Venous contrast

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. 

Fig. 5.27a,b. Cystic medial necrosis. Due to the high grade proximal stenoses, test bolus measurement prior to CE-MRA failed in this case, leading to superimposition of venous contrast (a). Evaluation is nevertheless possible on original source slices (b), showing stenoses and aneurysms along the proximal supraaortic vessels (arrows)... 

DCE-MRA using MS-325 displays a long blood pool retention and allows steady-state imaging, which improves the vascular, arterial, and venous contrast enhancement [108]. This combination has been used for imaging the whole heart [109] and for cardio-venous imaging [110]. The pattern of contrast enhancement for cardiovascular imaging also depends on the type of CAused [111]. 

MDCT with high-resolution CTA is an excellent technique for the diagnosis, therapy, and follow-up of aortic pathologies. The examination protocol must be adapted to the clinical question in order to avoid excessive radiation exposure. Modern techniques Hke modulation of the tube current along the x-, y-, and z-axes can routinely reduce radiation exposure. In specific cases, it can even be further reduced by the reduction of the tube voltage. Depending on the cHnical question, nonenhanced scans and venous contrast-enhanced phases can contribute important information for diagnosis. 

Fig. 15.4a-c. Fibrolamellar hepatocellular carcinoma in the left lobe in a 21-year-old woman, a Non-enhanced transverse CT scan shows a large mass (long thin arrows) replacing the left lobe of the liver, hypoattenuating to the non-neoplastic parenchyma, with central calcifications (short thick arrow), b Hepatic arterial contrast-enhanced transverse CT scan demonstrates moderate, heterogeneous enhancement within the tumor (arrows), with a central hypoattenuating scar, c Portal venous contrast-enhanced transverse CT scan shows isoattenuation of the tumor. Note septa (arrows) radiating from the center to the periphery... 

In general, arterial thrombi are platelet-rich ( white clots ) and form at ruptured atherosclerotic plaques, leading to intraluminal occlusion of arteries that can result in end-organ injury (e.g., myocardial infarction, stroke). In contrast, venous thrombi consist mainly of fibrin and red blood cells ( red clots ), and usually form in low-flow veins of the limbs, producing deep vein thrombosis (DVT) the major threat to life results when lower extremity (and, occasionally, upper extremity) venous thrombi embolize via the right heart chambers into the pulmonary arteries, i.e., pulmonary embolism (PE). 

Radiographic contrast studies are the most accurate and reliable method for diagnosis of VTE. Contrast venography allows visualization of the entire venous system in the lower extremity and abdomen. Pulmonary angiography allows visualization of the pulmonary arteries. The diagnosis of VTE can be made if there is a persistent intraluminal filling defect on multiple x-ray films. 

In the case of local administration, lipoplexes are generally retained at the site of injection, with poor dispersion (22). In contrast to small emulsions or neutral liposomes, which immediately appear in the venous outflow perfusate following intratumoral injection, the appearance of cationic liposomes is highly restricted to the injection zone (22). The authors deduced that the determining factor altering the pharmacokinetic properties is not the rate of transfer from the interstitial space to the vascular site but rather the rate of transfer from the injection site to the well-vascularized region (23). 

Sodium nitroprusside (SNP) is both a venous and an arterial vasodilator. An important part of its vasodilator action is caused by the release of nitric oxide (NO), similarly as for the organic nitrates. SNP can only be administered via the intravenous route. It is a rapidly and short acting vasodilator. It has been used in the treatment of hypertensive emergencies and in the management of myocardial ischaemia. In spite of its vasodilator action it hardly influences heart rate, in contrast to hydralazine and minoxidil. The dosage of SNP should not be higher than 3 pg/kg/min within 48 h, in order to avoid the rise of cyanide ions and thiocyanate in the blood. 

In contrast to hydralazine, minoxidil, and diazoxide, sodium nitroprusside relaxes venules as well as arterioles. Thus, it decreases both peripheral vascular resistance and venous return to the heart. This action limits the increase in cardiac output that normally follows vasodilator therapy. Sodium nitroprusside does not inhibit sympathetic reflexes, so heart rate may increase following its administration even though cardiac output is not... 

Other required substances are transported from the arterial blood through the walls ofthe arterial capillaries into tissues and organs. In contrast, waste products and unrequired substances produced by the organs and tissues are transported to the venous blood through the walls of the venous capillaries, which combine into venules, and then into larger veins. 

The compounds potently inhibit factor Xa in vitro with reversible binding kinetics and are able to inhibit not only free but also prothrombinase-bound factor Xa (Ki 41 nM, 0.1 I nM, and 0,5 nM, respectively) (58-60), In contrast, no direct effect on platelet aggregation has been described (60-62), Antithrombotic activity in arterial and venous thrombosis models has been demonstrated and it has a reduced effect on hemorrhage in comparison to standard therapy (58,60,63). Factor Xa inhibitors are able to reduce the endogenous thrombin potential in platelet-poor as well as in platelet-rich plasma (64,65). Thus, thrombin generation seems to be a suitable biomarker for clinical evaluation and has been evaluated in phase I studies (66,67). 

Clinically, PC-MRA is often used for laminar flow with few pulsations as for example in the cerebral venous sinuses. Many users apply it as a thick slab 2D technique with short acquisition times and primary projective vessel depiction. 3D PC-MRA demands relatively long measurement times for data acquisition and is somewhat sensitive to patient movements. Principally, phase contrast methods additionally enable a quantification of blood flow velocity and the assessment of flow directions. 

Many causes of acute spinal cord infarction (of arterial and venous origin) have been reported (Table 17.2). They include diseases of the aorta and aortic surgery, thromboembolic events and cartilaginous disc embolism, vasculitis, coagulopathy, radiation-induced vasculopathy, toxic effects of contrast medium, epidural anesthesia, periradicu-lar nerve root therapy with crystalline corticoids, decompression illness, shock or cardiac arrest, lumbar artery compression and other etiologies... 

Fig. 18.2. a,b Normal venous anomaly in a 3D phase contrast venous angiogram performed at 1.5 T. c,d 3D phase contrast venous angiogram in a patient with idiopathic intracranial hypertension displayed in different projections. The bilateral short stenoses (arrows) are well shown by MR venography, e digital subtraction angiogram in an oblique projection. Confirmation of the obstructed vessel lumen on both sides (arrows), but the finding at this location can only be demonstrated on special projections... 

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