Carotid artery stenosis
The NASCET study determined that it was beneficial, with respect to patient outcomes, to treat surgically symptomatic lesions with 70-99% stenosis. The current gold standard for diagnosing such lesions is with X-ray angiography. However, due to potential adverse effects from X-ray angiography, it is desirable to find a magnetic resonance angiography (MRA) technique that can accurately depict stenotic narrowing of the carotid arteries.
Coronary artery imaging is hampered by motion due to the beating of the heart during the cardiac cycle and gross translational motion due to respiration. In the quest for diagnostic quality images of the coronary arteries using MRI, both of these motions must be addressed.
Cardiac motion imaging
There are two basic strategies for dealing with cardiac motion
- gating: data acquisition is timed based on some form of detection of the cardiac cycle, e.g. ECG monitoring or plethysmograph signals
- adaptive averaging: images of the heart or arteries obtained in a short enough amount of time to "freeze" the motion of the heart are used to correct for motion
For cardiac-gated studies, the two critical issues are at what point during the heart cycle should data be acquired and for how long. Any gated study implicitly assumes that the heart motion is periodic and will return to the same position at the same time as measured to some reference point (such as the R-wave of an ECG trace).
Respiratory motion imaging
Cardiac gating to diastole and with narrow acquisition windows does a good job of restricting cardiac motion. Unfortunately, the heart continues to move at a longer period due to respiration. There are a number of different schemes that have been proposed to deal with respiratory-based motions. These can be roughly divided into three main categories:
- Breath holding
- Respiratory gating based on some measure of respiratory position
- Phase reordering techniques
The key issues with any respiratory compensation method involves how good the image quality is when using the technique and how efficiently the data can be acquired.
A comparison of 2-D time of flight has been undertaken:
- twirl-time of flight (ultra-short TEs): no flow dephasing, off-resonance blurring and long acquisition time
- 3-D gadolinium-enhanced MRA: limited flow sensitivity, good SNR in rapid acquisition, limited spatial resolution and limited robustness/timing.
An example of the quality of images obtainable using 3-D Gadolinium-enhanced MRA