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Medical Physics 2: Radiology > Mammography & screening > Flashcards

Flashcards in Mammography & screening Deck (28)
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1

Core issues of mammo

1) Low tissue contrast
2) Subtle distortions in breast architecture
3) looking for small object - micro calcs ~0.1mm

2

Mammo geometry

1) FIxed geometry - except in mag
2) Uses anode heel effect
3) Small focal spot

3

Mammo x-ray tube

- Small focal spot
- Low kV (25-35kVp)
- Mo, Rh or W annode
- Mo, Rh Ag or Al filter
- Anode heel effect reduces intensity at nipple edge.

Mostly W target now

4

Impact of filtration

- Removes low E photons
- reduces skin dose
- increases mean energy of beam
- K edge filters generally used for mammo

5

Effect of k-edge filter

- Filter is relatively transparent below the k-edge
-> char x-rays transmitted
- Signif atten of bremstrah above k-edge

- increases contrast

6

Benefits of compression

- Immobilises breast
- Reduces the dose
- Improves image quality
- Reduces scatter within the breast - thinner tissue

7

Compression systems

Pre-compression : Position before full compression
Tilting paddles: conform better.
Compressibility detection available
Shifting paddle: L-R adjustment

8

Define AEC

Controls amount of radiation at the detector

Ensures correct exposure regardless of thickness / composition

Not used with implants

Must work with vary low rad level due to attenuation

9

AEC types

Basic: mAs

kV selection: dose rate det select on test shot
Adjust based on dose rate

Full auto: filt/voltage based on test shot
target/ filt/volt based on breast thickness
All based on test shot

10

Mammo detector type

Mainly a-Se and CsI with a-Si

11

Advantages and limitations of DR mammo

• Better contrast
performance
• Larger dynamic range
• Improved dose efficiency
• Imaging processing
• PACS
• Lower resolution
• Start-up costs
• Image processing

12

Advantages and limitations of film mammo

Analogue
• Contrast-latitude
compromise
• Limited dynamic range
• No post processing
• Stability of film processing
• Proven technology
• High resolution
• Affordable

13

DR spectrum

Harder more pen beam
reduction in contrast but can inc depth dose to compensate
Film would get too dark, lower patient dose while maintaining im quality

14

Phillips sys

Collimator defs many thin beams which scan accross

No need for a grid

15

Photon counting det features

Electronic noise effectively eliminated
• Secondary carrier noise eliminated
• Digitisation noise eliminated
• DQE potentially improved due to good detection
efficiency coupled with less detector noise
• Response has to be very quick

16

Measurement of mammo dose

Mean glandular dose - at-risk dose

ESD doesn't take into account changes at depth

17

Measurement of breast dose

• Simulated with PMMA blocks - QA
• Dose surveys
• Calculate entrance surface air kerma
• Derive MGD using HVL and composition dependent
conversion factors

18

NDRL

for 50-60mm thick breast NDRL is < 3.5mGy

19

Define stereo-tactic mammo

Add-on for standard mammo or dedicated

Uses two images recorded at different angles, combined to give depth information - via ref point matching

For placing guide wires

20

Digital breast tomosynthesis

• X-ray tube moves in an arc about a pivot point
• Breast and detector (in general) do not move
• Acquire a series of low dose projections
• Computer reconstructs data into slices at different depths in the breast

removes effect of overlying anatomy

large datasets

21

Breast cancer incidence

1 in 8 women will develop breast cancer

22

Patient Screen requirements

Originally aged 50-64 every 3 years

2005 - 50-70 every 3 years, 2 views

2016 - 43-73 every 3 years

23

Issues with younger women

less common
harder to image
increased risk of induction from screening

24

Protocol for older women

- Routine invites stop at 70
- Can still attend though

25

Risk vs benefit

Risk : 1 induced per 14000 screened 3 times over 10 years (aged 50-70)

Benefit : 1 saved for every 400 over 10 years

26

IRMER refferal

Women invited (referred) for screening if they meet the referral
criteria in the Cancer Reform Strategy document. Invitation letter
is signed by director of screening centre (or lead radiologist). No
individual signed request.

27

Irmer just / auth

• Radiologist, breast clinician or breast screening radiographers
are practitioners
• Identity of woman and eligibility confirmed before authorising
exposure.
• Consider previous screening history
• Record kept that woman meets local and national criteria
• Record confirming that examination has been justified,
authorised and by whom.

28

Exemptions to standard protocol

Women who have recently been screened
• Women who attend without invitation
• Women over current age range
• Partial examinations
• Technical recalls and repeats