Neuroradiology And Ultrasound

Fig. 2.7. a-b Multiple cavernous hemangiomas. a Coronal T1 -subtle lesion in the temporal lobe (arrow). b Gradient echo T2. This sequence accentuates the low signal produced by calcium and hemosiderin, demonstrating multiple lesions.

room as they can be rapidly projected into the scanner with the possibility of injury to any adjacent personnel.

A comprehensive list of metallic implants, devices and materials tested for MR safety is listed in Shellock and Kanal's book [17] and up-to-date information is available on the Internet at

Magnetic Resonance Angiography

Magnetic resonance angiography (MRA) is performed using special sequences based on GE techniques. These sequences produce signal in flowing blood that can be distinguished from adjacent stationary tissue. The basic concepts involve either time of flight (TOF) or phase contrast (PC) techniques. With the TOF technique, stationary tissue is saturated with multiple radio-frequency pulses. The free protons within blood are unsaturated so can be excited and give back signal, i.e. appear white on the scan against a black background. PC techniques rely on change in phase of the transverse spin magnetization that occurs in moving protons (i.e. within moving blood) within the magnetic field.

The TOF technique is more generally used as it produces better spatial resolution and is more easily obtained than the PC technique, which is reserved for special applications such as assessment of velocities of flow and CSF flow studies. The PC technique is also helpful if there is a large amount of hematoma present as high signal from hematoma can interfere with the TOF images and obscure subtle pathology.

Once the acquisition has been performed, the "raw data" are processed by maximum intensity projection techniques, which pick out the high signal in the section (i.e. the flowing blood) and reconstruct it into an image that is similar to

Table 2.4. Some contraindications to MRI (see reference 17 for complete list)

Cardiac pacemakers Neurostimulators Intracranial aneurysm clip Artificial heart valves, e.g. Starr-Edwards valve Cochlear implants

Shrapnel - if close to vascular structure Metallic fragments in the globe of the eye a conventional angiography. Reconstruction in multiple planes can be produced and viewed through 360o. Other reconstruction techniques, such as multiple projection reconstruction and volume-rendered or endoluminal views, may be helpful in assessing complex vascular anatomy.

The use of gadolinium enhancement can reduce scanning time and is particularly useful where small vessels are being studied, e.g. spinal MRA. The patient does need to be cooperative and still for up to 10 minutes. This, and the noisy environment, mean that it is not ideal in the investigation of SAH, when CTA is quicker. Arterial digital subtraction angiography (DSA) remains the gold standard for angiography.

MRA is most useful in the detection and evaluation of aneurysms that have no history of SAH or where there has been a significant delay in the diagnosis. Standard MR sequences are also performed at the time of the examination as aneurysms may be seen as low signal. Aneurysms as small as 2 mm can be shown but an accuracy of at least 80% is seen for aneurysms of 5 mm and larger. Aneurysms that present with mass effect and cranial nerve palsy can be very accurately demonstrated (or excluded) by MRI with MRA. The non-invasive nature of MRA makes it very attractive as a screening test for aneurysms. This may be desirable in certain high-risk groups such as polycystic kidney disease or familial aneurysm disease. The implications of aneurysm screening for an incidental aneurysm have to be carefully discussed with the patient. Recent evidence suggests a much lower risk of hemorrhage (0.05%/year in small aneurysms) than have previous studies and also a higher morbidity/ mortality (up to 14%) for surgical treatment [18].

Follow-up of aneurysm after endovascular treatment may be performed with MRA. MRA can be helpful in the evaluation of AVMs, particularly when combined with standard MRI. Exact anatomical location and size of the AVM can be obtained from conventional spin-echo techniques, which greatly help management decisions about the possible permutations of surgery, radiosurgery and endovascular treatment.

Vascular stenosis, particularly of the carotid bifurcation, can be assessed by MRA usually as a confirmatory test following ultrasound.

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