Exam 1 Definitions Flashcards
PSF (Point Spread Function)
- System response to Delta Function
- Output determined by convolution of input and the PSF
- LSI: Linear Shift Invariant System
- The convolution of a signal with the PSF is a smooter version of the input signal
Aliasing can be prevented if…
You filter the analog sample before sampling
Contrast
- Intrinsic: Depends on object parameters
- Display is changed by Look Up Table
- LUT is window and level
- Local contrast between target and background
- C=(t-b)/b
Resolution
Reduced FWHM = improved resolution
Transfer Function
Fourier Transform of the PSF
MTF (Modulation Transfer Function)
- Compare the amplitude of incoming wave to output wave
- To calculate: Measure ERF, Take derivative to get LSF, Fourier Transform LSF to get H(u), normalize
- Cutoff: MTF(u_c)=0
SNR
Tells us about the overall image quality
CNR
Tells us about the detectability of the image
NPS (Noise Power Spectrum)
Frequency Dependance of noise variance
QDE (Quantum Detection Efficiency)
Reflects the efficiency of X-ray detection, ignoring other elements
DQE (Detective Quantum Efficiency)
- Describes overall frequency-dependent SNR performance of a system
- Describes dose efficiency of a system
- How well the system converts (SNR_in)^2 to (SNR_out)^2
NEQ (Noise Equivalent Quanta)
Number of Quanta that would give observed SNR
Rose Model
Fluence has to be high enough to see lesion in background
ROC curve (Reciever Operating Characteristic curve)
- Relates the quality of an image to how well it conveys diagnostic information to the physician
- True positive fraction vs. False positive fraction
- Sensitivity vs. 1-Specificity
Particle displacement and velocity
- Have different phase relations with pressure
- Pressure is in phase with velocity and out of phase with displacement
Acoustic Impedance
indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a specific frequency
Q factor
- Q=f/bandwidth
- High Q = narrow bandwith
- light damping = high Q
Time Gain Compensation
User adjustable amplification of the returning echo signals as a function of time to compensate for beam attenuation
Resolution Cell
- For Geometric approximation, the resolution cell extends in Z over the duration of the pulse n_e
- The pulse width in x and y is determined by the beam spread (q(x,y))
- The face function
Speckle
- Caused by overlap of resolution cells
- Recieved signals are correlated, therefore the Ralyeigh random variable produce a spatially correlated random pattern
- this multiplies the desired image of reflectivity
- Recieved signals are correlated, therefore the Ralyeigh random variable produce a spatially correlated random pattern
- Granule size depends on size of resolution cell
Imaging Modes
- A mode
- B mode
- M mode
A mode Imaging
- Display of processed information from receiver vs time
- As echoes return from tissue boundaries and scatterers, a digital signal proportional to echo amplitude is produced as a function of time
- One line is produced
B mode Imaging
- The electronic conversion of A-mode information into brightness modulated dots along the A-line trajectory
- Brightness of the dots is proportional to the amplitude of the echo signal
M mode imaging
- Uses B-mode information to display the echoes of a moving organ
Penetration depth rule
Signal is lost from absorption
Repetition time rule
signal “dies” then repeat
Frame Rate Limits
- N scan lines are needed to make an image
- To increase: restrict field of view or increase frequency
CW (Continuous Wave Doppler)
- Measuring Blood Velocity
- A velocity spectrum display is achieved by dividing the recieved Doppler frequency into time segments and calculating the Fourier Transform of the signal interval
PW (Pulsed Wave Doppler)
- Combines the velocity determination of CW doppler with the range discrimination of pulse echo imaging
- Aquires a sample of the pulse at a fixed delay (f_R) using several pulses
- Similar to CW, the recieved signal can be divided into segments to make a velocity spectra
- Doppler Aliasing
- Can be avoided if PRF is 2 times the Doppler Frequency
- High PRF limits the depth of penetration
CF (Color Flow Doppler)
Provides a 2D visual display of moving blood in the vasculature, superimposed on the conventional gray-scale image
