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25 Cards in this Set
- Front
- Back
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What imaging technique has the highest image contrast for soft tissues?
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MRI (nonionizing radiation!)
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Describe the precess of protons in water/fat. How does MRI take utilize this?
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Spin (Precess) of protons in water/fat is randomly oriented and creates no net magnetic vector
Upon application of a strong magnetic field (0.2-3.0 Tesla) there is a small net magnetization in longitudinal (Z-axis) The precessing protons change the magnetic field and induce a current which is detected by the conductor |
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How do protons influence signal intensity?
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Signal intensity determined by
Number of protons Relaxation properties of protons Biochemical environment |
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What occurs in T1 relaxation? How does it differ with tissue?
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External B shut off; magnetic field in Z plane increases and protons relax
Fat relaxes more quickly (highest Z plane magnetization; highest intensity) WM/GM about the same CSF has SLOWEST relaxation (lowest intensity) |
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What occurs in T2 relaxation? How does it differ with tissue?
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Magnetization in XY plane decrease over time
CSF has longest relaxation (highest intensity) WM/GM shorter (lower intensity) |
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Describe FLAIR imaging.
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T2-weighted but CSF signal nulled (CSF like T1-weighted image)
Fluid (CSF) Attenuated Inversion Recovery |
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What is the effect of gadolinium on MRI?
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Shortens T1 of tissues with deficient BBB, thus it enhances lesions (gives them greater intensity)
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Describe Diffusion-Weighted MR Imaging. Applications?
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Modifited T2-weighted image that detects water movement in tissue.
Thus, areas of decreased blood flow have increased intensity. Great for detecting areas of stroke. |
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Why is nephrogenic systemic fibrosis considered a contraindication for MRI?
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Contrast agent can make Gd detach from chelate and stick around longer in pt; decreased clearange (renal dz)
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What are the contraindications of MRI?
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Missile Effect (magnet is always on)
Pacemaker Internal Defibrillator Cochlear Implant (Generally safe: surgical clips, vascular stents, cardiac valces, joint prostheses, implantable pumps) |
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Describe the differences in x-ray attenuation based on tissue.
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Metal, Bone, Calcifications are RADIODENSE (RADIO-OPAQUE)--good attenuators of x-rays
Air, Fat, Muscle are RADIOLUCENT (poor absorbers) |
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Describe the differences on CT scans based on tissue.
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Same as x-ray:
Air is radiolucent CSF is radiolucent Bone and acute blood are radiodense (-opaque) (higher attenuation values = more radiodense) |
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Pixel vs Voxel
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Pixel: picture element
Voxel: cuboidal pixel |
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When are CT scans useful?
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Traumatic injury of head, face, spine (acute hemorrhage, fracture)
Acute stroke (exclude hemorrhage) Headache Temporal Bone and paranasal sinuses |
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When is Catheter Angiography useful? Risks?
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Diagnosis of aneurysm, arteriovenous malformations, vasculitis
Risks: Invasive procedure! Risk of neurologic complication (ischemia, stroke, death) |
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When is a Single Photon Emission CT useful?
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Diagnosis of dementia or seizure focus (ex: in AD would have decreased flow in temporal and parietal association areas)
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When is Pittsburgh Compound-B useful?
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Dx of AD (PCB binds amyloid proteins and emits positrons which are then detected by PET device)
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What are the limitations of ultrasound?
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Cannot penetrate ossified bone (can see brain if patient has carniotomy; or in neonates)
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