The theoretical use of MinIP for assessment of infarcted myocardium relies upon the unproven ability for Minip to enhance distinction of the low attenuated infarcted segment from surrounding normal myocardium. In this study we compared thin slice MPR, 5mm MPR, 5mm MIP, and 5mm MinIP techniques for their utility in the detection of myocardial infarction. The results suggest that MinIP and thick MPR were the optimal image reformat methods.
A prior study by Bouvier studied MinIP reformats for the identification and quantification of aortic valvular stenosis8
. The authors demonstrated a two stage process of planimetry of the aortic value, initially using MinIP to distinguish the aortic valve orifice from surrounding soft tissues, then using MIP to exclude calcifications from the planimetry. In contrast to cardiac CT, more research has been conducted on the application of MinIP methods for noncardiac imaging including the detection of emphysema, CT cholangiography, and in pancreatic imaging9,10,11
. Following from the work of Napel12
, Remy-Jardin and colleagues reported an analysis of 13 subjects in which thin slices were compared to 3-, 5-, and 8mm slices reconstructed with MinIP and compared to histology for the presence and extent of emphysema13
. The sensitivity for the detection of emphysema was significantly greater with MinIP than with thin slices (82% vs. 61%, p < 0.01). The authors concluded that the 8mm MinIP slices provided the most optimal suppression of vascular structures, thus improving the detection of emphysematous pulmonary parenchymal changes. Satoh and colleagues showed that 10mmMinIP was superior to1mm high resolution CT slices for the quantification of emphysema as determined by histopathology and pulmonary function testing14
Beyond use of cardiac CT for the detection of myocardial infarction, ongoing work provides promise for the application of CT for detection of inducible myocardial ischemia and prior infarction. Preliminary research in this area has been promising in the context of adenosine administration15,16
, however such pharmacologic stress imaging protocols for the concomitant visualization of coronary stenosis and myocardial perfusion will be expected to benefit from scanners with high temporal resolution and/or greater volume coverage, given the relative limitations with single source 64-slice CT. Further research will be necessary to refine the optimal settings to evaluate perfusion, which may not necessarily be the same settings for inducible ischemia and infarction.
As investigation into the use of cardiac CT for the detection of myocardial perfusion defects continues, our analysis yielded several helpful findings. Qualitatively, infarcted segments viewed with the MinIP and thick MPR techniques were rated by the experienced reviewers as having significantly greater visibility and definition on both rating scales, as compared to the thin MPR and MIP techniques. When defects were evaluated on a quantitative basis, the absolute difference in Hounsfield Unit attenuation between normal and infarcted segments was significantly greater for the MinIP and thin MPR techniques. The intensity of infarcted segments was significantly greatest for the MinIP technique alone. Despite this methodological superiority, segments viewed with the thick MPR and MinIP still corresponded to a scale guide of “visible but ill defined” indicating further refinements in imaging techniques should be pursued.
The finding that the MinIP technique yielded segments with significantly greater relative difference is expected, given that infarction manifests as hypoattenuation and that the MinIP technique is designed to translate the voxel with the lowest attenuation value in a given slice. Similarly, it would have been expected that the MIP technique would perform poorly in the identification of hypoattenuation from infarction given that it is designed to translate the voxel with the greatest attenuation value in a given slice. In consideration of the thin slice (0.75mm) and thick slice average weighted (5mm) MPR techniques, it can be postulated that the thick slice technique was quantitatively less sensitive for infarction than the thin technique as the thick slices translate the average voxel attenuation, which may blunt identification of segments with low voxel values. The significantly greater CNR of the MinIP and thick MPR techniques likely helps to explain why the MinIP and thick MPR techniques were rated by the experienced reviewers as having significantly greater visibility and definition on both qualitative rating scales.
Beyond the post-processing technique utilized, several other considerations have been described as helpful in distinguishing infarcted myocardium 17,18
. These considerations include presence of a regional wall motion abnormality in the same segment as the perfusion defect and persistence of the suspected defect across multiple different phases of the cardiac cycle (e.g., present in both systole and diastole). In addition, the presence of fatty infiltration, wall thinning, and/or myocardial calcification of an infarcted segment can support the chronic nature of an infarction. The presence of fatty infiltration can be quantified as attenuation discrepancies, as evidenced by our previous findings of significantly lower CT attenuation values in patients with long-standing MI (-13 ± 37 HU) than those with acute MI (26 ± 26 HU) and healthy controls (73 ± 14 HU; P < 0.001)6
, and as analyzed in the quantitative portion of this manuscript.
In consideration of limitations, subjectivity is inherent in qualitative rating scales. A lack of precision may have been introduced by the comparison of five to eight 8mm-thick short-axis sections of the LV obtained with CMR with CT images of different slice thickness. Also of note, readers were permitted to adjust window width and level settings as desired during the qualitative assessment and the window width and level settings ultimately selected for each segment are not controlled for in this analysis. We and others have observed that use of a narrow window width and narrow window level (e.g. W 200, L 100) are helpful in the detection of infarcted myocardium18
, as shown in and . However the most accurate settings and their relation to other parameters have not yet been determined. Future analysis should consider window width and level settings in addition to the image reformation technique and slice thickness as well as a comparison of the precise extent of perfusion defect seen on CT with the various post-processing techniques to that seen on MR. Finally, it should be noted that this study was not designed to study diagnostic accuracy. Future analysis with a balanced number of controls will be needed to fully determine the performance characteristics of these techniques.