Filters

Question 1

High Pass Filters

Answer 1

• ## allow high frequencies through and attenuate low frequencies

Question 2

Ramp filter

Answer 2

• high pass filter
• increases resolution
• decreases noise
• edge enhancement
• decreases ring artifact

Question 3

Cut-off frequency

Answer 3

• where the filter magnitude drops to zero
• determines the maximum frequency retained in the images

Question 4

Order

Answer 4

• varies the slope of a filter funciton
• a higher order=steeper slope
• the term power (2n) is sometimes used

Question 5

Low-pass filters

Answer 5

• allow low frequencies through and attenuates high frequencies
• reduce image noise
• butterworth, hamming, hanning, parzen, shepp-logan
• butterworth window

Question 6

Butterworth filters

Answer 6

• low-pass filter
• max weighting fact of 1
• versatile
• shape of the filter is determined by the order or power and critical frequency
• ramp filter + butterworth window results in butterworth filter

Question 7

Hamming and hann filters

Answer 7

• low pass filters
• smoothing filters
• commonly used
• cut-off frequency is used to define the level of noise accepted/rejected

Question 8

Band pass filters

Answer 8

• allow a specific range of frequencies to be used
• highest amplitude in mid frequency range
• combination filters (roll off filters)

Question 9

Adaptive filters

Answer 9

• wiener and metz
• also called retorative or resolution recovery filters
• use criteria about system resolution and collimation in mathematical fillter function
• Suppress noise like low pass filters but also prevent blurring and smoothing of edges

Question 10

Wiener and metz filters

Answer 10

• multiply the middle frequency data by a value greater than one
• wiener: based on the SNR of the image
• Metz: basedon the measure MTF of gamma camera collimation system

Question 11

When to filter

Answer 11

• Filtering can be incorperated at various stages of the reconstruction process
• appliedin relation to backprojection, prior, during, and after