Arterial enhancement

Digital x rays can be modified electronically to increase contrast, display certain structures in color or three dimensions, or subtract areas that interfere, allowing clearer pictures of structures such as veins and arteries. Very small differences in density may be amplified for even clearer viewing. This may be important in cases in which a contrast medium is injected into veins or arteries enhancing small differences allows the use of smaller amounts of the contrast medium, lessening the danger to patients. 

Flitney FW, Megson IL,Thomson JLM, Kennovin GD, Butler AR. Vasodilator responses of rat isolated tail artery enhanced by oxygen-dependent, photochemical release of nitric oxide from iron-sulphur nitrosyls. Br J Pharmacol 1996 117 1549. 

Figure 4.10 demonstrates the limited utility of a scan when the delay is not chosen correctly. Both mathematical and animal models have demonstrated that, when there is reduced cardiac output, intravenous injection will result in a delayed intra-arterial arrival time and a greater peak arterial enhancement [65]. In patients with reduced cardiac output, therefore, once again, a 35- to 40-s prep delay is typically employed. Increasing the degree of arterial opacification can be accomplished simply by utilizing either a larger injection volume or a faster injection rate. With a test bolus, the time density curve generally has a slightly different geometry from that of the main bolus, as shown in Fig. 4.11. This is likely due to injection of a smaller...

Fleischmann D, Hittmair K (1999) Mathematical analysis of arterial enhancement and optimization of bolus geometry for CT angiography using the discrete Fourier transform. J Comput Assist Tomogr 23(3) 474-484... 

T1 postcontrast shows arterial enhancement Occurs at 2 h and can last 7 days... 

T1 postcontrast shows arterial enhancement Peaks at 1-3 days and resolves by 1 week... 

Fig. 6.3 Arterial enhancement from infarct. T1 weighted postcontrast image shows enhancement of the left MCA vessels in this hyperacute infarct...

Reduce preload. Preload is the volume of blood that fills the Nitroglycerin reduces venous tone (It also dilates ventricle during diastole. Elevated preload causes over - coronary arteries, enhancing blood delivery to the heart),... 

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). 

Metastatic involvement of the penis can present with distinct tumor nodules or diffuse infiltration of the penile shaft. In patients with diffuse secondary involvement of the shaft, the lesions can be barely visible at ultrasound except for mild alteration of the penile echotexture, diffuse or focal infiltration of the tunica albuginea, or irregular bulking of the penis. Occasionally, isoechoic distinct nodules cannot be identified at grey-scale ultrasonography. After intravenous microbubble administration, patients with secondary involvement of the penis present with profound alteration of penile vasculature. A variable early arterial enhancement is followed by a washout phase in which the corpora cavernosa present with inhomogeneous enhancement. Distinct isoechoic nodules may become appreciable after microbubble administration (Fig. 21.7) because they present with different enhancement characteristics, compared with cavernous tissue (Bertolotto et al. 2005). 

Arterial enhancement depends on iodine administration rate (or iodine flux) and can be controlled by the injection flow rate (ml/s) and/or the iodine concentration of the contrast medium (mgl/ml) (Fleischmann et al. 2000) (Fig. 8.3). Most of the angiography protocols used on 64-detector-row scanners today utilize very... 

The challenge for spinal artery CTA is to provide sufficient arterial enhancement but to scan before arrival of contrast medium in the venous system. An ROI of the bolus tracking system placed in the ascending aorta might be affected by inflow artifacts of the SVC and may result in a mistimed early scan. Therefore, placement of the ROI in the aortic arch or descending aorta is recommended. In the presence of aortic dissection, caution should be taken that the ROI is not too big or positioned in the false lumen or across the dissection membrane, respectively. In these cases, manual start of the scan should be considered. The Hounsfield unit threshold should be around 100 HU above baseline. Scan start is usually delayed by time for table movement (<3 s), which is usually right above the origin of the vertebral arteries. An additional scan delay of 3 s is recommended for scanners with equal to or more than 16 rows and rotation time equal or less than 0.4 s. Hounsfield unit values of attenuated blood in the thoracic aorta should never be lower than within the pulmonary trunk. 

However, the scanning time of a single-slice spiral CT scanner is too slow to obtain a pure arterial phase the result is a hybrid phase, with mixed arterial and portal venous enhancement (usually shces acquired first in the volume have a pure arterial enhancement whereas subsequent slices show portal dominant enhancement). 

In order to take full advantage of MDCT capabilities adequate timing and flow rate of injection should be carefully evaluated. Hepatic arterial enhancement is primarily influenced by the rate of iodine injection and timing of contrast bolus, whereas venous phase enhancement is determined by total dose of iodine administered to patients. 

The main factor in relation to arterial enhancement is iodine flux (mg of iodine entering the cir-... 

Venous phase enhancement is not modified by changes in flow rate since it is determined by the total given dose of iodine. So when total iodine dose is fixed but higher concentration and lower volume of contrast medium is used, an earlier and higher arterial enhancement and an improved depiction of hypervascular HCCs is obtained this results in a greater diagnostic efficacy, employing an equal iodine dose without an increase in cost, because the cost of contrast material for CT examinations depends on the total amount of iodine in the contrast material (Awai et al. 2002). 

Dynamic MR imaging, after contrast medium administration, is able to demonstrate the early arterial enhancement that reflects the presence of subcapsu-lar feeding vessels, and a rapid wash-out of the lesion. On portal venous and equilibrium phases hepatic adenoma appears isointense or slightly hyperintense, with focal heterogeneous hypointense areas of necrosis, calcification or fibrosis if present (Fig. 10.15) (Chung et al. 1995). 

Due to the high temporal resolution achievable nowadays by dynamic studies performed with multislice CT or hypergradient MR, the different phases of liver enhancement can be clearly separated. Since portal and venous vessel enhancement occurs slightly later than arterial enhancement, it may occur at this stage to depict a non-opacified intrahepatic venous vessel. When seen end-on these vascular structures may simulate a hypodense focal nodule, thus mimicking a true focal liver lesion. Besides their typical anatomical distribution, the direct visual comparison with later phases of liver enhancement should be able to rapidly solve the problem and avoid this common pitfall (Fig. 11.3). 

---