Indication

This method, also known as a fast low-angle shot (FLASH), or fast-field echo (FFE), is the most extensively studied for assessing ventricular wall motion, volumes, mass, and EF. It is particularly useful for visualizing turbulent flow associated with valvular stenosis and regurgitation.

Limitation

In patients with severe ventricular dysfunction, slow blood flow along the endocardial surface reduces one’s ability to visualize low contrast in the LV myocardium.

Description

Using spoiled gradient-echo techniques, the movement of blood through the LV cavity provides bright contrast against the gray appearance of the myocardium.

Limitation

This technique underestimates turbulent flow associated with intracardiac shunts or valvular heart disease (Table 16-1).

Description

SSFP imaging exhibits high signal-to-noise ratio (SNR) and high contrast-to-noise ratio (CNR) with the blood-myocardial interface. With SSFP, cine MR images depict the endocardial surface, regardless of blood flow velocity.^7,8 This technique can be used with brief repetition times repetition times (TR) (<3 ms), leading to short scan times with high temporal resolution. Studies comparing FLASH and SSFP imaging have demonstrated significant improvements with SSFP in the blood-myocardial interface in patients with decreased LV ejection fraction.⁹ With short echo times of less than 1.5 ms, the blood pool within the cavity appears more uniform and image quality is not hampered by turbulent flow.

Indication

Echo-planar imaging (EPI) is useful for obtaining a qualitative assessments of LV wall motion in patients unable to perform breath-holding.

Limitation

Because of low spatial and temporal resolution, this technique is not well suited for precise determination of volumes and EF.¹⁰

Description

Ultrafast EPI is a fast imaging sequence in which multiple echoes are acquired following each excitation that are then used to perform single-shot imaging. “Snapshots” of LV wall motion acquired during relatively short imaging times (e.g., 30-40 ms/slice) are produced with this technique by using advanced hardware to switch the gradients rapidly during scanning. This fast gradient-echo technique exhibits a relatively low SNR and more chemical shift artifacts compared with conventional gradient-echo methods.

Indication

The sensitivity-encoding (SENSE) technique obtains images during short periods of breath-holding.

Limitation

There is a loss of SNR in some images.

Description

This technique uses sensitivity information from multiple coil elements to correct for k-space undersampling in the post-Fourier domain. Each coil has a different sensitivity profile so that undersampled acquisitions may be unfolded that result in decreased scan time. Images produced with the SENSE technique exhibit consistent contrast, with preserved spatial resolution (see Table 16-1).

Indication

This is highly accurate for assessing LV midwall myocardial function.

Limitation

With this technique, it is difficult to measure LV function along the LV epicardial and endocardial surfaces.¹¹

Description

Myocardial motion can be tracked in one, two, or three dimensions using tissue tagging.^12,13 With tagging, dark saturation bands, or “markers,” are placed across the myocardium for the purpose of quantifying LV function. These markers are induced by prepulse sequences applied immediately after the R wave, usually in planes perpendicular to the imaging plane. Quantification of intramyocardial deformations can be accomplished by tracking the intersection points of the tagging lines to demonstrate myocardial rotation, contraction, relaxation, and strain (Fig. 16-1). The SPAMM (spatial modulation of magnetization) technique,¹⁴ uses two perpendicular sets of parallel lines that form a rectangular grid on the image that can be tracked throughout the cardiac cycle. These tagging lines move with the myocardium during the contraction and relaxation phases of the cardiac cycle (see Fig. 16-1).

FIGURE 16-1 Assessment of LV strain by MRI tagging. Shown are tagged MRI images of the mid–left ventricle in short axis at end-diastole (left) and end-systole (right). The deformation of the grid can be seen visually and can be used to determine the quantitative circumferential strain. For two-dimensional analysis, motion of the heart wall that transformed a hypothetical unit circle during diastole into an ellipse during systole corresponds to the directions of the eigenvectors of the transformation. Their lengths are the eigenvalues shown in the diagram below the images. The radial thickening or displacement is the difference between c′ length and c length.

Another tagging technique, complementary SPAMM (C-SPAMM), results in myocardial tag persistence throughout the entire cardiac cycle by using a negative tagging signal during the diastolic phases of image acquisition. The C-SPAMM technique allows for the study of both systolic and diastolic myocardial deformation resulting from improved tag contrast relative to the background myocardium. A disadvantage of C-SPAMM involves a longer image acquisition time. However, this technique is useful for patients with diastolic dysfunction or elevated heart rates (e.g., during dobutamine stress).

As soon as the images are acquired, there are three methods for extraction of motion data from tagged images: (1) tracking the dark tag lines as intensity minima; (2) eusing optical flow analysis; or (3) applying harmonic phase (HARP) determinates. All three of the tracking methods (Table 16-2) are highly accurate for assessing midwall myocardial function but exhibit some difficulty for discriminating tag intersection points near the epicardial and endocardial surfaces.¹¹

TABLE 16-2 Technique for Assessing Quantitative Analysis of Myocardial Motion