Wall motion defects and sometimes signal enhancement in infarcted regions were discernible in the RT images obtained before Gd-DTPA injection. This helped us to choose the initial postinjection slice, and simplified the search for first-pass perfusion defects and DHE. The integrated approach to imaging all of these effects is thus beneficial from a workflow standpoint, and may reduce the total scan time.
It is apparent in some of the experiments (e.g., ) that infarcted tissue is visible in the RT-SSFP images without any magnetization preparation (12
). While we were able to demonstrate this consistently in the pig experiments with large infarct areas, smaller infarcts in the human studies were less readily apparent. This may be due to the close proximity of the infarct to the bright blood signal, and could perhaps be improved by the use of image postprocessing or blood saturation methods. To null the myocardium in steady-state imaging, one could use window/level adjustments or other postprocessing methods.
Since the RT-IR-SSFP image stream contains images of differing contrast, one could improve the user interface by displaying the myocardium-nulled image in a separate window from the view-shared images. However, for interventional applications, some observers found it preferable to view a single image stream containing the infarct enhancement, wall motion, and device in the same window. The myocardium-nulled image could also be color-coded and overlaid translucently on the view-shared images for more persistent guidance to infarcted regions.
For diagnostic applications, this technique could be extended to a multiple-slice RT method whereby images from each slice could be displayed in a separate window. With multiple-slice imaging, increased CNR would be expected, since the magnetization would recover more completely in a given slice while data are being acquired on other slices.
The longer data acquisition window used in RT imaging increases the difficulty in nulling the myocardium. It also increases temporal blurring of wall motion, and may be a cause of the observed overestimation of infarct size. One of the concerns about this RT technique is whether it can visualize small, nontransmural infarcts, given the effects of the longer acquisition window and the lower spatial resolution. Our initial experiences suggest that nontransmural infarcts are well visualized, but further study is needed to determine the smallest infarct that can be detected with the proposed technique. The data acquisition window can be shortened using parallel imaging methods, such as sensitivity encoding (SENSE) (13
). Temporal SENSE (TSENSE) has been shown to exhibit less temporal blurring of wall motion than view-sharing (15
), which may reduce this overestimation of infarct size. Parallel imaging should also eliminate the ghosting seen in the first view-shared image obtained after the myocardium-nulled image is acquired (16
In the proposed technique, spatial and temporal resolution are sacrificed for increased interactivity during scanning without gating or breath-holding. A protocol that does not require breath-holding should increase comfort, especially for very sick patients, and could result in better compliance. In addition, there is potential clinical value in the ability to observe wall motion and DHE in the same scan. In some studies with small infarcts (e.g., ), we observed DHE with no appreciable wall-thickening abnormality. This highlights the value of combining different types of cardiac MRI studies to obtain a more complete picture of an individual patient's heart structure and function.
The TIs required in RT-IR-SSFP for TD2 = 0 were shorter than those needed for IR-GRE. This is a consequence of playing the inversion pulse when tissues are at their SSFP values, before the magnetization is allowed to recover fully. This reduces the achievable SNR, but has the positive effect of increasing the overall frame rate, since a shorter TD2 reduces the required TI. However, SNR increases greatly when SSFP is used, resulting in satisfactory image quality. Another consequence of playing the nonselective inversion pulse with TD2 = 0 is that it is almost always possible to null both the normal myocardium and the LV blood pool. This facilitates identification of the endocardial border of infarcted regions.