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Structural Imaging Modalities
Advantages and Limitations
· CT offers excellent spatial resolution (,1 mm) and is effec-tive at distinguishing tissues with markedly different X-ray at-tenuation properties (e.g., bone versus soft tissue versus fluid versus gas).
· CT is useful for the detection of acute bleeding (less than 24 to 72 h old) but less helpful in subacute bleeding (more than 72 h old) and in severely anemic patients (hemoglobin be-low 10 g/dL) (Osborn, 1994). In addition, CT is an excellent modality for imaging bone. CT is the imaging modality of choice for acute trauma or when an acute bleed or ischemia is suspected.
· CT is not helpful in visualizing subtle white-matter lesions due to CT’s poor ability to distinguish between the X-ray attenuation properties of different soft tissue densities.
· CT uses ionizing radiation and thus is strongly contraindi-cated in pregnancy. Women of childbearing potential should undergo a pregnancy screen prior to having a CT scan.
· Patient anxiety is usually less during a CT scan than during an MRI scan because the scanning environment is more open, there is less noise associated with the procedure, and scanning time is relatively brief ( 10 minutes for the brain).
· CT is best for patients with metallic implants (e.g., foreign bodies, some aneurysm clips, pacemakers, etc.) as MRI is contraindicated in this patient population
Advantages and Limitations
1) MRI provides excellent spatial resolution and superior soft-tissue contrast in comparison to CT (i.e., more useful for the visualization of white matter and white matter lesions). Also, while CT images are always axial images, MRI data can be resliced in any plane.
2)MRI is superior for surveying the posterior fossa and brain stem.
3) Fresh blood from an acute bleed (less than 48 to 72 hours) is not easily distinguished from gray matter. However, diffu-sion-weighted MRI may soon change this. Subacute bleeding (greater than 48 to 72 hours) or chronic hematomas are easily identified with MRI.
4) MRI does not use ionizing radiation, and so it is preferable to CT in pregnancy, but still is relatively contraindicated.
5) MRI is contraindicated in patients with metallic implants for the following reasons:
a) Metal can cause artifacts in MR images.
b) Metal can shift position or absorb heat within the magnetic field, causing burn injuries.
c) Mechanical devices such as pacemakers can malfunction within the magnetic field.
6)As many as 10% of patients undergoing MRI scanning ex-perience significant anxiety during the imaging session. This is likely due to a number of factors. MRI scanners are much deeper (often several feet deep) than CT scanners; patients generally perceive the MRI scanner as more tunnel-like and may experience claustrophobia. Pretreatment of potentially anxious patients with benzodiazepines or other sedatives/ hypnotics before undergoing MRI may be helpful in these situations. Also, the noise associated with the procedure is much louder than with CT. Lastly, the scanning time required
for MRI is longer than that required for CT (20–40 minutes versus 10 minutes for a study of the brain).
1) CT is the modality of choice for patients with acute bleeds or acute trauma, though diffusion-weighted MRI may soon become the modality of choice for assessing suspected acute brain ischemic events.
2)MRI is superior to CT for the differentiation of white from gray matter and the identification of white matter lesions.
3) MRI is superior to CT for the detection of posterior fossa and brain stem pathology.
4) CT is recommended if MRI is contraindicated (i.e., paramag-netic protheses; inability to tolerate scanner time, noise, or confinement).
5) MRI is recommended if radiation exposure is contraindicated (i.e., young children or women of childbearing potential).
General Guidelines for Structural Neuroimaging
1) On the basis of existing data, we have suggested criteria for ap-propriate structural brain imaging Patients with acute changes in mental status (including changes in affect, behavior, or personality) plus at least one of three additional criteria:
a) Age greater than 50 years;
b) Abnormal neurological exam (especially focal abnormalities);
c) History of significant head trauma (i.e., with extended loss of consciousness, neurological sequelae, or temporally re-lated to mental status change in question).
2)New onset psychosis.
3) New onset delirium or dementia of unknown cause.
4) Possibly for treatment refractory patients.
5) Possibly prior to an initial course of electroconvulsive therapy (may be helpful in identifying lesions that may lead to an ad-verse outcome such as aneurysms, tumors, arteriovenous mal-formations, hydrocephalus and basal ganglia infarction).
We estimate that adherence to the criteria listed above should yield positive findings in 10 to 45% of cases. However, only 1 to 5% will produce findings that lead to specific medical intervention. Lastly, if structural neuroimaging is indicated, one should use MRI unless the problem is an acute trauma or an acute bleed is suspected.
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