Arterial phase

Choroidal fluorescence appears early and usually precedes the arterial phase by about 1 second. Depending on the origin of the new vessels, neovascular fluorescence coincides with the arteriolar or venous phase of fluorescence. Enhanced capillary permeability (leakage) delays fluorescence, followed by a slow increase in fluorescence as the dye recirculates and stains the affected tissues. 

M. Hypervascular hepatocellular carcinoma detection with double arterial phase multi-detector row helical CT. Radiology 2001 218 763-767... 

Bader TR, Prokesch RW, Grabenwoger F (2000) Timing of the hepatic arterial phase during contrast-enhanced computed tomography of the Uver assessment of normal values in 25 volunteers. Invest Radiol 35(8) 486 92. 

Shimizu T et al (2000) Helical CT of the liver with computer-assisted bolus-tracking technology scan delay of arterial phase scanning and effect of flow rates. J Comput Assist Tomogr 24(2) 219-223. 

Fig. 4.10a-c. Hepatocellular carcinoma, a US irregular echo texture with areas of shadowing. Enhanced CT of the liver arterial phase (b) and portal phase (c) shows the slightly enhanced lesion with a suggestion of a central scar (arrows)... 

On CT, an area of low attenuation with calcifications and disruption of the normal contour can be identified. On enhanced CT, the tumor will show a momentary enhancement that will be higher in the arterial phase compared with the portal venous phase. 

The appropriate examination technique is critical for sensitive detection and specific characterization of focal liver lesions. A biphasic examination of the liver with a late-arterial and a portal venous phase can be regarded as standard today. For specific indications, like the follow-up of hepatocellular carcinoma (HCC) after transarterial chemoembolization (TACE) or for the depiction of the arterial vessels prior to angiography, an early arterial phase scan, which can be post-processed into CT angiography, is helpful (Fig. 3.2). The value of delayed scans (e.g. 5 min after contrast agent injection) is controversial in the literature mainly centers with a focus on imaging in liver cirrhosis consider the use of late phase images as necessary, whereas other authors see no added value for it [21,45]. 

Fig. 3.2. Axial and coronal sections in the early arterial phase (left), late arterial phase middle) and portal venous phase (right) in a female patient suffering from hepatocellular carcinoma (HCC) under treatment with transarterial chemoembo-lization (TACE). In the early arterial phase only the liver arteries are properly enhanced the portal vein and also the liver parenchyma are not yet opacified. The two HCC nodules in segment 2/3 and segment 6 (marked by large arrows) are also both not properly demarcated. The early arterial phase, therefore, is not suitable for detection of hypervascular tumors it is rather a CT angiographic phase and can be omitted in most cases. The most important phase for detection of hypervascular tumors is the late-arterial phase (also called arterial-dominant phase or phase of portal venous inflow). In this phase there is already enhancement in the portal-vein (small arrow in the upper row) and in the liver parenchyma. The liver veins are not yet opacified in this phase (small arrow in the lower row). Most hypervascular tumors reach their highest attenuation in this phase. In the portal venous phase enhancement of the liver parenchyma is highest, the vascular enhancement in the portal venous system and in the hepatic vein is similar. Hypervascular tumors show decreased attenuation compared to the late-arterial phase depending on the degree of wash-out they can be still hyperdense (as in this case), isodense (see Fig. 3.4) or even hypodense...

Fig. 3.4a,b. MDCT in the late arterial (a) and portal venous phase (b) in a male patient suffering from a neuro-endocrine carcinoma with liver metastases (arrows). Note the strong wash-out of the metastases to nearly isointensity, so that even the larger lesions can retrospectively not be properly detected in the portal venous phase in contrast to the excellent conspicuity of the lesions in the arterial phase. This example strikingly demonstrates the importance of a correctly timed late arterial phase... 

Fig.3.7a-d. MDCT in the arterial phase (a) and MRI with the hepato-biliary contrast agent Gd-EOB-DTPA in the arterial phase (b), portal venous phase (c) and liver-specific phase 20 min after injection (c) each examined with a Tl-w 3D GRE sequence with fat-saturation in a patient with a neuroendocrine tumor and liver metastases. The hypervascular metastases were not detected with MDCT. In the arterial phase after Gd-EOB-DTPA injection hyperintense lesions are demarcated. The hypervascular metastases show a wash-out to isointensity in the portal venous phase. With Gd-EOB-DTPA liver-specific phase imaging can be performed in addition to the early dynamic phase. Note the signal increase in the normal liver parenchyma in (d) caused by the physiological Gd-EOB-DTPA up-take whereas the suspected liver metastases are demarcated as areas spared from specific Gd-EOB-DTPA up-take... 

Fig. 3.8. Male patient suffering from a multifocal hepatocellular carcinoma (HCC), treated with repeated sessions of transarterial chemoembolization (TACE). Dyna-CT (Siemens Medical Solutions, Erlangen, Germany) image started together with injection of 10 ml iodinated contrast agent via a super-selective catheter system placed in the right hepatic artery compared with a corresponding MDCT section in the arterial phase after i.v. injection of 120 ml iodinated contrast agent. Note the excellent, direct depiction of arterial blood supply of the HCC nodule in liver segment 5/8 (arrow). The artifacts in the left and right liver lobe are caused by spots of Lipiodol in already treated HCC nodules after earlier transarterial chemoembolization...

Fig. 10.2. Prolonged stable disease after whole-liver RE. More than 3 years after the procedure and without any evidence of radiological activity in the main noduie (enhancement in the arterial phase after contrast injection on CT or MRI), 20% of tumor was found to be viable on histological examination after liver transplantation...

Typical metastatic liver tumors have a thick hypoechoic rim with a relatively hyperechoic center, known as the huU s-eye sign (Fig. 16.10a) however, this finding cannot he applied to all metastatic liver tumors. Penetration of the normal vascular structure through the tumor proven hy color/power Doppler, ring-shaped enhancement in the arterial phase, and loss of enhancement in the postvascular phase as determined hy contrast ultrasound, are helpful findings for the diagnosis of metastatic liver tumors. 

It has been demonstrated that delayed acquisition uncovered up to 11% of endoleaks that were missed by arterial phase alone [19,20]. An optimal CT protocol for the monitoring of the aorta after endolu-minal therapy should include a delayed acquisition (Fig. 14.7). 

Fig. 14.7a,b. Diphasic helical CT. a Arterial phase demonstrates no endoleak. b Delayed phase showed a type II endoleak (white arrow)... 

In patients scheduled for a pelvic scan only, the start delay should be at least 60 s practical experience suggests that 70-90 s are optimal. With this start delay, there is contrast enhancement of the iliac, external, internal, and common veins already at the beginning of the scan and it is thus possible to exclude venous thrombosis or tumor infiltration of the veins. Since perfusion of the pelvic organs such as the uterus is not a major diagnostic concern, it is not necessary to perform a scan of the early arterial phase. 

An additional late scan 120-180 s after intravenous contrast administration may be helpful to exclude deep pelvic vein thrombosis or if there is poor visualization of pelvic veins on the arterial phase scans (Fig. 2.2). Depending on the patient s pelvic circumference, the late scan may be acquired with a reduced tube current, e.g. 50 mAsec. Late scans obtained after 3-5 min are also suitable for evaluation of the bladder, which, at this time, contains part of the renally eliminated contrast medium. Even filling of the bladder with contrast medium is achieved by rotating the patient about her longitudinal axis. 

On unenhanced CT scans, the layered anatomy of the wall cannot he distinguished. After administration of contrast medium, the myometrium can he distinguished in most cases during the arterial phase as it shows early and strong enhancement while the junctional zone/endometrium is of relatively low... 

By performing the pancreatic phase of contrast enhancement 40 s after the bolus of contrast injection and not during an arterial phase (20-25 s) of contrast enhancement, excellent arterial enhancement and adequate venous enhancement are usually obtained. This allows us to evaluate the peri-pancreatic vessels optimally (see Fig. 3.2). 

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