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PHYSICS of modalities > Mammography > Flashcards

Mammography Flashcards

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1
Q

mammography equipment

A
  • angled tube head
  • c-arm design
  • fixed focus-detector distance
  • compression device
  • fixed field size
  • grids
  • AEC
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2
Q

difference between conventional and mammography x-ray HOUSING tubes

A

conventional: glass envelope

mammo: metal tube

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3
Q

difference between conventional and mammography x-ray ANODE

A

conventional: tungsten

mammo: grounded Mo, Rh anode

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4
Q

difference between conventional and mammography x-ray axis of rotation

A

C: horizontal

M: vertical

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5
Q

difference between conventional and mammography x-ray filters

A

C: Al filter dose reduction

M: Mo or Rh filters for spectral shaping

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6
Q

difference between conventional and mammography x-ray anode angle

A

C: 7-16deg

M: 0deg; tube tilt 26deg

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7
Q

mammography x-ray tube operating voltage

A

23-40kVp

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8
Q

dual filaments in a focusing cup

A

0.3mm (contact) and 0.1mm (magnification) focal spot size

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9
Q

why use small focal spot

A
  • minimize geometric blurring
  • maintains spatial resolution
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10
Q

typical tube currents

A

100mA +/- 25mA for large focal spot

25mA +/- 10mA for small focal spot

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11
Q

what track is used for anode

A

Mo/Rh targets, sometimes combination with Tungsten

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12
Q

reason for choosing Mo and Rh as anode targets

A

charateristics xray production
Mo - 17.5 and 19.6 keV
Rh - 20.2 and 22.7 keV

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13
Q

why is tube tilted by 26deg

A

minimize effective focal spot size

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14
Q

what are the 3 factors that xray beam spectrum depend on

A
  • anode material
  • selected filter
  • kV
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15
Q

why is low kVp used

A
  • minimise compton scattering and maximise photoelectric effect -> enhance differential absorption by various tissues of the breast
  • reduce radiation dose to breast -> glandular tissue of breast is high radiosensitive
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16
Q

use of a filter of ____ element as x-ray tube target is designed to

A

same; suppress the lower and higher energy bremsstrahlung x-ray and allow transmission of characteristics x-ray energy

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17
Q

what does the lower and higher energy bremsstrahlung xray contribute to

A

lower - radiation dose

higher - loss of contrast

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18
Q

xray tube window is made of

A

Berylium

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19
Q

breast thickness and its recommended target, filter and kVp

A

fatty breast ~4cm thick:
- Mo target
- 30micron Mo filter
- 24-26kVp

glandular breast ~5-7cm thick:
- Mo target
- 25micron Rh filter
- 27-31kVp

> 7cm:
- Rh target
- 25micron Rh filter
- 27-31kVp

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20
Q

what does HVL depend on

A
  • kvp
  • compression paddle thickness
  • added tube filtration
  • target material
  • age of tube
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21
Q

relationship between kvp, HVL, atomic number targets and filters

A

HVL increases with higher KVP and higher atomic number targets and filters

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22
Q

AEC modes of operation

A
  • auto time
  • auto kvp
  • full automatic
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23
Q

why is there a need for breast compression

A
  • reduces overlapping anatomy and decreases tissue thickness
  • less scatter, more contrast, less geometric blurring of the anatomic structures, less motion and lower radiation dose to the tissues
  • better visualisation of tissues near the chest wall
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24
Q

what is spot compression

A
  • apply compression to a smaller area of tissue using a small compression plate or cone
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25
Q

what does scatter radiation do to the images

A

degrades subject contrast and detail

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26
Q

relationship between scatter, breast thickness and area

A

scatter increases with breast thickness and breast area

27
Q

main variables of a mammo imaging system

A
  • contrast
  • sharpness
  • dose
  • noise
28
Q

why and how is half field xray beam used in mammo

A
  • angled such that the central ray comes down along the chest wall edge perpendicular to the detector
  • eliminated unnecessary dose to the patient’s torso
  • prevents non-visualisaion of tissue in the chest wall region
29
Q

explain the anode heel effect

A
  • lower xray intensity is on the anode side of the field and should be placed at where nipple edge is.
  • more tissue at chest wall, so require more intense xray beam
  • a more uniform exposure is achieved
30
Q

what is the optimal viewing conditions of a mammo image

A

soft copy:
- ambient light intensity <20lux
- display monitor 5MP
- magnifiers available to view fine deatils such as microcalcifications

hard copy:
- films are exposed to high optical densities to achieve high contrast
- luminance of mammo viewbox should be at least 3000cd/m^2
- film masking - preserve visibility of low contrast details

31
Q

advantages of analog mammo over DM

A
  • more affordable
  • superior spatial resolution
  • can be transformed into digital images with CR and saved as DICOM
  • no digital detector -> which is a fragile and expensive part
  • easier to maintain
32
Q

disadvantage of analog mammo over DM

A
  • uses film so need a CR reader to convert image to digital –> takes a longer time to receive the image
  • 2 systems working together (machine and CR reader, if one breaks, the entire process is down
  • difficult to archive images unless have CR
33
Q

advantages of DM over analog in terms of lower patient dose and reduced recalls

A
  • lower dose
  • wider dynamic range
  • higher contrast resolution
  • better SNR
  • reduced false positive and increased PPV
  • image manipulation tools
34
Q

advantages of DM over analog in terms of improved efficiency

A
  • simplifies storage and retrieval of images
  • elimination of lost films
  • images can be easily transferred electronically to a central location (PACS) for diagnosis
  • availability of CAD
35
Q

advantages of DM over analog in terms of enhanced patient care and productivity

A
  • elimination of need to leave exam room to process films
  • eliminates processing issues
  • faster interventional procedures
  • faster patient throughput
36
Q

disadvantage of DM over analog

A
  • more expensive
  • delicate digital detector
  • more sensitive to ambient temp
  • spatial resolution is limited to pixel size
  • harder to service
37
Q

does digital or film screen have a better contrast resolution

A

digital - wider latitude = greater dynamic range = more shades of grey

38
Q

what target material is used for digital mammo

A
  • tungsten (W) is better as the kV is more for penetration since the algorithm drives contrast
  • W offers lower dose with longer scale contraste
39
Q

what filtration is used for digital mammo

A

Rh and Ag

40
Q

what are the 2 types of digital mammo

A

direct and indirect

direct: convert xray directly to electronic signals
indirect: convert xray -> light -> electronic signals

41
Q

what are some image quality measurements

A
  • modulation transfer function (MTF)
  • detector quantum efficiency (DQE)
42
Q

explain MTF

A
  • measure of signal transfer over a range of spatial frequencies -> image sharpness
  • high MTF = more transfer = sharper image and superior contrast
43
Q

explain DQE

A
  • measure of dose efficiency
  • high DQE = superior image quality and reduced patient dose
  • high DQE = lower image in noise = reduced missed cancers
  • high DQE = flexibility to reduce dose
44
Q

what is the preferred target filtration combination of a full field digital mammo

A

W/Rh

45
Q

what is tomosynthesis

A
  • set of rapidly acquired low-dose projections taken at multiple angles through the compressed breast
  • then used computer reconstructions to create 3D images of the breast
46
Q

compare narrow and wider sweeping angle for tomosynthesis

A
  • wider sweep angle = gives more complete blurring outside of the focal plane
  • narrow sweep angle = lesion margins appear sharper
47
Q

advantages of DBT compared to 2D

A
  • better depiction of the smallest calcifications
  • better delineation of the lesion border
  • less compression
48
Q

disadvantage of DBT compared to 2D

A
  • motion artifacts more likely to occur because of longer exposure time
  • large calcifications cause significant artifacts
  • reconstructed images lengthens interpretation time
49
Q

how is breast dose calculated

A

mean glandular dose (MGD) - measured in mGy or mrad

MGD (mrad) = DgN x ESE

50
Q

what is the acceptable dose

A

for a 50-50 glandular adipose tissue breast, with 4.2cm compression thickness, MGD must be less than 3mGy per view

51
Q

factors affecting breast dose

A
  • photon energy
  • breast composition and thickness
  • equipment
52
Q

what is DgN

A

factor that converts ESE to mean dose
depends on:
- HVL, filter, target, kVp
- breast composition and thickness

53
Q

what are some patient related artefacts

A
  • motion
  • skin line
  • hair
  • anti-perspiration
  • superimposition of body parts
  • eye glasses
  • jewelry
54
Q

what are some hardware-related artefacts

A
  • dust/cracks on compression paddle
  • under exposure
  • field inhomogeneity
  • collimation misalignment
  • noise obscuring calcification
  • grid lines
  • grid misplacement
  • vibration artifact
  • detector-associated:
    ghosting
    gouging
    horizontal line artifact
    dead pixel
    unread lines
55
Q

what are some software processing artefacts

A
  • high density
  • loss of edge
  • breast within breast
  • vertical processing bars
  • detector interface line
  • skin line processing
56
Q

what is a storage related artefact

A

reconstruction artifact

57
Q

why is laser printer sensitometry QC performed

A
  • ensure that the film sensitometry is operating within tolerance level
58
Q

why is SMPTE QC performed

A
  • to determine whether the contrast and brightness setting of the monitors are acceptable
  • to check for limitations in spatial resolution and aliasing of displays
  • to check for non-uniformities, scratches and other defects
59
Q

what is ACR phantom used for

A
  • to assess the quality and consistency of the mammographic images
60
Q

what do SNR and CNR measure

A
  • both are a measurement of image quality

SNR: ratio of a signal power to the noise power corrupting the signal
CNR: ability of an imaging modality to distinguish between various structures of an acquired image by the perception of differences in their apparent signal intensities

61
Q

what is a compression thickness indicator QC for

A

to ensure that the indicated compression thickness is within tolerance

62
Q

what is the flat field uniformity test

A
  • an analysis of the homogeneity of detector field
  • to ensure that the images are uniform in intensity and free of artifacts
63
Q

what is gain and geometry calibration for

A

to ensure that the system is calibrated properly

PHYSICS of modalities (4 decks)

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