Arteriovenous transit time

Acquisition times of CE-MRA are determined by the desired spatial resolution. Given the physiologic arteriovenous transit time of approximately 8 s the use of an adequate read-out technique allows a total acquisition time of25-30 s without impairment due to venous contrast. This leads to a maximum spatial resolution of 0.5-0.8 mm with partition thicknesses below 1 mm. 

A novel technique is now available, CT perfusion (see Sect. 31.2.3), which offers a measure of tumor neoangiogenesis. It provides functional information represented by blood volume, blood flow, mean transit time, and permeabihty surface area (Fig. 31.8). In tumors, compared with normal tissue, blood volume, blood flow, and permeability surface area are usually increased, whereas mean transit time (representing an indirect measure of arteriovenous shunts) is reduced because of the higher number of shunts compared with normal tissue (Fig. 31.9) (Sahani et al. 2005). 

Fig. 31.8a-l. (continued) MDCT of rectal adenocarcinoma before and after neoadjuvant chemoradiotherapy. Mean transit time (seconds), increased from 4 (i) to 7 s (I), due to the presence of arteriovenous shunts... 

Microbubbles are also ideal tracers because of the small injected volumes and this has been exploited in the liver to identify conditions characterised by arteriovenous shunting such as cirrhosis and metastases an early hepatic vein transit time indicates a haemodynamic abnormality (Blomley et al. 1998). The dynamics of the wash-in and wash-out phases following a bolus injection can be calculated and used to form functional images which are truly quantitative (Eckersley et al. 1998). 

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