Andersons pediatric cardiology 199

FIG 7.3 Fetal magnetic resonance oximetry using T2 mapping The top panel reveals the individual T2 preparation images targeting the descending aorta of a normal late-gestation fetus The signal intensity from within the vessel is plotted against the interval between the excitation and echo (T2 preparation time), yielding a T2 decay curve (bottom left) Oxygen saturation (SaO2) can be calculated from the corresponding T2 time based on the relationship shown (bottom right) a.u., Arbitrary units FIG 7.4 Fetal T2 mapping (A) Imaging planes for targeting short-axis images of the umbilical vein and descending aorta (B) A corresponding T2 map, which shows the higher signal in the umbilical vein than descending aorta, indicating the higher oxygen saturation in the blood returning from the placenta compared with blood supplied to the placenta by the fetus FIG 7.5 Validation of T2 mapping in vivo Vessel T2 was measured using T2 mapping and compared with conventional blood gas analysis of samples taken from indwelling vascular catheters in the pregnant ewe and sheep fetus, showing a similar relationship to the in vitro human calibration shown in Fig 7.3 To enhance the accuracy of human fetal MR oximetry, the magnetic properties of human fetal blood have been characterized at 1.5 and 3 T using in vitro preparations of umbilical cord blood obtained from elective caesarian sections and prepared through a range of oxygen saturations through graded exposure to nitrogen gas The T2 of human fetal blood is slightly longer than adult blood Importantly the relationship between T2 and oxygen saturation is dependent on hematocrit.13 This is analogous to clinical cyanosis, which is much more obvious in the setting of polycythemia because of the greater amount of deoxyhemoglobin in the blood for the same degree of desaturation The effect of changes in hematocrit on the relationship between T2 and oxygen saturation is shown in Fig 7.6 Thus, to perform accurate oximetry based on T2 mapping, it is necessary to quantify hematocrit The quantification of hematocrit also allows for an accurate measurement of the oxygen content of blood To measure hematocrit, the relationship between the T1 recovery of blood and its hematocrit can be utilized T1 is similar to T2 in that it represents another fundamental magnetic property, this time describing its longitudinal magnetization recovery The T1 of blood can be measured in a similar fashion to the way we measure T2, by making a series of images with different intervals between and inversion of the magnetization of the blood and the quantification of its signal A T1 recovery curve for blood is shown in Fig 7.7 However, while the T2 of blood is mainly dependent on oxygen saturation but also influenced by hematocrit, the T1 of blood is mainly dependent on hematocrit but also influenced by its saturation The effect of variation in oxygen saturation on the relationship between T1 and hematocrit are shown in Fig 7.8.13 Using a cubic polynomial solution, it is possible to calculate both oxygen saturation and hematocrit of a blood sample from a combination of its T1 and T2.14 This is illustrated by the graph in Fig 7.9, where the semicircular line represents all of the possible combinations of oxygen saturation and hematocrit for a given R2, where R2 is the T2 relaxation rate or 1/T2 The vertical line represents all the possible combinations of oxygen saturation and hematocrit for a given R1, where R1 is the T1 relaxation rate or 1/T1 The point at which the lines intersect is the only possible solution for hematocrit and oxygen saturation for this particular combination of R1 and R2 Therefore this approach yields blood oxygen saturation, hematocrit, and oxygen content (ignoring oxygen dissolved in plasma), where: FIG 7.6 Impact of variation in hematocrit on the relationship between T2 and oxygen saturation (SaO2) With increases in hematocrit (Hct), T2 shortens This is true for adult human blood (A) and fetal blood (B)