2 advantages of CT over plain film x-rays
1) CT can image in slices
2) CT avoids problems of superposition of structures
Is spatial resolution better in CT or plain film X-ray?
Spatial resolution is actually better in plain film X-ray
However, CT has extremely good low contrast resolution, enabling detection of very small changes in tissue type (CT can distinguish soft tissue, fluid, blood, fat, calcification)
What does CT measure?
CT measures linear X-ray attenuation of each pixel, which depends mostly on electron density in that pixel
Higher electron density = higher X-ray attenuation = brighter on CT
X-ray attenuation coefficients
There is an x-ray attenuation coefficient assigned to each pixel
Higher number = brighter on CT!
Water = 0
CSF = 4
White matter = 28
Gray matter = 34
Air = 1000
Bone = ~1000
(Fat = -87??)
CT with contrast
Can inject iodine-based contrast to detect leaky BBB
Iodine has high atomic number and is bright on CT
Normal brain does not enhance but vessels, tumors, infections do
Average linear attenuation coefficient for CT
The average linear attenuation coefficient between tube and detectors, "mu"
Reflects the degree to which x-ray intensity is reduced by a material
Each pixel is given a shade of gray corresponding to its "mu"
Generate Hounesfield numbers from "mu"
We get more info from CT compared to plain film x-ray, but it comes at a cost--what is that?
Much bigger doses of radiation with CT
What is the leading cause of atraumatic subarachnoid bleed?
Ruptured aneurysm causes subarachnoid hemorrhage (SAH)
Can see basal cisterns in front of brainstem have hyperdense (white) material
Baby with ventricles that are too big
Hydrocephalus because of congenital aqueductal stenosis
Large 3rd ventricle but normal 4th ventricle
Males have this more often
Ring enhancing lesions
Bacterial abcess: bright on DWI (ring enhancing lesion on MRI)
Tumor: dark on DWI (ring enhancing lesion on MRI)
Three easy steps of MRI
1) Bring hydrogen protons into equilibrium with big magnetic field
2) Disturb equilibrium (with radiofrequency pulses; create horizontal magnetization that wasn't there before)
3) Allow system to "relax" back to equilibrium
What does MRI depend on?
Hydrogen protons and their magnetic properties
Speed of relaxation varies for hydrogen protons based on their chemical environment (hydrogen in water vs. fat vs. protein)
Relaxation is governed by exponential time constants (T1 or T2)
At equilibrium, what are protons doing?
Protons spinning "out of phase" and produce net vertical magnetization but no horizontal magnetization
T1 vs. T2
T1: exponential time constant for spin-lattice relaxation; how quickly vertical magnetization recovers in that tissue; shorter T1 means brighter (fat bright and water dark)
T2: exponential time constant for spin-spin relaxation; how quickly horizontal magnetization disappears in that tissue; longer T2 means brighter (fat dark and water bright)
3 things that MR signal and appearance on MRI images depend on
1) Proton density per voxel (how many hydrogen protons are there to generate a signal)
2) T1 time of protons
3) T2 time of protons
What has greater tissue contrast, MRI or CT?
MRI has better tissue contrast!
Why is MRI gold standard imaging technique?
Even though it has lower spatial resolution (only 1mm), much higher contrast resolution
3 main types of MR images
1) Proton density (PD) weighted
2) T1 weighted (reflect differences in PD and T1 times)
2) T2 weighted (reflect differences in PD and T2 times)
How does MRI contrast (gadolinium) work?
Gadolinium shortens the T1 time of adjacent hydrogen protons, making them brighter
Note: MRI cannot "see" gadolinium because it can only see hydrogen protons, BUT gadolinium changes the environment of the hydrogen protons around it to create bright contrast in the area that it's in!
When is CT preferred over MRI, and why?
CT is fast and less motion sensitive so is used for fast screening
CT is better at detecting Ca2+ and acute blood so used for trauma
Used to rule out SAH, subdural hematomas, hydrocephalus
Positron emission tomography (PET)
Use radioactive tracer that emits protons, which annihilate electrons to create 2 photons given off at 180 degree angle and are detected to create an image
Measures tissue metabolism using F18 attached to glucose molecule (18-FDG)
More metabolically active group of cells will be "hot" on PET
Used to study brain activity, oncologic imaging, dementias, seizures, neurodegenerative disease, tumors
Lower spatial resolution (6mm)
Red = hypermetabolic; blue (and yellow) = hypometabolic
CT vs. PET
On CT, nodule looks nonspecific but on PET can see hypermetabolism which tells you that it's a tumor
(If no metabolic activity it might just be a benign nodule)