RTC on RADIATION PROTECTION OF PATIENTS FOR RADIOGRAPHERS

RTC on RADIATION PROTECTION OF PATIENTS FOR RADIOGRAPHERS Accra, Ghana, July 2011 Optimization of Protection in Mammography IAEA International Atomic Energy Agency

Topics • • • Important physical parameters The mammographic X-ray tube Focal spot size High voltage generator Anti-scatter grid Automatic Exposure Control Dosimetry Quality control Digital mammography IAEA 2

Mammograms are the most technically demanding of all x-ray examinations • Low contrast objects • Normal tissues and cancers have similar density • Means we need low k. Vp, which in turn means high dose and need for compression • Very small high contrast objects • Means we need high spatial resolution • Breast has high radiosensitivity • Screening for cancer can start at age 45 -50 – cumulative dose can be high • Even for diagnostic mammograms, dose must be considered • Result is that QC is of great importance – best image at lowest dose IAEA 3

Film Characteristics – Tied to H&D Curve IAEA 4

What do we want to see ? Mass with spiculated margins Architectural distortion IAEA Clusters of micro/macro calcifications 5

• IF WE CAN’T SEE THESE OBJECTS, WE ARE WASTING PATIENT DOSE, AND PUTTING THE PATIENT AT RISK FROM UNDIAGNOSED DISEASE! IAEA 6

…………and more than most x-ray imaging, mammography demands attention to quality control to optimise dose, and to produce diagnostic images IAEA 7

X-ray Equipment • Dedicated x-ray • • equipment Compression paddle Grid Image receptor (screen /film, CR, DR) AEC IAEA 8

Main variables of the mammographic imaging system • Contrast: capability of the system to make visible small differences in soft tissue density • Sharpness: capability of the system to make visible small details (calcifications down to 0. 1 mm) • Dose: the female breast is a very radiosensitive organ and there is a risk of carcinogenesis associated with the technique • Noise: determines how far the dose can be reduced given the task of identifying a particular object against the background IAEA 9

Contrast • Linear attenuation coefficients for different types of breast tissue are similar in magnitude and the soft tissue contrast can be quite small • The contrast must be made as high as possible by imaging with a low photon energy (hence increasing breast dose) • In practice, to avoid a high breast dose, a compromise must be made between the requirements of low dose and high contrast IAEA 10

Variation of contrast with photon energy Contrast 1. 0 Ca 5 (PO 4)3 OH Calcification of 0. 1 mm 0. 1 0. 01 • The glandular tissue contrast falls below 0. 1 for energies above 27 ke. V Glandular tissue of 1 mm 0. 001 10 IAEA • The contrast decreases by a factor of 6 between 15 and 30 ke. V 20 30 40 50 Energy (ke. V) 11

Breast dosimetry in screen-film mammography • There exists a small risk of radiation induced cancer associated with X-ray examination of the breast • Achieving an image quality at the lowest possible dose is therefore required • Hence breast dosimetry is crucial IAEA 12

Radiation dose to the breast • Dose decreases rapidly with depth in tissue due to the low energy X-ray spectrum used • Relevant quantity: The average glandular dose (AGD) related to the tissues which are believed to be the most sensitive to radiation -induced carcinogenesis IAEA 13

Radiation dose to the breast • The breast dose is affected by: § the breast composition and thickness § the photon energy § the sensitivity of the image receptor • The breast composition has a significant influence on the dose • The area of the compressed breast has a small influence on the dose § the mean path of the photons < breast dimensions § majority of the interactions are photoelectric IAEA 14

Mean Glandular Dose (arb. Units) Variation of AGD with photon energy • The figure demonstrates the rapid increase in dose with decreasing photon energy and increasing breast thickness 20 10 8 cm 2 1 • For the 8 cm thick breast there is a dose increase of a factor of 30 between photon energies of 17. 5 and 30 ke. V 2 cm • At 20 ke. V there is a dose increase of a factor of 17 between thicknesses of 2 an 8 cm 0. 2 10 IAEA 20 30 40 (ke. V) 15

Focal spot size • For the screening unit a single-focus X-ray tube with a 0. 3 focal spot is recommended • For general mammography purposes, a dual focus X-ray tube with an additional fine focus (0. 1) to be used for magnification techniques exclusively is required • The overall system resolution should be measured yearly or when resolution decays rapidly IAEA 16

X-ray tube filtration • The beam quality is defined by the HVL • HVL with compression paddle in place at 28 k. V Mo/Mo to be over 0. 30 mm Al equivalent and typically < 0. 4 mm Al (see for example European Guidelines for QA in mammography screening for more exact numbers) IAEA 17

Mammography x-ray generators • A nearly constant potential waveform – usually • • medium or high frequency generaors The tube voltage range should be 25 - 35 k. V The tube current should be at least 100 m. A on broad focus and 50 m. A on fine focus. The range of tube current exposure time product (m. As) should be at least 5 - 800 m. As It should be possible to repeat exposures at the highest loadings at intervals < 30 seconds IAEA 18

Why an anti-scatter grid ? • Effects of scatter may significantly degrade the contrast of the image and the need for an efficient anti-scatter device is evident • Mammography should ONLY be performed with a (moving) grid IAEA 19

Automatic exposure control (AEC) • The system should produce a stable optical density (OD variation of less than 0. 2 ) in spite of a wide range of m. As • Hence the system should be fitted with an AEC located after the film receptor to allow for quite different breast characteristics • The detector should be movable to cover different anatomical sites on the breast and operate effectively over the full possible range of k. Vp and breast thicknesses IAEA 20

Measurement of breast dose • The AGD cannot be measured directly but it is derived from measurements with the standard phantom for the actual technique setup of the mammographic equipment • The Entrance Surface Air Kerma (ESAK) freein-air (i. e. without backscatter) is the most frequent used starting quantity for patient dosimetry in mammography IAEA 21

ACR phantom MGD – NSW screening Mean 1. 24 m. Gy 3 rd quartile 1. 28 m. Gy IAEA 22

Mammography Quality Control • BSS requires Quality Assurance for medical exposures • Guidelines prepared by EC, PAHO, ACR etc • A Quality Control program should ensure: • The best image quality • With the least dose to the breast • Hence regular check of important parameters IAEA 23

Parameters to be considered by a QC program (1) § § Focal Spot size (star pattern, slit camera) OR System resolution (line pair gauge) Tube voltage (reproducibility, accuracy, HVL) AEC system (k. Vp and object thickness compensation, OD control, short term reproducibility) § Compression paddle (compression force, alignment to patient chest wall) § Screen-Film (inter-cassette sensitivity, screen/film contact) IAEA 24

Parameters to be considered by a QC program (2) l Film Processing § Baseline (temperature, processing time, film OD) § Film and processor (screen cleanliness, sensitometry) § Darkroom (safelights, light leakage, film storage) l Film Viewing § Viewing Box (brightness, homogeneity) § Environment (room illumination) IAEA 25

Parameters to be considered by a QC program (3) System Properties § Reference Dose (average glandular dose) § Image Quality (image contrast, threshold contrast visibility, exposure time) IAEA 26

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Compression Typical force used = 100 to 200 Newtons = 10 -20 da. N (~10 -20 kg) IAEA 28

• NO (OR INSUFFICIENT) COMPRESSION MEANS: • Too much tissue to be penetrated by the beam ie. too much attenuation • Higher dose • Less visibility of soft tissue objects eg. Masses • More overlay of objects in the beam • Need typically >10 kgf (da. N) compression IAEA 29

Towels to protect compression plate and breast support Bathroom scales IAEA 30

Distribution of actual compression force IAEA 31

Image Quality • Uses special phantom (eg. RMI 156) • Tests visibility of “rods” (fibrosis), “specks” (calcifications), and “masses” (tumours) • Image taken with AEC IAEA 32

Image Quality Phantom (ACR) - X-Ray Should be able to see: • 4 thickest fibres • 3 largest specks • 3 largest masses Chest Wall Edge IAEA 33

Phantom Image Quality Evaluation • Measure the optical density at the centre of the phantom and in and outside the disc to calculate difference in OD. • Circle any artifacts or grid lines on the film. Investigate the source of artifacts. IAEA 34

Phantom Image Viewing Conditions • Phantom images should be read under optimal viewing conditions • Images should be masked to eliminate extraneous light around the film • Use a large field of view magnifying glass to assist in the visualization of specks. IAEA 35

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Routine measurements • All repeated measurements related to the image receptor should be performed with the same film cassette (or CR plate) to rule out AEC variations and differences between screens and cassettes, or CR plates • Mark the cassette as “Test” for example IAEA 37

Routine measurements - OD The optical density (OD) of the processed image taken under AEC with a uniform PMMA phantom 45 mm thick, is measured at a reference point, which lies 40 -60 mm from the chest wall side and laterally centered. The reference optical density is ~1. 6 -1. 8 including (base + fog) density (~0. 2) IAEA 38

Viewbox requirements • Mammo films have much higher optical density than conventional films • CANNOT effectively use a conventional film viewbox for mammo films • You will not see subtle density differences • You may miss seeing a tumour IAEA 39

Measurement of Viewbox Brightness or Luminance (cd. m-2) IAEA 40

Measurement of Room Brightness or Illuminance ( lux) IAEA 41

Viewbox requirements • Viewbox brightness (luminance) measured • • in Cd/m 2 A conventional viewbox might be 800 – 1500 Cd/m 2 A mammo viewbox should be >3000 Cd/m 2 …. and must be more uniform in brightness …. and (for the doctor) must have a mask to exclude light at the edge of the film IAEA 42

Other measures • Correct patient positioning • Retake analysis • Record QC data IAEA 43

Digital mammography • Film/screen mammography rapidly being replaced by computed radiography (CR) or direct radiography (DR) • These have special features compared to film, and require some changes in QA (requires a separate talk!!) IAEA 44

Mammography Technologies Screen/film Analogue CR Plates Computed radiography X-Ray Image Capture CCD A-Si (amorphous silicon) Indirect image detection (phosphor) Digital Integration Digital detector systems Direct image detection (no phosphor) IAEA Adapted from KCARE UK Direct photon counting A-Se (amorphous selenium) Si wafer 45

Schematic: CR Plate Readout Rotating Polygon è Mirror ë Stimulating Laser Photomultiplier Tube ê ADC ë Fibre Optical Coupling CR Plate Moved Translationally IAEA Dual-sided readout also possible (Fuji) Digitized signal

CR plate erasure • Required even after image readout due to scattered and background radiation storage (so-called ghost images) • Secondary erasure of all plates daily • Primary erasure of all plates weekly IAEA 47

a-Si Indirect Detectors Cs. I-scintillator with a-Si switching diodes or TFT-read out 100 m pixel size giving ~5 lp/mm X-rays scintillator Pixel matrix Used by GE and others IAEA line driver contacts photo diode switch amplifier, multiplexer & ADC

Direct Detection : a-Se Used by Fuji and others Top electrode Charge collection electrode Gate pulse IAEA X-rays IN a-Se layer Signal Out TFT Charge storage capacitor 49

Use “Roam and Zoom” - 50μ del example 18 cm 3600 pixels Each image: Typically 32 MB of information ! 24 cm Display: 4800 pixels 2400 x 1700 pixels IAEA Display resolution = 12 lp/mm

Monitor and Image QC • • • Monitor QC Monitor cleanliness Viewing conditions Printer area cleanliness Printer QC IAEA 51

Monitor QC • Monitors now all flat panel • 1. 3 (normal PC monitor), 3, and 5 (dedicated) megapixel (MP) • Use software test pattern (TG 18), appropriate resolution (eg. 2 k version) • Weekly test, evaluation preferably by same person IAEA 52

TG 18 -QC Phantom IAEA 53

Check for jitter Check resolution elements are resolved Resolution elements Confirm 20 discrete shades of grey all present 95% IAEA 5% Check number of visible letters in “Quality Control” 54

Monitor cleanliness • Cleanliness of the monitor can have an effect on the quality of the displayed images • Recommend WEEKLY cleaning of monitors to ensure they are free of dust, fingerprints and other marks that might interfere with image interpretation • The manufacturer’s specific instructions should be adhered to when choosing cleaning agents IAEA 55

What the radiographer can do – digital mammography • CR plate erasure • To remove “stored” image remnant • Daily, before imaging – secondary erase (what is done at each read cycle) • Weekly (first working day) – primary erase (deeper level of erasure) IAEA 56

• Viewing conditions (CR and DR) • Daily - Check monitor screens for dirt/dust/fingerprints and clean • Make sure there are no reflections on the monitor visible to the reader IAEA 57

• Printer check (CR and DR) • • At least weekly – print test pattern Check that all features visible No artifacts such as lines If wet laser printer, also check densities as per film if possible IAEA 58

• Full-field artifact (CR and DR) • Daily or before patient examination if infrequent • Need uniform absorber (4 cm perspex/PMMA), so may be difficult • Simple AEC exposure, look for “dead” pixels, streaks, other artifacts • Use narrow window for viewing IAEA 59

• Detector calibration (flat field test) – DR only • As recommended by manufacturer, and using their test materials • Recalibrates detectors automatically IAEA 60

• Phantom image quality (CR and DR) • Weekly • Use ACR or other phantom, expose with AEC • Score as recommended for that phantom IAEA 61

• CR plate cleaning – monthly or as recommended (particularly in dusty areas) • Follow manufacturer’s method IAEA 62

• CR plate uniformity • Expose perspex eg 4 cm, under AEC • Record each cassette’s exposure index (S#, lg. M, EI etc) • Should be within manufacturer’s limits (these differ from manufacturer to manufacturer) IAEA 63

QC of the radiographer! • Check images for correct positioning, visibility of chest wall, nipple correctly placed, no skin folds etc. IAEA 64

Summary • To achieve the best image quality while keeping the breast dose as low as reasonably achievable is the final goal to be reached when consistently using mammography equipment. • Implementing a well defined QC protocol can effectively contribute to the achievement of such a goal. IAEA 65

How good is your mammo. QC program? IAEA 66

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