonstrate increased signal intensity. In children, rhabdomyosarcoma is the most common primary cardiac malignancy and may involve the valves. Undifferentiated sarcoma tends to occur in the left atrium. Primary osteogenic sarcoma commonly occurs in the left atrium and may demonstrate calcification. Leiomyosarcoma not only tends to occur in the left atrium, but may also invade pulmonary veins and the mitral valve. Fibrosarcoma often occurs in the left atrium and may be necrotic. Liposarcoma is rare and is usually a large, infiltrating mass that may have foci of fat. Primary cardiac lymphoma often involves the pericardium. Atrial myxomas tend to be attached to the interatrial septum and have low MR signal. ³¹ Myxomas may demonstrate Gd enhancement in the core, which at pathology has dense neovascular channels.

Cardiac thrombus

The MR image characteristics of cardiac thrombus differ depending on the age of the clot. ³¹ Subacute clots exhibit homogeneously low MR signal, do not enhance with Gd, and demonstrate magnetic susceptibility effects. Histopathology has demonstrated that these clots are avascular and contain dense iron deposition. Chronic clots exhibit intermediate and heterogeneous MR signal as well as multiple areas of Gd enhancement. Using black blood imaging, it may be difficult to differentiate areas of slow blood flow from thrombus. In these cases, Gd imaging is recommended to delineate blood pool from myocardium more clearly.

Aneurysms

Using CVMR, it is possible to identify true ³² and pseudo-aneurysms ³³ of the left ventricle, atrial septal aneurysm, ³⁴ and sinus of Valsalva aneurysm. ³⁵ A cardiac pseudoaneurysm is a rupture of the ventricular myocardium that is contained by the pericardium. If the pericardium also ruptures, this condition can be fatal, hence the importance of its timely diagnosis.

Future directions

With CMRI, it is possible to pursue many exciting avenues of investigation. These include coronary artery imaging, plaque characterization, and tissue characterization using spectroscopy. It is currently possible to image anomalous coronary arteries. ³⁶ Routine, accurate imaging of coronary arterial stenosis is an area of active development and research. ^37,38 The ability to assess atherosclerotic plaque accurately, with the goal of identifying "vulnerable" plaque at high risk of rupture and causing acute coronary artery closure, is of great importance. ³⁹

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Clinical Quiz

1. Which of the following cardiac MR imaging techniques cannot be used to detect ischemia prior to infarction?

A. Myocardial tagging

B. Ventricular wall

C. Delayed hyperenhancement motion

D. Perfusion imaging

E. Coronary flow reserve

 

2. Which of the following is not characteristic of right ventricular dysplasia?

A. Fatty replacement

B. Akinesis

C. Dysrythmia

D. Wall thinning

E. Edema

 

3. Delayed hyperenhancement is seen in:

A. Infarction

B. Ischemia

C. Dyskinesis

D. Arrhythmia

E. All of the above

 

4. Optimal cardiac MRI requires:

A. Strong gradients

B. Fast gradients

C. Segmented coverage of

D. Phased-array surface k-spacecoils

E. All of the above.

 

5. Which of the following is NOT true regarding evaluation of coronary flow reserve by MRI?

A. It requires an injection of dobutamine.

B. It can be used to assess blood flow in the left main and LAD coronary arteries.

C. It is based on phased contrast MRA to quantitate blood flow.

D. It has 100% sensitivity for predicting a >=70% stenosis.

E. It can preselect which patients will require angioplasty or stent.

Answers

1. C. Delayed hyperenhancement only demonstrates infarcted myocardium, not ischemia;. 2. E.; 3. A. Delayed hyperenhancement is only positive with infarction.; 4. E; 5. A. Coronary flow reserve is performed during vasodilator stress using adenosine or dipyridamole, not inotropic stress (dobutamine).

Note: No contrast agents are approved by the U.S. Food and Drug Administration for use in imaging of the heart.

 

EDITORIAL STAFF

William G. Bradley, Jr., MD, PhD, FACR * Editor-in-Chief

O. Oliver Anderson * Publisher

Elizabeth A. McDonald * Editor

Beverly Harris Assistant * Editor

Felice Ponger-Shaloum * Art Director/Production Manager

Applied Imaging is published by Anderson Publishing, Ltd. It is supported by a grant from Amersham Health. The views and opinions expressed in this publication are those of the authors and do not necessarily reflect those of the publisher or sponsor. All inquiries should be addressed to: Anderson Publishing, Ltd., 1301 W. Park Avenue, Ocean, NJ 07712.