First-Pass Imaging
How to do first-pass imaging?
Optimal implementation of 3D contrast-enhanced first-pass MRA generally requires accurate timing of the contrast bolus. As with other gadolinium-based extracellular contrast media (ECCM), delayed imaging may result in suboptimal arterial enhancement and venous overlay. Alternatively, premature imaging may result in severe artifacts caused by a rapid change in the concentration of the intraluminal contrast medium concentration during the critical phase (contrast-sensitive phase) of the acquisition period.
This contrast-sensitive phase is based on the Fourier nature of MR image data and is determined by the low spatial frequency data (known as center of k-space) in opposition to the high spatial frequency data (known as peripheral k-space) which contains primarily information on spatial resolution. Some hardware manufacturers are offering alternative sequences with non-linear readout of center of k-space. Nevertheless, exact bolus timing remains essential for high-quality first-pass MRA. Optimal synchronization of the respective MR sequence and contrast injection is important to increase the image quality. This is achieved by collecting the center of k-space during maximal and constant arterial concentration of the blood pool contrast agent.
Why do blood pool contrast agents show excellent results at and beyond first-pass imaging?
Albumin-binding blood pool contrast agents have been specifically developed for vascular imaging. Gadofosveset eg, possesses additional chemical groups which allow reversible binding to plasma proteins (serum albumin) in the blood stream.
The binding action results in prolonged and significantly increased vascular enhancement – allowing both first-pass / dynamic contrast-enhanced MRA and a high-resolution approach with a long acquisition window.
Due to the fact that they permit repeated acquisition of MR angiograms, these blood pool contrast agents offer clear advantages over all other Gd-based agents on the market, which can only be employed for first-pass MRA. In clinical practice, the risk of a 'missed bolus' , which can necessitate a second examination – typically more than 24 hours later – is virtually non-existent with those blood pool agents.
Are there differences in first-pass MRA with blood pool agents compared to extracellular contrast media (ECCM)?
Performance of first-pass MRA with blood pool contrast agents is very similar to conducting it with ECCM. The experience gained to date has shown that no adjustment of sequence parameters is needed. A bolus-timing test procedure with 1 ml of contrast is recommended. To avoid venous enhancement in the arterial phase, more contrast should not be used. Alternatively, ’fluoroscopic' methods of real-time bolus tracking by means of fast dynamic 2D reconstructions, such as BolusTrak or CareBolus, can be applied, depending on availability and individual preferences.
In order to achieve comparable bolus lengths and adopt the proven ECCM dynamic relations between injection duration, scan delay and data acquisition, the rate of the blood pool contrast agent injection should be decreased by 1/3 to 1/2 compared to standard ECCM.