Andersons pediatric cardiology 198

FIG 7.1 Metric optimized gating The colors represent the cardiac phases and the numbers represent the lines of data comprising the magnetic resonance image An artificial trigger is used to collect gated imaging data Initially there is a misgating artifact, represented by the blurring of colors in the final images After iterative reconstruction where superfluous imaging data are discarded, the reconstruction matches the R-R interval that was present during the acquisition, represented by the uniform colors in the final images The correct reconstruction was identified through the lack of artifact in the resulting images FIG 7.2 Human fetal cine phase contrast magnetic resonance imaging with metric optimized gating (A) Imaging plane (short axis of ascending aorta) (B) Midsystolic phase from the cine, with anatomic images on the left and velocity encoded information on the right (C) Flow curves generated from the phase data The comparison between 1.5 and 3.0 T reveals improved signal-to-noise ratio (SNR), although the flow quantification is similar AAo, Ascending aorta; DAo, descending aorta; MPA, main pulmonary artery; RV, right ventricle Magnetic Resonance Oximetry MR oximetry exploits the different magnetic properties of oxygenated and deoxygenated hemoglobin, whereby the transverse or T2 relaxation time of the MRI signal returned from blood is proportional to its oxygen saturation The T2 of blood can be measured using an approach called T2 mapping A series of images of the blood vessels are obtained with different intervals between the excitation of the tissues and the measurements of the returning signal, as shown in Fig 7.3 With increasing intervals between excitation and echo, the signal returned from the blood decreases in an exponential manner with time constant T2 By plotting the signal intensity against the time interval, or T2 preparation time, the T2 curve and its time constant T2 can be obtained The use of adequate spatial resolution and accurate prescription of the imaging plane in the short axis of the vessel are also critical to avoid contamination of the signal being returned by blood with signal from the tissues surrounding the vessel of interest Fig 7.4 shows an example of a T2 map made in a normal late-gestation human fetus showing recommended imaging planes for the umbilical vein and descending aorta The signal is higher in the umbilical vein than the descending aorta, reflecting the higher oxygen saturation in the blood returning from the placenta compared with the blood being supplied to the placenta In a late-gestation fetus the larger vessels can be imaged using an in-plane resolution of 1.5 × 1.5 mm and a slice thickness of 8 mm, providing ample signal-to-noise ratio For smaller and more tortuous vessels, higher resolution is ideal, although this may compromise signal-to-noise ratio However, we have demonstrated the reproducibility of MR oximetry based on T2 mapping in human fetal vessels with a diameter of 5 mm or more.12 In fetal sheep, in which movement artifacts can be overcome using general anesthesia, we have demonstrated a high degree of correlation between vessel T2 and oxygen saturation measured using conventional blood gas analysis of samples collected using intravascular catheters, as shown in Fig 7.5