Vascular phase

Vascular phase The vascular phase occurs 10 to 15 minutes after the tissue is injured. When blood vessels dilate (vasodilation), they become more permeable, enabling fluid and white blood cells to leave the plasma and flow to the injured tissue. 

The site of the injury becomes red, swollen, and warm and loses its normal function. This happens in two phases. The vascular phase is where blood vessels dilate and become permeable so fluid and white blood cells can leave the vessel. The delayed phase is where white blood cells infiltrate the tissue. 

Contrary to gadolinium-based and to iodinated contrast agents, microbubbles do not diffuse out of the blood circulation. As a consequence, after intravenous microbubble administration fibrotic tissue shows no enhancement in all vascular phases. At contrast-enhanced ultrasonography, localized cavernosal tissue fibrosis presents as a circumscribed perfusion defect. Diffuse fibrotic changes present with inhomogeneous enhancement ofthe corpora cavernosa. Contrary to the normal cavernosal tissue, delayed peripheral enhancement of variable degree is often appreciable, probably by the presence of viable subalbugineal cavernosal tissue fed by peripheral vascular pathways, while poor or no contrast enhancement is appreciable in the central portion of the corpora cavernosa (Fig. 21.10). 

The criteria for the characterization of focal liver lesions are derived from their behavior and degree of contrast agent enhancement in the different vascular phases of liver CT. For a valid characterization, usually at least two different phases are mandatory, so that changes of enhancement over time (e.g., wash-in and wash-out of contrast agent) can be appreciated. Nowadays, the necessity for an additional precontrast scan is usually denied however, some centers still use it, depending on the clinical situation and referring diagnosis. 

Fig.1.2.1a-f. Normal MR urogram in 3-month-old boy with antenatal hydronephrosis. Images a-c show same slice from each of three separate volume acquisitions, whereas d-f show MIP projections derived from the same three separate time points, a and d show the cortical phase, b and e were acquired 60 s later and demonstrate enhancement of both the cortex and medulla with the signal intensity of the medulla exceeding the cortex, c and f were acquired 115 s after the vascular phase and show excretion into the calyces, renal pelvis and ureters. The renal transit time was 2 min and 20 s bilaterally and the volumetric DRF was 51 49... 

Herts BR, Coll DM, Lieber ML et al (1999) Triphasic helical CT of the kidneys contribution of vascular phase scanning in patients before urologic surgery. AJR Am J Roentgenol 173(5) 1273-1277... 

Fig. 2.2. Time-density curves representing enhancement of aorta, portal vein and liver parenchyma during each scan. Please note clear separation of different vascular phases with early arterial phase demonstrating exclusive enhancement of arterial vessels with absolutely no venous contamination. EAP, early arterial phase LAP, late arterial phase PVT, portal venous phase P, equilibrium phase...

Microbubbles are less than 10 pm in diameter and are safe and effective echo enhancers. They are blood pool agents and do not diffuse into the ex-travascular fluid compartment. When given intravenously microbubbles produce marked signal augmentation on insonation for several minutes, in grey scale and Doppler, with up to 25 dB enhancement in echo strength. In addition to the vascular phase some agents exhibit a delayed liver-specific phase. 

Conversely, more recent low MI imaging, which is performed with SonoVue, can be performed in real time without substantial bubble destruction. This allows a lesion to be imaged continuously during its vascular phase as well as comprehensive surveying of the liver in multiple planes. Low MI imaging is now preferred in most instances. 

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