Basis of Mammography F. Milano Dept. Clinical Physiopathology University of Florence

Diagnostic sign characteristics - 1 Breast tissue typeDensity (g/cm3) Linear Attenuation Coefficient at 20 keV (cm-1) Mammary tissue Fibroglandular tissue Adipose tissue Skin Average breast (50% glandular – 50% fat) Lesion Carcinoma Calcifications

Diagnostic sign characteristics - 2 Low contrast visible mass centimeter in diameter Calcifications 100 microns

The mammography image

Variation of the linear attenuation coefficient with energy

Linear attenuation coefficient and interaction components

The photoelectric effect in breast The photoelectric effect is the dominant interaction at energy below 22 keV IMAGE CONTRAST The photoelectric effect is responsible for most of the energy imparted to the breast DOSE

Scatter radiation The scattered photons reduce the contrast in the image grid compression of the breast

Scatter / primary ratio in mammography

Spatial resolution Focus dimension Geometry Breast compression Detector characteristics

Mammography equipment Dedicated x-ray unit Dedicated detectors Dedicated image processing

The x-ray unit

X-ray spectral considerations for mammography The quality of the image and the dose with which it is produced are critically dependent on the x-ray spectrum used to form the image The shape of the spectrum is determined by the anode material, the applied voltage to the tube and the type of added filtration to the tube

Molibdenum k edge lines: 17.4 – 19.6 keV Molibdenum anode and filtration

28 kVp 25 kVp X-ray tube applied voltage and image quality

Contrast – Noise - Unsharpness

Signal-to-Noise ratio and photon energy

Spectrum suited to largest breast thickness Optimal energy band and breast thickness keV 2 cm keV 4 cm keV 6 cm keV 8 cm K edge Rhodium 20.2 – 22.8 keV

Mo/Mo – Mo/Rh – Rh/Rh spectra

Scatter radiation The effect of scatter is to reduce contrast The fraction of the possible contrast imaged in the presence of scatter is given by the Scatter Degradation Factor (SDF) SDF = 1 / ( 1 + S/P ) In mammography S/P range from 0.33 to 1.5 Only 40% (S/P=1.5) to 75% (S/P=0.33) of the possible contrast is imaged unless scatter is controlled If scatter is controlled image contrast can be improved by a factor of 1.4 – 2.5

Grid in mammography

Spatial resolution – Geometry – Focus size

Direct magnification No grid ( air gap ) Microfocus – 0.1 mm

Compression Breast compression is critical to optimizing image quality and dose Scatter Spatial resolution Movement artifact Absorbed dose

Compression A correct B incorrect Compression permits a breast uniform thickness

Effect of compression on scatter

Compression The breast compression makes the structures to be imaged closer to the detector improving geometrically the spatial resolution It also makes more visible the edge of the lesion

Compression plate and photon attenuation

Automatic Exposure Control (AEC)

AEC compensation for kV and breast thickness AEC is critical in the mammographyc practice

Film-screen system

Exposure mAs of 50 Exposure mAs of 100 Exposure mAs of 150 Image quality and AEC

AEC film-screen Film screen and AEC

Higher speed, darker image Higher gradient, greater contrast Effects of film-screen settings and processing

Gradient mAs mAs mAs mAs 80.7 mAs 93.1 mAs mAs 58.7 mAs 67.8 mAs 1.47 Speed Higher gradient, higher contrast image. Higher speed, darker image.

AEC to backAEC to front AEC and image quality degradation

Contrast and film sensitometry Gradient are different for different exposure

Patient dose It is important: To evaluate the risk to the patient To compare imaging systems To assess performances of the imaging equipment To comply patient radioprotection regulations

Dose and exposure parameters Exposure at breast entrance Dose to the entrance surface Dose to the midline Mean dose to the glandular tissue

Mean Glandular Dose (MGD) It provides the best indicator of the risk to the patient It cannot be measured directly It must be calculated from the results of simple measurements and tabulated values

Mean Glandular Dose (MGD) D = K p g K is the incident air kerma without backscatter to a 4 cm PMMA phantom p converts the air kerma from PMMA phantom to that for standard breast g converts the air kerma for the standard breast to MGD

p/g table and PMMA/breast thickness equivalence

MGD – kV – Breast thickness – Half Value Layer

Mean Glandular Dose

MDG – Entrance Skin Exposure (ESE) – kVp – Subject Contrast (Cs) – Signal to Noise ratio (S/N) t is the imaging time for a 5-mm cubic inflitrating ductal carcinoma Table 1 - Mo/Mo Table 2 - W/Mo

Dose – Mo/Mo – Rh/Rh

The END Thanks for the attention !