Exam 1 Definitions

Question 1

PSF (Point Spread Function)

Answer 1

• 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

Question 2

Aliasing can be prevented if…

Answer 2

You filter the analog sample before sampling

Question 3

Contrast

Answer 3

• 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

Question 4

Resolution

Answer 4

Reduced FWHM = improved resolution

Question 5

Transfer Function

Answer 5

Fourier Transform of the PSF

Question 6

MTF (Modulation Transfer Function)

Answer 6

• 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

Question 7

SNR

Answer 7

Tells us about the overall image quality

Question 8

CNR

Answer 8

Tells us about the detectability of the image

Question 9

NPS (Noise Power Spectrum)

Answer 9

Frequency Dependance of noise variance

Question 10

QDE (Quantum Detection Efficiency)

Answer 10

Reflects the efficiency of X-ray detection, ignoring other elements

Question 11

DQE (Detective Quantum Efficiency)

Answer 11

• 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

Question 12

NEQ (Noise Equivalent Quanta)

Answer 12

Number of Quanta that would give observed SNR

Question 13

Rose Model

Answer 13

Fluence has to be high enough to see lesion in background

Question 14

ROC curve (Reciever Operating Characteristic curve)

Answer 14

• 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

Question 15

Particle displacement and velocity

Answer 15

• Have different phase relations with pressure
  
  • Pressure is in phase with velocity and out of phase with displacement

Question 16

Acoustic Impedance

Answer 16

indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a specific frequency

Question 17

Q factor

Answer 17

• Q=f/bandwidth
  
  • High Q = narrow bandwith
  • light damping = high Q

Question 18

Time Gain Compensation

Answer 18

User adjustable amplification of the returning echo signals as a function of time to compensate for beam attenuation

Question 19

Resolution Cell

Answer 19

• 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

Question 20

Speckle

Answer 20

• 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
• Granule size depends on size of resolution cell

Question 21

Imaging Modes

Answer 21

• A mode
• B mode
• M mode

Question 22

A mode Imaging

Answer 22

• 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

Question 23

B mode Imaging

Answer 23

• 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

Question 24

M mode imaging

Answer 24

• Uses B-mode information to display the echoes of a moving organ

Question 25

Penetration depth rule

Answer 25

Signal is lost from absorption

Question 26

Repetition time rule

Answer 26

signal “dies” then repeat

Question 27

Frame Rate Limits

Answer 27

• N scan lines are needed to make an image
• To increase: restrict field of view or increase frequency

Question 28

CW (Continuous Wave Doppler)

Answer 28

• 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

Question 29

PW (Pulsed Wave Doppler)

Answer 29

• 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

Question 30

CF (Color Flow Doppler)

Answer 30

Provides a 2D visual display of moving blood in the vasculature, superimposed on the conventional gray-scale image