MRA Techniques

When performing 3D CE MR portography some important technical issues have to be considered. By the time a conventional extracellular MR-contrast agents reaches the portal vein, it is considerably diluted. This dilution is caused by the contrast extraction at the capillary level for redistribution into the extracellular compartment and Gd extraction in the liver [5]. Table 1 lists the MR contrast agents currently available in Europe and elsewhere for CE-MRA. Current commercially available gadolinium-based agents are extracellular in nature and most have similar T1 relaxivity values of between approximately 4.4 and 5.6 mmol-1 sec1. The one agent with truly unique physicochemical properties among the contrast agents listed in Table 1 is gadobenate dimeglumine (MultiHance®, Gd-BOPTA, Bracco Imaging SpA, Milan, Italy) which is currently approved in Europe and elsewhere for MR imaging of the CNS and liver and under investigation for applications in CE-MRA. Gadobenate dimeglumine differs from the other agents in two major respects. Firstly, unlike the other available gadolinium-based contrast agents which are excreted exclusively by glomerular filtration through the kidneys [6-9], Gd-BOPTA is eliminated from the body through both the renal (96-98% of the injected dose) and hepatobiliary (2-4% of the injected dose) pathways [10,11]. Secondly, due to a unique capacity among current agents for weak and transient interaction with serum albumin [ 12], Gd-BOPTA possesses a T1 re-laxivity in plasma (9.7 mmol-1 sec-1) which is approximately twice that of most of the conventional gadolinium chelates [13].

These facts have to be taken into account when determining the contrast dosage. Thus, when using conventional extracellular MR-contrast agents (i.e. agents with no albumin binding) a dosage of 0.2 mmol/kg body weight is recommended for dedicated portal vein imaging. This dosage can be lowered when employing Gd-BOPTA [14].

A rather lower flip angle of 20┬░-30┬░ is advantageous as it improves visualization of the diluted gadolinium in the portal vein. The images can be acquired in a coronal or axial slice orientation. Coronal imaging has the advantage of including the mesenteric arteries including the inferior mesenteric artery on the arterial phase (see chapter VI.3) and also including the superior and inferior mesenteric veins and retroperitoneal collaterals. The axial plane has the advantage of imaging the main portal vein and its branches "in-plane", which usually results in higher resolution compared to reformations. Another advantage of the axial acquisition is the fact that the entire liver is depicted allowing the detection and characterization of hepatic tumors. Finally, axial imaging has a smaller field-of-view without wraparound artifacts if frequency encoding is right-to-left. However, one limitation of the axial plane is wrap-around in the slice direction (superior to inferior). Bright fat adjacent to the imaging volume hampers image quality. This problem can be eliminated by utilizing a fat suppression technique and by using an imaging volume that matches the entire volume of the coil sensitivity. In this way, tissue above and below the coil will have limited signal to wrap into the image volume [5].

3D axial imaging volumes can easily be prescribed from coronal localizers. The axial 3D volume should extend from just above where hepatic veins enter the inferior vena cava down to well below the spleno-portal confluence. On slower MR scanners it may be necessary to use a slice thickness of 5-6 mm to obtain adequate coverage.

When imaging in the coronal plane, it is crucial to extend sufficiently anteriorly to include the entire portal and mesenteric venous system in the imaging volume. For planning of the 3D acquisition the portal vein should be readily identified on the localizer. Usually the main portal vein is depicted well on axial T1,T2 or gradient echo images.

High-performance gradient MR-scanners in combination with partial Fourier imaging can provide up to 50 imaging sections in a convenient 20 second breath-hold. MR scanners with inferior gradient performance may require thicker sections of up to 4 or 5 mm in order to extend far enough anteriorly (for coronal volumes) to include the portal vein and still be fast enough to be acquired during a breath-hold.

For portal venous phase imaging the breath-hold interval needs to be kept rather short. A patient who can suspend breathing for 40 seconds during the arterial phase may be too winded for another 40-second breath-hold during the portal venous phase. Therefore, it is best to keep the acquisition time under 30 seconds per phase to enable patients to suspend breathing twice in a row with only a few seconds rest in between [5].

Analysis of the portal venous and equilibrium phase images can be accomplished rapidly by performing a series of overlapping thick maximum-intensity-projections (MIP). Volume rendering may not work as well because of hepatic parenchy-mal enhancement.

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Essentials of Human Physiology

Essentials of Human Physiology

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