Saturday 20 October 2012

Imaging of Neck vasculature with emphasis of non contrast MR Angiography

In Neck atherosclerosis of the carotid arteries accounts for the majority of cervical vascular pathology and the most common indications for imaging include transient ischemic attack (TIA), ischemic stroke or carotid bruit. 
Different imaging modalities which are commonly used for evaluation of neck vasculature include DSA, Doppler, CT Angiography (CTA) and MR Angiography (MRA).
Each modality has its own pro and con, however DSA remains the gold standard.

Doppler is most commonly used screening test for carotid stenosis, due in part to the superficial location of the cervical carotid arteries and the ability of doppler to provide functional information. Again its relatively inexpensive, easily available, rapid and safe. Its main disadvantage is Operator and equipment dependent.

CT Angiography has rapidly evolved over the past decade with the advent of multidetector row CT scanners. CT Angiography can be performed at high spatial resolution and is better at detecting calcification than DSA or the MRA. Further, CTA does not suffer from the flow-related artifacts that can affect MRA interpretation. Unlike MRA, CTA is not limited by the presence of implanted devices such as cardiac pacemakers. Most patients can tolerate the short scan times of CTA. The main disadvantages of CTA are contrast-based technique and ionizing radiation. 

Magnetic Resonance Angiography
There are two main techniques TOF and PC, among them TOF is most commonly used.
We use non contrast 3 D TOF for brain as well as Neck as a routine screening along with parenchymal sequences for Brain, let's see the technical details in brief.

Time-of-flight (TOF) imaging, is based on differences in the excitation of flowing protons in blood and the stationary protons in background tissues, can be performed as 2D TOF as well as 3D TOF technique. 2 D TOF is rapid but susceptible to stair-step artifacts due to motion between the two adjacent slices. 3 D TOF technique offers ability to achieve a smaller voxel size, resulting in higher resolution, as compared to 2D TOF. However, the smaller voxels result in lower signal-to-noise ratio (SNR), and the use of a larger imaging volume predisposes the technique to signal loss from saturation effects. In addition, 3D TOF imaging offers a shorter allowable echo time (TE), which can minimize signal losses from phase dispersion.

Phase contrast (PC) imaging uses two gradients of equal strength but opposite polarity to induce a phase change in excited protons.  PC can thus measure flow velocity as well as directionality. Technical limitations and issues related to scan time limit the clinical application of this technique for neck angio.

Contrast-enhanced magnetic resonance angiography (CE MRA) refers to the injection of an intravascular contrast agent, to shorten the T1 of blood and thus provide contrast against surrounding tissue. A fixed delay time, a timing run, an acquisition triggered off detecting the arrival of contrast and temporally resolved strategies can all be employed.

Artifact and Signal Loss
Two major sources of artifact and signal loss in MRA are saturation effects and phase dispersion. The different MRA techniques have their own particular strengths and weakness is as well as differing susceptibility to the saturation and dephasing effects.

Saturation Effects
Slower blood protons may become saturated and lead to a signal loss. This effect is important for clinical assessment as it can simulate near-occlusive stenosis as complete occlusion. A similar phenomenon is noted in areas where blood recirculates or produce edie's current, as in the carotid bulb. The third example of such saturation effect is signal dropout from in-plane blood flow when blood is flowing parallel to the imaging plane rather than perpendicular to it, the blood will be within the imaging slice for a longer period of time, resulting in loss of signal. Such saturation effects can be minimized by making the imaging slice thinner or to increase the TR to prolong the time between excitation pulses. The ability of 2 D TOF to obtain a thin slice has an important advantage over 3 D TOF in demonstrating slow flow. However, a thinner slices will require more slices to cover a similar anatomic area of interest and has the potential to increase sensitivity to motion and, thus, stair-step artifacts. Prolonging the TR is problematic, as this will decrease the degree to which the signal from background stationary tissue is suppressed. MOTSA (multiple overlapping thin-slab acquisition) is a 3D TOF technique that uses a relatively thin-slab thickness to minimize signal loss from saturation while maintaining the other advantages of 3D TOF imaging. TONE (titled optimized nonsaturating excitation), is another technique used to reduce signal loss from saturation, which uses a variable flip angle to reduce the saturating effect of the excitation pulse as protons enter the image volume. The use of iv contrast in CE MRA is another very effective means of reducing saturation effect by shortening the T1 of the blood, allows for the use of thick-slab without the consequence of saturation. Therefore, a very large field of view (FOV) can be acquired parallel to the vessels of interest, such as a coronal acquisition to cover the aortic arch to the intracranial extent of the cervical arteries without signal loss from in-plane flow. 

Phase Dispersion
Phase dispersion, also referred to as dephasing artifact, is another source of signal loss in MRA. One solution to phase dispersion artifact is to decrease the size of the voxel so that each voxel contains a smaller range of velocities of flowing blood protons. The smaller distribution of velocities will result in less intravoxel signal loss from dephasing. Another approach to decreasing the dephasing artifact is to decrease TE. Among TOF techniques, a 3 D technique offers smaller voxels and shorter TE values than are generally achievable with 2 D techniques. Administering gadolinium compensates for the signal loss in 3D TOF imaging from the saturation effects described earlier while compensating for signal losses from dephasing.

Imaging of vessels that have undergone stenting. 
Both MRA and CTA will suffer from artifacts from stents. MRI-incompatible stents may demonstrate complete dropout of signal along the stented segment of vessel. Thin-section CTA can visualize the contents of a stented vessel, though the stent will cause streak artifact. CTA will overestimate the degree of in-stent stenosis and that CTA cannot reliably determine patency of a stent smaller than 4 mm in diameter.

Reference : CT and MR Angiography: Comprehensive Vascular Assessment edited by Geoffrey D. Rubin, Neil M. Rofsky.

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