Myocardial tagging in polar coordinates with use of striped tags

Abstract

Regional deformation abnormalities in the heart wall provide a good indicator of ischemia. Myocardial tagging with magnetic resonance imaging is a new method of assessing heart wall motion during contraction. Current methods of myocardial tagging either do not provide two-dimensional information or lack a coordinate system well adapted to the morphology of the heart. In this article, the authors describe a new tagging method that provides a true polar coordinate system, with both radial and angular dimensions. This is accomplished with use of a section-selective version of spatially modulated magnetization resulting in striped tags (STAGs). These STAG planes are placed in the myocardium in a star pattern so that they intersect on the long axis of the heart and stripes appear through the width of the heart wall. In the short-axis view during contraction, rotation around the long axis yields angular information such as shear and twist, while separation of the stripes within the myocardium permits measurement of radial thickening. Therefore, this method provides a coordinate system for calculating two-dimensional strain that is adapted to the morphology of the left ventricle.

Figure 1

(a) MR image of canine left ventricle tagged with the preinversion pulse sequence. The line of intersection of the tag planes occurs along the long axis of the heart. (b) MR image of a canine heart tagged with the SPAMM sequence to create an orthogonal grid. Resolution allows one to three intersecting lines in the heart wall.

Figure 2

The preinversion pulse sequence. The RF inversion pulses are section selective. The angle of each tag plane is set by the x and y gradients (XGRAD and YGRAD), which, in this case, rotate in the tag plane.

Figure 3

The one-dimensional SPAMM sequence. The first RF pulse excites the whole sample with a 90° pulse. The gradient pulse then provides a phase shift in the transverse magnetization that is linearly dependent on position. The second RF pulse results in a longitudinal component of magnetization that sinusoidally varies with position. The spoiler pulse cancels signal from the remaining transverse magnetization.

Figure 4

The STAG pulse sequence. A phasing gradient is used after the first 90° section-selective pulse to linearly distribute the transverse phase in a direction along the selected section. The second 90°pulse rotates this transverse magnetization, with the magnitude of the longitudinal component dependent on the previous phase in the transverse plane. The spoiler pulses disperse any remaining transverse magnetization. RFI = in-phase channel of the RF, RFQ = the quadrature of the RF, XGRAD = x gradient, YGRAD = y gradient.

Figure 5

The STAG sequence is followed by a standard spin-echo imaging sequence. The beginning of the tagging sequence is gated by the upslope of the QRS complex. The tagging sequence is repeated at different angles for each STAG and the imaging sequence can be repeated at different delays after tagging to measure motion. ECG TRIG = electrocardiographic triggering, RFI = in-phase channel of the RF, RFQ = the quadrature of the RF, XGRAD = x gradient, YGRAD = y gradient, ZGRAD = z gradient.

Figure 6

MR image obtained with use of the STAG sequence of a 1-L water phantom. The quadrature head coil and a 16-cm field of view were used. The angular separation of the tags is reduced in this example to show the coherence in the sinusoidal striping in the tags. This effectively sets up a polar coordinate system.

Figure 7

MR images of a human heart obtained with use of the STAG sequence. Images were obtained 90 msec apart, with short-axis views centered on the long axis. Here, a 20-cm field of view was used and as many as four tag points occurred in the myocardial wall. As the heart contracts—a to b—points closer to the endocardium separate more than the epicardial points (arrow), demonstrating transmural dependence of wall thickening.