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32 Cards in this Set
- Front
- Back
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What is the scan time on a CT?
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Less than 1 second
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What is the major limitation of CT scan?
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Imaging the posterior fossa, where lucent linear artifacts (caused by x-ray attenuation by thick osseous structures at the skull base) project across the brainstem and cerebellum, and may obscure underlying lesions.
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What does CT allow for?
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Differentiation of gray matter from white matter and it shows the main divisions of the basal ganglia and thalamus
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Describe CT contrast agents
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Iodinated water soluble
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What is CT used for?
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Initial evaluation of stroke, head injury, or acute infection. Especially useful for patients with pacemakers or other metallic implants (cochlear implants, old aneurysm clips, metallic foreign bodies in the eye and implanted neurostimulators).
CT is prefered for early detection of acute intracranial hemorrhage, especially subarachnoid hemorrhage |
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When is CT superior to MRI?
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Evaluating cortical bone structures of the skull and spine
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When is MRI superior to CT?
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-At studying bone marrow
-CT is less sensitive in showing nonhemorrhagic infarction during the first 48 hours after -Infarcts within the brain stem and cerebellum |
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Compare CT and MRI at detecting acute infarcts in the first 24 hours
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CT: 58%
MRI: 82% Diffusion-weighted MRI: 95% |
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What is Contrasted Enhanced CT (CECT) used for?
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Detection of lesions that involve breakdown of the BBB, such as brain or spinal tumors, infections, or other inflammatory conditions.
Often used to rule out cerebral metastases, but is considered less sensitive than gadolinium enhanced MRI, especially in the detection of certain intracranial tumors, infections, and other inflammatory lesions that are associated with lesser degrees of BBB breakdown |
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Compare/contrast new and old CT contrast agents
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Older ones are high-osmolar contrast media (HOCM) while new ones are low-osmolar contrast media (LOCM)
LOCM are more expensive, but less allergenic and are associated with less morbidity than HOCM |
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What does an MRI measure?
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T1 and T2, tissue-specific relaxation constants
Proton densite |
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How do you differentially weight T1, T2, and proton density
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By varying:
1) The imaging technique (spin-echo, fast spin-echo, gradient echo, etc) 2) The repetition time (interval between repetition of the pulse sequence) 3) The echo time (the interval between radiofrequency excitation and measurement of the radiowave emission or signal) |
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How long does it take to acquire a spin-echo image with MRI?
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4-7 min for T1 weighted imaged
8-12 minutes for both proton density and T2 weighted images |
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How long does it take to acquire a fast spin-echo image?
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2-3 minutes
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What is FLAIR imaging?
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Fluid-Attenuated Inversion Recovery imaging suppresses signal from normal CSF and allows for many lesions to be more easily detected
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What are the major advantages of MRI?
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1) Greater soft tissue contrast which provides better definition of both normal anatomic structures and pathologic lesions
2) Multiplanar capability (i.e. axial, coronal and sagittal) and 3) Visualization of blood flow or CSF flow Other advantages include better visualization of the posterior fossa and spinal cord and the lack of ionizing radiation |
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Describe MRI contrast agents
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Based on chelates of gadolinium, a rare heavy metal
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How does gadolinium work as a contrast agent?
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The accumulation of Gd-media within a specific region of the brain shortens both T1 and T2 relaxation times, and appears as an area of increased signal intensity on T1-weighted images, even when precontrast images show no evidence of abnormal signal
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What modality do you use in the first evaluation of stroke patients
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Either CT or MRI
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Describe the use of MRI in stroke
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-Infarcts can be readily identified within the first 24 hours in over 95% of patients
-Earlier changes of cerebral infarction may be seen within the first 3 hours after the onset of stroke on diffusion-weighted images -This is related to the visualization of cytotoxic edema within affected cells in the zone of acute infarction -On FLAIR and proton-density-weighted images, hyperintensity may be identified later than 3 hours after onset within the affected cortical gray matter -During the first 5 days after stroke onset, Gd-enhancement may be seen within the small arteries of the ischemic cerebral territory, with gyral enhancement present 5 days to several months after onset -The focal reversible lesions of transient ischemic attacks are also seen more frequently on MRI than on CT |
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When is Gd-MRI better than CECT?
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It has greater sensitivity in detecting neoplastic nad inflammatory lesions
-Known to have higher sensitivity in detecting certain primary brain tumors that are often difficult to detect on CT, such as small schwannomas, optic nerve and hypothalamic gliomas, meningeal carcinomatosis and cerebral metastases |
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Describe imaging MS
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-MRI is better at finding the demyelinating plaques associated with multiple sclerosis.
-MS plaques are characteristically seen on T2-weighted images as multifocal hyperintense lesions with periventricular white matter and corpus callosum -Additional lesions within the optic nerves, brainstem, and spinal cord may be detected -Gd-MRI helps distinguish acute demyelinating plaques from more chronic ones |
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When is Gd-MRI the imaging modality of choice?
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-MS
-Meningitis, encephalitis, myelitis -Epidural abscess of emypema -Seperating recurrent disc herniation from post surgical scarring or fibrosis on patients who have received spine surgery |
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Describe imaging of AIDS patients
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-Gd-MRI can enhance lesions seen on non-contrast T2 weighted images in the cerebral white matter with AIDS patients
-Large single enhancing lesion -> cerebral lymphoma -Multiple small enhancing lesions -> cerebral toxoplasmosis -Symmetric, non-enhancing -> HIV encephalitis -Asymmetric non-enhancing -> progressive multifocal leukoencephalopathy |
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Describe the use of Gd-MRI in spine imaging
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-Herniated discs and degenerative spondylosis can be evaluated on MRI
-In patients who have been operated on, Gd-MRI is needed to separate recurrent disc herniation from postsurgical scarring and fibrosis -Also useful in identification and delineation of spinal tumors and infections |
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When is Gd-MRI not useful?
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-Not so useful when there are relatively few contrast-enhancing lesions are found
-Patients with complex partial seizures -Headache -Dementia -Head trauma -Psychosis -Congenital craniospinal anomalies |
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Describe imaging of the major arteries and veins of the neck and brain
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-On standard spin-echo images there are usually seen as areas of signal void due to relatively fast blood flow through vessels
-Gradient echo pulse sequence enables visualization of flowing blood as areas of increased signal intensity -After obtaining a series of contiguous thin sections with gradient-echo techniques, a map of blood vessels is reconstructed as a set of projection angiograms |
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Describe MRA images
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-Show vascular anatomy like conventional angiogram
-Advantage of providing views in nonstandard angiographic orientations -Avoids hazards of intra-arterial injection |
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Compare MRA vs conventional angiogram
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-Conventional angiogram is gold standard for cerebrovascular imaging
-Conventional angiogram has better resolution -Conventional angiogram is associated with 0.5-3% risk of neurologic complications |
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When is MRA used?
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-Stroke
-Transient ischemic attack -Possible venous thrombosis -Arteriovenous malformation -Vascular tumors -Cerebral aneurysms as small as 3mm -Useful for determining patency of arteries of circle of Willis in patients with acute stroke -Used on extracranial carotid arteries for detection of stenosis |
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When is MR venography used?
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-Visualize the cerebral venous sinuses and other major cerebral veins
-Particularly useful in diagnosis of venous sinus thrombosis |
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What are the problems with MRA?
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-Occasionally overestimates the degree of carotid stenosis
-Does not clearly demonstrate areas of ulceration within atherosclerotic plaques |
