IMAGE ACQUISITION TECHNIQUES

“White blood” imaging with fast field gradient-echo, or steady-state free precession (SSFP), sequences provides the basis whereby cine images of LV and RV mass, volume, and ejection fraction (EF) are determined with MRI.⁶ By grouping the phase encodes and coordinating image acquisition with the rhythmic contraction of the heart, crisp, clear images of cardiac contraction can be obtained in 2 to 4 seconds.

Steady-State Free Precession

Indication

This method, also known as true fast imaging with steady-state precession (TrueFISP), is also used for assessing ventricular wall motion, volumes, mass, and EF.

Limitation

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

TABLE 16-1 Imaging Techniques

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.

Echo-Planar Imaging

TECHNIQUE FOR ASSESSING QUANTITATIVE ANALYSIS OF MYOCARDIAL MOTION

Tagged Imaging

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

Technique	Advantage	Limitations
Tagged imaging	Provides the data suited for strain assessment.	Difficulty resolving subendocardial and subepicardial differences. Requires high temporal resolution to avoid motion blurring Uses cine gradient-echo techniques
Phase contrast imaging	Provides comparable data with TDI for assessment of diastolic function and LV dyssynchrony. Can be used with retrospective gating to view the entire complete cardiac cycle.	Difficult to calculate myocardial strain. Requires high temporal and spatial resolution Susceptibility to motion artifact (such as blood related artifact and beat to beat variation) Cannot used to measure large movements
DENSE	Provides regional LV myocardial function assessments across the endocardium, mid wall, and epicardium. Provides data needed for tissue tracking for strain assessments.	Requires experienced center and specialized software