Mammography and fluoroscopy

Question 1

Mammography: how is the anode heel effect used?

Answer 1

-Use of anode heel effect with cathode over chest wall to capture thicker end of breast and anode over nipple (lower energy beam can penetrate thinner area).

Question 1

What does PACS stand for? What is the name of files stored in PACS?

Answer 1

-Stands for Picture archiving and communication system
-Images are stored as Digital Imaging and Communications in Medicine (DICOM) files. A file also contains the patient, display preferences and imaging modality.

Question 2

Mammography: what is the focus detector distance?

Answer 2

-Fixed focus-detector distance (65-66cm)

Question 3

Mammography: how does the AEC work?

Answer 3

-Automatic exposure control: the detector acts as AEC

Question 4

Mammography: what is the target material? What x-ray energies are required?

Answer 4

-Needs to produce characteristic x-rays at energies of 17-20 kEv (20-30 for large breasts)
-Usually use Molybdenum

Question 5

Mammography: what filter material is used for larger and smaller breasts? What is the aim of the filter?

Answer 5

-Usually Molybdenum filter (and target) but not always enough energy for larger breasts.
-Larger breasts: molybdenum target and Rhodium filter
-Aim of filter: removes high energy x-ray photons and to try and make the beam as monoenergetic as possible.

Question 6

Mammography: Why can you not use a rhodium target and Molybdenum filter?

Answer 6

-The filter would attenuate the rhodium characteristic x-rays.

Question 7

What are the atomic numbers and k-edges of Molybdenum a Rhodium?

Answer 7

-Molybdenum: K edge 20keV, atomic number 42
-Rhodium: K edge 23keV, atomic number 45

Question 8

How is magnification in mammography achieved?

Answer 8

-Increasing object-film distance  this occurs through greater divergence of the transmitted beam.

Question 9

Mammography: How does the k-edge compare to the characteristic radiation?

Answer 9

-The k-edge of an element is just higher than its characteristic radiation t/f any element is relatively transparent to its own characteristic radiation.
-Mammography exploits this by using molybdenum as both anode target and filter.
-Digital spatial resolution is better than film mammography (partial volume effect of DR allows higher contrast of smaller details eg micro-calcifications).

Question 10

What is the typical exposure for glandular breast tissue? What is the dose/risk of inducing a fatal cancer in women of screening age?

Answer 10

-1.5-3mGy
-Dose of 2mGy is the dose that can induce a fatal cancer in 1:50 000 50–65-year-old women

Question 11

Mammography: how do focal spot size, compression and anti-scatter grids affect spatial resolution?

Answer 11

Spatial resolution is improved by:
-Focal spot size: can be small (0.1-0.15mm) for spot views or can be broad (0.3mm) for full views
-Compression: good b/c leads to less attenuation, reduces scatter, reduces geometric and movement unsharpness, spread tissues out so less overlaying of features.
-Anti-scatter grids

Question 12

How do thicker breasts affect mammography? How does this affect the kV used?

Answer 12

-Thickness of breast: more radiation absorbed, more scatter, increased beam hardening (less contrast), longer exposure time required.
-Thin breasts: MoMo at kV25
-Thicker breasts: MoRh at kV 32

Question 13

What is breast tomosynthesis (tomography)?

Answer 13

Breast tomography uses digital radiography to reconstruct planar images of sections of the breast. There are two main methods of acquiring breast tomosynthesis:
1. The x-ray tube traverses along an arc acquiring images as it travels and the detector remains stationary
2. The x-ray tube traverses along an arc and the detector also rotates
The images are then reconstructed using filtered back projection or iterative reconstruction

Question 14

What are pros of breast tomosynthesis? 3

Answer 14

• Provides enhanced lesion detection
• Reduces false positive recalls
• Allows more precise lesion localisation

Question 15

What are cons of breast tomosynthesis? 4

Answer 15

• Higher radiation dose (approximately double)
• High contrast objects (e.g. surgical clips) can cause significant artefacts
• Longer interpretation time
• Requires substantially more data storage

Question 16

What is fluoroscopy?

Answer 16

Fluoroscopy is the use of real time x-ray imaging. It used to utilise image intensifiers, which have been in use since the late 1950’s, but now uses flat panel detectors, which are similar to the digital radiography used in projection radiology.

Question 17

What is fluorography? What is the matrix size for fluorography?

Answer 17

• Images usually formed and viewed after the x-ray exposure is complete
• Better quality images acquired than in fluoroscopy but at higher doses
• Matrix is typically 1024 x 1024 pixels with each pixel representing 10 bits of grey scale information
  *Acquisition of single diagnostic quality images.

Question 18

fluo: how do you measure brightness gain?

Answer 18

Brightness gain = minification gain x flux gain

Question 19

fluo: how do you measure minification gain?

Answer 19

o Minification gain = (Dinput / Doutput)2 (where D is diameter of input and output screen respectively)

Question 20

fluo: how do you measure the image intensifier conversion factor?

Answer 20

o Image intensifier conversion factor (Gx) = L / X’ (where L = luminance of II output, X’ = II entrance dose rate)

Question 21

fluo: what factors affect brightness gain?

Answer 21

o More minimisation = higher gain
o Gx falls in proportion to the reduction in the area of the input field in zoom setting
o Higher voltage applied across II tube = more flux gain

Question 22

Xray tube in fluo: input screen - describe the MOA

Answer 22

o X-ray photons enter tube through aluminium or titanium window
o II input window: aluminium or titanium foil to allow x-rays to enter tube and (maintains vacuum).

Question 23

What is the material of the input phosphor of x-ray tube?

Answer 23

o Layer of Caesium iodide CsI:Na for good x-ray absorption efficiency. Each x-ray photon produces ~3000 light photons in blue spectrum

Question 24

Xray tube in fluo - What is the role of the input phosphor?

Answer 24

Each x-ray photon produces ~3000 light photons in blue spectrum
o X-ray photons converted into light photons in the input phosphor layer.

Question 25

What is the role of the photocathode in fluo?

Answer 25

o Fluorescent emission from phosphor absorbed by light-activated photocathode made of SbCs3 which then releases electrons into body of II tube.
o photons are converted into electrons in the photocathode layer.

Question 26

Xray tube in fluo: What is the role of the electron optics?

Answer 26

• Electron focusing: positively charged electrodes along tube direct electron path to create exact but minified and inverted image on input screen
• Magnification: achieved via electronically focusing electron beam. Magnified images use less signal and so need a higher dose but improve the resolution

Question 27

Xray tube in fluo - What is the charge differential between the input screen and output screen?

Answer 27

• Input screen at negative charge compared to output screen to direct electron towards output screen.

Question 28

Xray tube in fluo - Output screen: what is the phosphor made of?

Answer 28

• Output screen made of thin layer of silver activated zinc cadmium sulphide crystals (ZnCdS:Ag) that convert electrons into light photos that then leave through the output window

Question 29

Xray tube in fluo - Output screen - what is halation and how can this be minimised?

Answer 29

• Scatter of light (halation) in the output window can seriously degrade the contrast of the output tube. This can be minimised with: smoked glass, very thick glass, fibre-optic bundle.

Question 30

Display of image in fluo: explain how a TV camera works

Answer 30

o Electronic TV camera uses electron beam that scans across photoconductive target to create flow of electrons, the rate of which corresponds to the amount of light photons striking that area. (not really used anymore)

Question 31

Display of image in fluo - how is the resolution determined in a TV camera?

Answer 31

-Vertical resolution is determined by the number of scan lines
-Horizontal resolution is determined by the bandwidth of the system (higher bandwidth = lower resolution)

Question 32

Display of image in fluo: CCD device sensors - explain MOA

Answer 32

-Each pixel has an associated electrode. Each light photon gives rise to an electron-hole pair. The negative electrons accumulate in a potential well. These packages are then transferred to the light shielded storage array. Data is read from storage section line by line. The accumulation of charge is directly proportional to the intensity of the incident light
-Then sent to flat panel detector

Question 33

Display of image in fluo - explain how a flat panel detector works

Answer 33

-Utilises same technology as digital radiography -there is a flat panel of detectors that provide a direct electronic readout instead of requiring the conversion of analogue to digital like in IITV.
-Most commonly indirect dynamic FP detector with CsI:Tl x-ray scintillator layer superimposed onto a-Si high resolution active matrix

Question 34

T or F: CCD + flat panel has an improved DQE compared to IITV camera

Answer 34

True

Question 35

T or F: CCD + flat panel has video signal that emerges in digital form, reducing electronic noise

Answer 35

True

Question 36

T or F: II ITV camera has better temporal resolution than CCD, with a matrix size of 2048 x 2048

Answer 36

False - this is the resolution of CCD
IITV has poorer temporal resolution and has a circular field, meaning it doesn’t cover corners.
CCD produces better quality images

Question 37

What is the approximate range of bits per pixel for CCD?

Answer 37

-Greyscale of 12-14 bits per pixel

Question 38

T or F - Detective quantum efficiency for CCD is 10-20% better than IITV so can afford to reduce patient dose

Answer 38

True

Question 39

T or F - Zoom can be used for CCD and it increases spatial resolution as ir does in IITV

Answer 39

False - zoom is available but it doesn’t increase the spatial resolution as it does in IITV

Question 40

How does AEC work in fluorography?

Answer 40

o Alters kV and mA to ensure stable quality of images. This, in turn, alters the patient dose
o Done by measuring II light output with a photo-sensor or electronically sampling video signal

Question 41

What is the ABC/AEC for fluroscospy and fluorography?

Answer 41

-ABC: automatic brightness control - used in fluoroscopy
-Automatic exposure control: used in fluorography and radiography

Question 42

Describe the use and the curve in fluoroscopy: A curve

Answer 42

-Curve A: Anti-isowatt curve: increase kV and mA as radiological thickness increases.

Question 43

Describe the use and the curve in fluoroscopy: B curve

Answer 43

-Curve B: iodine contrast imaging -holds kV at a 60-65kV (optimum spectrum for imaging iodine). Curve has high imaging quality but high dose

Question 44

Describe the use and the curve in fluoroscopy: C curve

Answer 44

-Curve C: high kV curve – dose minimisation curve (low dose) – mostly for paeds.
*see curve page 65 Farr’s 3rd ed.

Question 45

Digital processing: fluo - describe greyscale processing

Answer 45

Grey scale processing with greyscale range compression: suppress or highlight intensities and improve contrast balance. Uses analogue (video) circuit or via a look-up-table (LUT)

Question 46

Digital processing: fluo - describe spatial filtering

Answer 46

Spatial filtering: similar to edge enhancement in projection radiography. Improves displayed spatial resolution. Best used for high image contrast (barium GI studies).

Question 47

Digital processing: fluo - describe temporal filtering

Answer 47

Temporal filtering: decreases level of dose by summing current image with previous frames, averaging out signal and resulting in smaller proportion of noise.

Question 48

Dose to patient is measured in?

Answer 48

Measured in skin dose rates

Question 49

What is the greatest contribution to staff dose in fluo?

Answer 49

Greatest contribution to staff dose is from scatter, amounts to ~0.1% of patient dose at a distance of 1m

Question 50

What is a Leed’s test and when is it used?

Answer 50

Used for quality control in fluoroscopy – consists of many circular details of different diameters and thicknesses.

Question 51

Describe how an image is taken in DSA

Answer 51

1. Acquire mask image (IM) to record anatomical background.
2. Contrast injected. Series of images acquired which show arrival and run-off of contrast (contrast medium enhanced image, IC)
3. Image frames subtracted via digital processor. Any structures that are common to set 1 and set 2 are subtracted (i.e. all background anatomy but not the contrast filled structures)
4. Amplify contrast signal to boost displayed contrast of the vessels

Question 52

Describe misregistration - artefact in DSA

Answer 52

The movement of a structure by even 1 mm can cause misregistration - structure in wrong place

Question 53

Dual energy radiography - how can you get an image with high soft tissue contrast?

Answer 53

Uses a composite of two images obtained with different tube voltages.
They can then be subtracted from one another to form an image with high soft tissue contrast (subtract low kV from high kV)

Question 54

Dual energy radiography - how can you get an image with high bone contrast?

Answer 54

Uses a composite of two images obtained with different tube voltages. They can then be subtracted from one another to form an image with high bone contrast (subtract high kV from low kV).