Physics 300: Wk 2

Mammography

Why is mammography challenging?

• Breast pathology can be very subtle.

• Soft-tissue masses may have similar attenuation to surrounding tissue.

• Microcalcifications can be very small but are important early markers.

• Therefore, mammography requires high contrast, high spatial resolution and low dose.

Why does mammography require high contrast and high spatial resolution?

• To visualise subtle breast pathology.

• High contrast → distinguish small differences in tissue attenuation.

• High spatial resolution → visualise fine details such as microcalcifications.

What focal spot sizes are used in mammography and why?

• 0.3 mm and 0.1 mm

• Small focal spot → reduces geometric unsharpness.

• Therefore → improves spatial resolution.

• 0.1 mm is used for magnification studies.

What is the disadvantage of a small focal spot?

• Limits tube current.

• Requires longer exposure times to achieve sufficient mAs.

• Longer exposure → increased risk of patient motion.

Why does mammography use low kVp (kilovoltage peak)?

• Produces lower-energy X-rays.

• Increases subject contrast.

• Helps distinguish subtle differences in breast tissue.

Why are molybdenum (Mo) and rhodium (Rh) used as target materials?

• Produce characteristic X-rays in the useful mammographic energy range.

• Mo → 17.5 and 19.6 keV

• Rh → 20.2 and 22.7 keV

• These energies are suitable for mammography.

Why is tungsten (W) commonly used in digital mammography?

• Higher X-ray production efficiency.

• High melting point.

• Provides higher X-ray output.

• Common filters include Rh, Ag (silver) and Al (aluminium).

What is the heel effect and how is it used in mammography?

• X-ray intensity is:

  • Higher at cathode

  • Lower at anode

• Cathode is positioned over the chest wall.

• Anode is positioned toward the nipple/anterior breast.

• This helps provide more uniform exposure.

Why is the X-ray tube physically tilted in mammography?

• To provide adequate field of view (FOV).

• Needed to cover the large breast field.

• Effective anode angle = actual anode angle + physical tube tilt.

• Smaller anode angle → greater tube tilt required.

Why is beryllium (Be) used for the tube port?

• Approximately 1 mm thick.

• Low atomic number (Z = 4).

• Provides low X-ray attenuation.

• Maintains structural integrity.

Why is filtration important in mammography?

• Optimises the X-ray energy spectrum.

• Removes unwanted X-rays.

• Allows useful characteristic X-ray energies to pass through.

• Helps balance image quality and breast dose

What is K-edge absorption?

• Sharp increase in photon absorption when photon energy is just above an element's K-shell binding energy.

• Probability of the photoelectric effect is very high just above the binding energy.

What are the important binding energies for Mo and Rh?

• Mo: K = 20 keV

• Rh: K = 23.2 keV

• These K-shell energies determine how the filters shape the spectrum.

What are the Mo/Mo, Mo/Rh and Rh/Rh combinations used for?

• Mo/Mo → thin/fatty breasts

• Mo/Rh → intermediate thickness

• Rh/Rh → thick/dense breasts

• Increasing from Mo/Mo → Rh/Rh produces a more penetrating spectrum.

What should you know about Mo/Mo?

• Mo target + Mo filter.

• Characteristic X-rays: 17.5 and 19.6 keV.

• Mo filter K-edge = 20 keV.

• Produces a relatively soft spectrum.

• Best suited to thinner breasts

What should you know about Mo/Rh?

• Mo target + Rh filter.

• Rh K-edge = 23.2 keV.

• Allows some bremsstrahlung between 20–23.2 keV through.

• More penetrating than Mo/Mo.

• Used for thicker/denser breasts.

What should you know about Rh/Rh?

• Rh target + Rh filter.

• Characteristic X-rays: 20.2 and 22.7 keV.

• Highest effective energy of the three.

• Used for thickest/densest breasts.

• Never use a Mo filter with a Rh target because the useful Rh characteristic X-rays would be absorbed

What does Automatic Exposure Control (AEC) do?

• Measures the radiation reaching the detector.

• Terminates the exposure when a preset signal is reached.

• Helps achieve appropriate image quality and exposure.

What are the three AEC modes?

• Auto Time: operator selects kVp and target/filter.

• Auto kVp: kVp selected based on breast thickness.

• Full Automatic: system selects kVp and target/filter.

What can cause inaccurate AEC response?

• Heterogeneous breast tissue.

• Too much or too little compression.

• Defective cassette.

• Faulty photo timer.

• AEC failure can also be protected against by a backup timer.

Why is breast compression essential?

• Reduces overlapping anatomy.

• Decreases breast thickness.

• Reduces patient motion.

• Reduces scatter.

• Reduces geometric blurring.

• Reduces radiation dose.

What is spot compression?

• Further compression of a suspicious area.

• Approximately 5 cm diameter.

• Spreads breast tissue.

• Helps eliminate superimposed anatomy.

• Main disadvantage of compression overall → patient discomfort.

Why is scatter undesirable in mammography?

• Reduces image contrast.

• Can increase noise in digital mammography.

• Increases with breast thickness and field size.

How can scatter be reduced in mammography?

• Anti-scatter grid.

• Air gap.

• Grids improve image quality but increase exposure and therefore breast dose

Why is the grid removed for magnification mammography?

• Increased object-to-detector distance creates an air gap.

• Air gap reduces scatter.

• Therefore, a grid is generally unnecessary

Why is magnification mammography performed?

• Improves visualisation of fine detail.

• Particularly useful for microcalcifications.

• Typical magnification = 1.5×–2×

How is magnification mammography performed?

• Breast positioned closer to focal spot.

• Use 0.1 mm focal spot.

• Remove anti-scatter grid.

• Increased object-to-detector distance creates an air gap.

• Magnification = SID / SOD

  • SID = Source-to-Image Distance

  • SOD = Source-to-Object Distanc

What are the disadvantages of magnification mammography?

• Increased object-to-detector distance can increase geometric blur.

• 0.1 mm focal spot limits tube loading.

• Can increase exposure time.

• Longer exposure → greater motion-blur risk.

• May increase breast dose.

What are the advantages of digital mammography compared with screen-film?

• Higher dynamic range.

• Better image quality at low doses.

• Rapid image acquisition and evaluation.

• Reduced patient waiting time.

• Post-processing can improve contrast.

• Easier storage and display.

How does an indirect flat-panel detector work?

• X-rays → light → electrical charge.

• Cesium iodide (CsI) scintillator converts X-rays to light.

• Amorphous silicon (a-Si) photodiodes convert light to electrical charge.

• Thin-film transistor (TFT) array reads the signal.

• Light spread can reduce spatial resolution.

How does a direct flat-panel detector work?

• X-rays → electrical charge directly.

• Common photoconductor = amorphous selenium (a-Se).

• X-rays create electron-hole pairs.

• No intermediate light stage → reduced lateral signal spread.

What is the difference between "For Processing" and "For Presentation" images?

• For Processing:

  • After corrections for detector imperfections.

• For Presentation:

  • After post-processing.

  • Contrast and detail enhanced.

  • Optimised for viewing on a calibrated diagnostic monitor.

What are Computer-Aided Detection (CADe) and Computer-Aided Diagnosis (CADx)?

• CADe: identifies and flags suspicious features.

  • Masses

  • Microcalcifications

  • Asymmetries

  • Architectural distortion

• CADx: analyses suspicious findings and estimates likelihood of malignancy.

• Both support, rather than replace, radiologist interpretation.

What is stereotactic breast biopsy and how does it work?

• Used to accurately localise a lesion and guide biopsy.

• Breast is compressed.

• Two images taken at different angles, commonly ±15°.

• Lesion shifts between images.

• Shift is used to calculate lesion x, y and depth coordinates.

• Particularly useful for microcalcifications not easily seen on ultrasound.

What is Digital Breast Tomosynthesis (DBT)?

• Produces 3D-like breast imaging.

• Multiple low-dose projections are acquired at different angles.

• Reconstruction creates thin slices through the breast.

• Reduces tissue superimposition.

How does DBT reduce tissue overlap?

• X-ray tube moves in an arc, approximately 15°–50° depending on system.

• Multiple projections are acquired.

• Reconstruction produces images at different breast depths.

• Structures in the selected plane appear sharp while structures above/below are blurred

What are the advantages and disadvantages of DBT?

Advantages:

• Reduces tissue overlap.

• Improves cancer detection.

• Better visibility of masses and distortion.

• Particularly useful in dense breasts.

• May reduce false-positive findings/recalls.

Disadvantages:

• Multiple images and reconstruction required.

• May have higher dose than 2D alone.

• Microcalcifications may be better assessed on 2D.

• Longer reading time and greater storage requirements.

Why are dense breasts difficult to image?

• Dense breasts contain more fibroglandular tissue.

• Fibroglandular tissue appears white.

• Many cancers also appear white.

• Therefore, lesions can be obscured.

• May reduce mammographic sensitivity.

• DBT, ultrasound or MRI may be considered in selected patients.

What is Average Glandular Dose (AGD) and what is its formula?

• Preferred dose index for mammography.

• Used to optimise radiation dose to glandular tissue.

• AGD = K × g × c × s

• K = incident air kerma

• g = dose conversion factor

• c = breast composition correction

• s = X-ray spectrum correction

• Standard breast: 4.2 cm, 50% glandular/50% adipose → maximum 3 mGy per image.

What are the key mammography Quality Control (QC) requirements?

• QC and Quality Assurance (QA) are essential.

• Mammography phantom simulates a standard compressed breast.

• Phantom contains:

  • 6 fibres

  • 5 calcification groups

  • 5 low-contrast discs/masses

• To meet the stated quality standard at <3 mGy:

  • ≥4 fibres visible

  • ≥3 calcification groups visible

  • ≥3 masses visible.