Acronyms
A measure of photon attenuation: HU
Housfield Units
This is how much SNR changges if 20 mCi instead of 5 mCi is injected for a PET scan
2x higher (SNR ~ √N)
Spin-lattice relaxation is associated with this time constant in MRI
T1 (spin-spin relaxation is associated with T2)
Treatment planning images
Simulation CT (taken on a dedicated CT scanner with flat bed, larger bore, and fancy laser setup)
Notably worsens as an object moves away from the detector and towards the x-ray tube
Resolution
Provided in the scanner dose report: CTDI and DLP
CT Dose Index (average dose output for a single bed position based on QC measurements), and dose length product (CTDI x length of scan)
This is roughly how long it takes for a sample of 20 mCi of 18F-FDG to decay to 2.5 mCi
Six hours (~ 2 hour half life, thus 3 half lives, 20/2/2/2 = 2.5)
True, Scatter, Random
Types of PET events
Angioplasty (placement of a stent in a coronal artery)
Fluoroscopy (provides an x-ray "video" to guide the placement)
It is improved with the addition of an anti-scatter grid
Contrast - fewer scattered photons (note: the grid stops photons, so unless the dose is increased the SNR worsens)
Reconstruction algorithm most commonly used in CT: FBP
Filtered back projection (PET and low dose CT use iterative reconstruction)
This is the physical size of an image with a matrix of 512 x 512 x 200 pixels and 2.0 mm isotropic voxels
25.6 x 25.6 x 10.0 cm
These two photon interactions result in an ionized atom
Photoelectric effect and Compton scattering (note: ionization only happens for photons of sufficient energy to knock electrons out of their orbits)
Detection of metastasis
Positron emission tomography
Improves with the introduction of iodine in CT imaging or gadolinium in MRI
Contrast
This replaced physical film storage solutions when medical imaging became digital: PACS
Picture Archiving and Communications system
CTDIvol in a helical CT scan changes from 4 mSv to this when changing the pitch from 1 to 2 (all else being equal)
2 mSv
This percent of electrons moving from cathode to anode in an x-ray tube produce usable x-rays in the target
1% (the rest goes to heat, which is dissipated by a spinning anode connected to a copper heat sink and surrounded by oil or water to absorb heat)
Blood flow in the heart
Doppler Ultrasound
This metric must often be sacrificed to improve spatial resolution
SNR
Software correction for the streaking seen on CT scans near dense structures: MAR
Metal Artifact Reduction
A 256 second spin-echo MRI scan becomes this long when the number of phase encoding steps increases by a factor of 2
512 seconds (note: changing the number of frequency encoding steps does not dramatically increase scan time because, unlike phase, it isn't repeated multiple times for each TR)
Beam hardening (the filter also helps even out the dose so it is not so high at the edges)
Verification of kidney stones
CT or US
Worsens for deep structures at high frequencies in US
SNR