Neuroimaging

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When was CT imaging developed?: 1970's
When were MRI scans developed?: 1980's
Plain film analysis is used to evaluate...: structures of the skull, facial bones, and sinus
Plain film analysis is still used to detect...(5): fractures; abnormal calcifications; developmental abnormalities; osteolytic disorders; osteoblastic disorders
CT & MRI imaging planes are similar to...: the horizontal (axial), coronal, and sagittal planes used to describe basic neuroanatomy
Sometimes the axial slices in CT scans are adjusted a few degrees off of the true axial plane. Why? (2 reasons): This enables the whole brain to be covered using fewer slices. This procedure also reduces radiation exposure to the eyes.
MRI axial slices are usually true horizontal slices: True or False: True
Scout, or localizer images do what?: They help the CT or MRI viewer see where the image slices are. These images should be included on all CT/MRI scans so that the exact angles can be documented.
CT was developed directly from what technology?: X-ray technology. So, similarly, CT measures the density of tissue.
CT scans differ in what two ways from conventional X-rays?: 1) Rather than taking only one view, the CT X-ray beam is rotated around the patient and takes many different views; 2) The CT X-ray data are reconstructed by a computer to obtain a detailed image of all the structures in the slice (e.g., liquid, air, bone, soft tissue)
Four facts about the single slice CT: 1) the scanner is shaped like a large ring; 2) the patient lies on the table & moves through the ring in small steps; 3) at each stop, X-rays are scanned through the patient and picked up by detectors on the opposite side of the ring; 4) the amount of energy absorbed depends on the density of the tissues traversed
Three facts about the Helical CT: 1) the patient does not have to stop when moving through the scanner; 2) instead of single slices, up to 4 rows of detectors provide multiple overlapping slices; 3) These advances greatly improved the resolution and speed of the CT scan
On a CT scan, white represents...: dense structures, like bone
On a CT scan, black represents...: less dense materials, like air
On a CT scan, hyperdense refers to...: bright areas
On a CT scan, hypodense refers to...: dark areas
On a CT scan, isodense refers to...: intermediate density materials similar to brain tissue
What color is CSF on a CT scan?: Dark gray
What color is fat tissue (subcutaneously just outside the skull) on a CT scan?: nearly black
What color is white matter on a CT scan?: Slightly darker than celluar gray matter due to its high myelin content (gray matter is slighly lighter than white matter due to its high water content)
Density in CT scans is often expressed in what unit?: Hounsfield units (HU)

Water = 0 HU
Air = -1000 HU
What do hemorrhages look like on a CT scan?: It depends on how old the hemorrhage is.
What does a fresh intracranial hemorrhage look like on a CT scan?: It shows up as a hyperdense area relative to brain tissue.
What does a one-week-old hemorrhage look like on a CT scan?: isodense
After a hemorrhage is 2-3 weeks old, what does it look like on a CT scan?: hypodense
Acute cerebral infarcts are readily detectable on CT scans: True or False: False. Acute infarcts (the first 6-12 hours) are often not seen on CT scans. After 12 hours, cell death and edema lead to an area of hypodensity seen in the distribution of the occluded artery.
What does a cerebral infarct that is weeks to months old look like on a CT scan?: The brain tissue surrounding the infarct may shrink, producing a local area of prominent sulci or enlarged ventricles. However, persistent areas of hypodensity may be the result of gliosis and brain necrosis with replacement by CSF.
What do neoplasms look like on a CT scan?: They may be hypodense, hyperdense, or isodense, depending on the type or stage.
What is helpful for visualizing neoplasms on a CT scan?: intravenous contrast dye
How do you detect mass effect on a CT scan?: Look for localized compression of ventricles, effacement of sulci, or distortions in other brain structures (e.g., herniations)
How does intravenous contrast in CT scans work?: The contrast material contains iodine which is denser than brain and will therefore appear hyperdense in areas of increased vasularity or breakdown of the blood-brain barrier. Often, images are obtained with and without contrast for comparison.
What is one caveat regarding intracaranial hemorrhage and CT contrast?: In suspected intracranial hemorrhage, it is very important to obtain a NONCONTRAST CT scan b/c small hemorrhages often appear on CT scans as whitish areas at the base of the brain. These areas could be masked by the normal hyperdense contrast material.
What is myelography?: A combination of a CT scan with a procedure in which a needle is introduced into the CSF which sends a contrast dye into the CSF. This allows for a better visual of nerve roots and impingements on the spinal CSF space.
For a head trauma, which is better, CT or MRI?: CT
For lower cost, which is better, CT or MRI?: CT
For a subtle tumor or infarct, which is better, CT or MRI?: MRI
For a brainstem lesion, which is better, CT or MRI?: MRI
For a fresh hemorrhage, which is better, CT or MRI?: CT
For an old hemorrhage, which is better, CT or MRI?: MRI
When speed is needed, which is better, CT or MRI?: CT
When there is a skull fracture, which is better, CT or MRI?: CT
When a person has a pacemaker, which is better, CT or MRI?: CT
When anatomical detail is needed, which is better, CT or MRI?: MRI
When assessing low-contrast or small lesions such as MS plaques or low-grade astrocytomas, which is better, CT or MRI?: MRI
What is the only cranial nerve observable by CT scan?: optic nerve
In a non-urgent situation in which a single more definitive image method is desired, which is better, CT or MRI?: MRI
Which is better for first screening for head trauma or supsected intracranial hemorrhage, CT or MRI?: CT
MRI was developed from...: Nuclear magnetic resonance.
What happens during an MRI?: A poweful magnetic field is applied that causes protons in the person to align their intrinsic spins in parallel with the magnetic field.
Which protons are the ones relevant to conventional MRI?: those protons forming hydrogen nuclei
Brighter areas on an MRI are known as...: hyperintense
Darker areas on an MRI are known as...: hypointense
T1-weighted versus T2-weighted MRI images: T1- looks like an anatomical brain section; it is better for identifying anatomy because the resolution is better.

T2- looks like a film negative; it is better for detecting pathological changes because while the resolution is poor, the contrast is good
Gray Matter appearance on T1- vs. T2-weighted MRI: T1- Gray
T2- light gray
White Matter appearance on T1- vs. T2-weighted MRI: T1- white (bright)
T2- dark gray
CSF/water appearance on T1- vs. T2-weighted MRI: T1- Black
T2- white
Fat appearance on T1- vs. T2-weighted MRI: T1- White
T2- Black
Air appearance on T1- vs. T2-weighted MRI: T1- black
T2- black
bone/calcification appearance on T1- vs. T2-weighted MRI: T1- black
T2- black
Edema appearance on T1- vs. T2-weighted MRI: T1- Gray
T2- White
Demyelination/gliosis appearance on T1- vs. T2-weighted MRI: T1- Gray
T2- White
In Proton density weighted images (a.k.a. first echo images)...: The contrast between gray & white matter is reduced. But it is better at detecting abnormalities in the parenchyma such as small areas of edema or infarction.
What are two factors that affect the intensity of MRI images/signals?: 1) the amount of hydrogen protons

2) Metallic implants can distort images
Intracranial hemorrhage on MRI: What is the characteristic course?: 1) On both T1 and T2, acute hemorrhage is hard to see b/c it is gray and looks like CSF

2) subacute hemorrhage looks white

3) Chronic hemorrhage looks dark. They also usually have a bright center with a dark rim.
Neuroangiography: definition: The study of the central nervous blood vessels and related vasculature using radiographs during injection of a intravascular contrast medium. It is the gold standard for evaluating vasular diseases in the head, neck, & spine.
Before the advent of MRI & CT, neuroangiography was used to...: detect slight distortions in the patterns of blood vessels suggestive of intracranial mass lesions
Now, neuroangiography is used to...(2): 1) visualize lesions of blood vessels

2) assess the vascular anatomoy of tumors when planning neurosurgery
Lesions optimally seen by angiography include:: Atherosclerotic plaques and other vessel narrowings

aneurysms

AVMs
Is angiography invasive?: Yes: It requires local anesthesia and catheter insertion, typically in the femoral artery
Provide three techniques for visualizing blood vessels that are less invasive than an angiography:: 1) Doppler ultrasound
2) Magnetic Resonance Angiography (MRA)
3) Spiral CT Angiography (CTA)

However, these techniques are not as sensitive or specific.
Doppler ultrasound: definition: used to measure flow and lumen diameter of large blood vessels in the head & neck; it is best for assessing atherosclerotic narrowings & is not good at detecting aneurysms or other vascular abnormalities
Magnetic Resonance Angiography (MRA): definition: detects direction and speed of blood flow; major vessels can be seen but smaller, branches cannot; it is used to detect decreased or absent arterial blood flow caused by atherosclerotic narrowing, thrombosis or dissection; can detect some aneurysms
Spiral CT angiography (CTA): definition: involves the rapid injection of IV contrast in conjunction with a helical CT to obtain images of blood vessels quickly; it can be used in patients where an MRA is contraindicated
Therapeutic Neuroradiography (3): 1) Wada Test

2) Treatment of brain aneurysms and AVMs- fill them with glue-like material via the angiography catheter

3) Stroke- current trials are underway in which thrombolytic agents are infused at the site of a clot to try and reestablish perfusion
Ultrasonography: definition: Uses sound waves above audible levels to generate diagnostic medical images
When is ultrasonography indicated?: When the target area is not blocked by bone or air. It is very useful in pediatrics b/c of the lack of ionizing radiation.
Ultrasonography is not useful for detecting...: extra-axial hemorrhages, masses, or white matter
In adults, ultrasonography can be used to detect...: vascular lesions of the head and neck
EEG: It is the original method for measuring brain activity and is useful in evaluating for seizure activity and in detecting widespread abnormality in brain function.
Is an EEG good at detecting focal brain lesions?: No. Its sensitivity and spatial resolution for detecting focal brain lesions is poor.
Evoked potential: It is similar to an EEG. The brain's electrical activity is recorded in response to specific stimuli.
Name five imaging techniques that produce images based on blood flow or dynamic blood flow volume (i.e., metabolic activity):: 1) Xenon regional cerebral blood flow mapping (Xe rCBF);

2) Positron Emission Tomography (PET);

3) Single photon emission computerized tomography (SPECT);

4) Dynamic contrast functional MRI (perfusion MRI);

5) Blood oxygen level dependent functional MRI (BOLD fMRI)
What neuroimaging technique based on metabolic activity is particularly useful in evaluating brain tumors?: PET scans: high grade neoplasms demonstrate increased metabolism while low grade neoplasms demonstrate decreased activity.
Name four functional MRI (fMRI) techniques:: 1) Perfusion MRI;
2) BOLD fMRI;
3) Diffusion MRI;
4) Magnetic Resonance Spectroscopy (MRS)
How does Perfusion MRI work?: It involves the rapid injection of gadolinium and rapid measurements with "echo planar" imaging to produce an image of dynamic blood volume.
How does BOLD fMRI work?: It measures relative changes in oxy- and deoxyhemoglobin, which occur with changes in regional cerebral blood flow.
How does Diffusion MRI work?: It uses rapid echo planar imaging and strong gradients to measure the diffusion coefficient of water in brain tissue.
How does MRS work?: It can be used to detect local concentrations of certain chemicals in the brain including some neurotransmitters.
Which fMRI technique may eventually replace the WADA test?: BOLD fMRI
Which fMRI technique can detect early areas of ischemia long before conventional MRI imaging?: Diffusion MRI
Ictal-Interictal SPECT Difference Imaging:: It can be used to indirectly measure regional brain activity during seizures and to help localize their region of onset.
What does Ictal-Interictal SPECT Difference Imaging look like between seizures?: It shows decreased regional cerebral blood flow between seizures.
What does Ictal-Interictal SPECT Difference Imaging look like ictally?: It shows increased regional cerebral blood flow ictally.

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