case studies

Contrast Enhanced MRi of the Carotid Arteries

Contrast enhanced Magnetic Resonance Angiography (CE-MRA) of the carotid arteries enables excellent demonstration of carotid artery stenosis. CE-MRA has replaced non-contrast enhanced MRA using Time-of-Flight techniques as it reduces acquisition time and increases the signal-to-noise ratio (SNR) in the resultant images (Townsend et al., 2003).

A fast 3D Gradient Echo fat saturation sequence (see Figure 1) with a short TE and TR with a dynamic contrast injection of Gadolinium is used. The presence of gadolinium shortens the T1 of the blood, this phenomenon controls the image contrast. CE-MRA is therefore independent of flow artifacts and associated saturation problems.

The timing of the contrast injection plays a critical part in the data acquisition (see Figure 2). For a successful CE-MRA examination the average ‘delay time’ post injection for the carotid arteries is around 7 seconds.

To ensure maximum arterial contrast it is important that the centre of k-space is acquired when arterial contrast is at it's peak. There are different ways of filling k-space (see Figure 3). When centrically ordered filling is used an entire kss line is filled before the next kpe line. When elliptical centric ordering is used k-space is filled in a spiral fashion. This is the preferred method for CE-MRA as it is the centre of k-space that contributes to image contrast.

Excellent CE-MRA images (see Figure 4) can be achieved. A study undertaken by Wutke et al., 2002, found that when compared to conventional angiography CE-MRA demonstrated a 100% sensitivity and 92% specificity for the detection of carotid artery stenosis (Wutke et al., 2002). CE-MRA does have several limitations; firstly CE-MRA has in the past overestimated the extent of carotid artery stenosis. Secondly the technique is dependant upon successful co-operation of the patient by keeping still as any movement during the data acquisition could result in a non-diagnostic examination.

References
Townsend, T.C. & Saloner, D et al., (2003) Contrast material-enhanced MRA overestimates severity of carotid stenosis, compared with 3D time-of flight MRA Journal of Vascular Surgery 38, 1, 36-40 Wutke, R & Lang, W et al., (2002) High-Resolution, CE MRA with Ellipitical Centric k-space Ordering of Supra-aortic Arteries Compared with Selective X-ray Angiography Stroke 33, 1522-1529.
Ann Heathcote DC(R), BSc (hons), PgC (CT) – Mobile MRI/CT Technologist North Region
(poster presentation completed in part fulfillment for PgC MRI)

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