The fast imaging method used in this study relies on a gradient echo sequence with a very fast repetition time (TR) and a reduced number of phase encoding steps. A very short echo time (TE) minimizes signal loss due to local magnetic field inhomogeneities (T2* effects) and flow-related dephasing. To emphasize T1 contrast, the data acquisition interval is prefaced with a 180° inversion pulse. Because the total time for data acquisition is short relative to the T1 of the myocardium, tissue contrast is similar to a standard inversion-recovery sequence with an infinite TR. The inversion time (TI) is selected to null the signal from the unenhanced myocardium. During contrast administration myocardium perfused by Gd-DTPA will produce nonzero signal. Low flip angle ECG gated MR imaging with predominantly Tl-weighted imaging parameters performed 10 to 45 and up to 90 seconds after rapid bolus injection of Gd-DTPA with images every 3-4 seconds, each within a breathhold (at end inspiration), similar to the technique reported by van Rugge [10] with the Turbo-FLASH technique (as a single shot) was the basic acquisition technique [11] using a short axis double oblique angulation (as demonstrated in figure 1) at a level mid way between the cardiac apex and base. The number of R-R intervals between each acquisition was chosen to be as close to 3 sec as possible ( for example if heart rate is 75 then R-R interval is 800 ms and 4 R-R intervals is 3.2 sec) to allow full T1 relaxation between each image [18]. Twenty minutes later (to allow dominant clearance of most of the Gd-DTPA) the subject was administered 0.56 mg/Kg of Dipyridamole over a 4 min period under constant physician supervision with ECG , blood pressure, and pulse monitoring using a Marshal TM94 digital BP monitor (Omron Health, Vermont Hills, Il) and oxygen saturation measurements with a 4500 MRI Pulse oximeter (In-vivo medical instruments Winter Park, Fl). Two minutes later an additional 0.04mmol/Kg gadopentate dimeglumine was given by bolus technique as was an additional 10-30 mCi (370-1110 MBq) of Tc99m-sestamibi. Stress MRI images were therein obtained with the same technique and angulation. Sixty minutes after stress the SPECT images of the heart were obtained back in the nuclear medicine area.
Figure 1
Figure 1: Short axis slice at mid heart level half way between apex and base is where MRI studies were performed and compared to comparable level scintigram data.
All patients were imaged on a Siemens Magnetom 1.0T unit (Siemens medical systems, Islin NJ). Short axis turboFLASH images of the heart were obtained with the double oblique method described by others [5]. Imaging parameters included a bandwidth of 350 HZ/pixel, a repetition time (TR) of 12msec, and an echo time (TE) of 6 msec. The flip angle was 12°. Representative image is demonstrated in figure 3. An inversion time of 400 msec was selected. Since the myocardial T1 relaxation time at 1.0T of approximately 750 msec the null point for the tissue calculates to 500 msec. Because the net imaging time was 380 msec, the central views in k space, would be acquired at 590 msec (400+380/2), within the theoretical null point for myocardium. The images obtained were 10m thick and 64 X 128 interpolated to 256 x 256 with a 35-cm field of view (FOV). After the intravenous (via antecubital vein) bolus of 0.04mmol/Kg of gadopentate dimeglumine was administered, imaging was begun immediately so that 20-30 short axis images were obtained at the same level every 3-4 heart beats with interpolated voxel size of 1.37 x 1.37 x 10mm. All images were prospectively triggered from the R wave of the cardiac cycle and all were obtained within 380 msec during the RR interval. The patients were instructed to breathe quietly and to stop breathing during each brief acquisition which provided reproducible slice positioning. The relationship between signal intensity and contrast concentration [12] and thus perfusion has been shown by others using similar MR techniques [13].
Figure 2
Figure 2: Radial sectors are measured in 45 degree increments and the time signal curves are collected for each sector. This data is normalized and interpolated to create the radial analysis presentation. Each concentric shell represents a time point, hence the signal intensity at radial position 0 corresponds with the signal intensity as a function of time at the lateral wall. In this example 4 shells correspond to 4 sequential time measurements.
No comments:
Post a Comment