Image analysis acquisition protocols

The technical considerations and interpretation of the second portion of the acute stroke protocol, CTA, is discussed in detail in Chap. 4. Importantly, however, the source images from the CTA vascular acquisition (CTA-SI) also supply clinically relevant data concerning tissue level perfusion. It has been theoretically modeled that the CTA-SI are predominantly blood volume, rather than blood flow weighted [20, 27,70], The potential utihty of the CTA-SI series in the assessment of brain perfusion is discussed in detail below. This perfused blood volume technique requires the assumption of an approximately steady state level of contrast during the period of image acquisition [27], It is for this reason - in order to approach a steady state - that protocols call for a biphasic contrast injection to achieve a better approximation of the steady state [71, 72]. More complex methods of achieving uniform contrast concentration with smaller doses have been proposed that may eventually become standard, such as exponentially decelerated injection rates [73] and biphasic boluses constructed after analysis of test bolus kinetics [72, 74]. 

Sequences must be optimized to achieve a high spatial resolution and a fast acquisition time. As sequences must allow analysis of the cartilage with high contrast to the surrounding tissue, thin sliced 3D spoiled gradient echo sequences with spectral fat suppression (Eckstein et al. 1998) or with selective water excitation (Hyhlik-Durr et al. 2000) are usually performed. The water excitation protocol allows the water-bound protons in the cartilage to be excited selectively and directly. Therefore, no prepulse is required as in conventional fat-suppressed imaging protocols. 

---