other Flashcards
(27 cards)
What is the Nyquist limit in Doppler ultrasound?
It is half the pulse repetition frequency (PRF/2); exceeding it causes aliasing.
What parameter is varied to suppress fat in STIR MRI?
The inversion time (TI).
What determines slice thickness in MRI?
gradient strength and RF pulse width.
What is the optimal Doppler angle for velocity estimation?
0 degrees, but clinically less than 60° is preferred.
What is the T1 recovery time of fat compared to water?
Fat has a shorter T1 recovery time than water.
What causes Gibbs artefact in MRI?
Undersampling of high spatial frequencies in k-space.
How does THI improve image quality?
By using harmonic frequencies, it reduces artefacts and improves lateral resolution.
What TR and TE settings produce a T1-weighted image?
Short TR and short TE.
What TR and TE settings produce a T2-weighted image?
Long TR and long TE.
What TR and TE settings produce a PD-weighted image?
Long TR and short TE.
What is the purpose of the 180° RF pulse in a Spin Echo sequence?
It refocuses dephased spins to form an echo at time TE.
What does T2* relaxation include that T2 does not?
Magnetic field inhomogeneities.
What is the purpose of the inversion time (TI) in inversion recovery sequences?
To null the signal from specific tissues (e.g., fat in STIR, CSF in FLAIR).
What causes Gibbs (truncation) artefact in MRI?
Undersampling of high spatial frequencies in k-space.
What causes aliasing artefact in MRI?
Field of view (FOV) is smaller than the body part → wraparound in phase-encoding direction.
What improves axial resolution in ultrasound?
Shorter pulse length (higher frequency).
What improves lateral resolution in ultrasound?
Narrow beam width (via focusing or THI).
What is the difference between Colour Doppler and Power Doppler?
Colour Doppler shows direction and velocity; Power Doppler shows amplitude only (more sensitive to low flow).
What is the main advantage of Continuous Wave (CW) Doppler?
Can measure high velocities without aliasing.
What is the formula for total spatial resolution in gamma cameras?
R^2 = R_{\text{intrinsic}}^2 + R_{\text{collimator}}^2
What improves collimator resolution?
Narrower, longer holes (but this reduces sensitivity).
What type of collimator produces a magnified, inverted image?
Pinhole collimator.
What limits intrinsic resolution in gamma cameras?
Crystal thickness and PMT resolution.
What is coincidence detection in PET?
Detection of two 511 keV photons within a time window (~5–10 ns), used to define a Line of Response.
