AIM: Ch 8: Mammography Flashcards
T/F: The diagnostic mammographic examination may include additional x-ray projections, magnification views, spot compression views, ultrasound, magnetic resonance imaging (MRI), or mammoscintigraphy.
True
Its clinical utility is in the evaluation of suspected breast cancer in patients for whom mammography is nondiagnostic, equivocal, or difficult to interpret (e.g., the presence of scar tissue, mammographically dense breast tissue, implants, or severe dysplastic disease). It is also used to assist in identifying multicentric and multifocal carcinomas in patients with tissue diagnosis of breast cancer.
Mammoscintigraphy utilizing Tc-99m sestamibi
Subject contrast is highest at low x-ray energies (____) and reduced at higher energies (e.g., greater than 30 keV)
10 to 15 keV
Low x-ray energies»_space; good contrast
Higher doses with increased exposure time
The mammography x-ray tube is configured with dual filaments in the focusing cup to produce____ and ____ (mm) focal spot sizes, with the latter used for magnification studies to reduce geometric blurring
0.3- and 0.1-mm
The filament current is restricted to limit the tube current, typically to ____ for the large (0.3 mm) focal spot and ____ for the small (0.1 mm) focal spot so as to not overheat the Mo or Rh targets due to the small interaction areas.
100 mA, 25 mA
Most common anode target material used in mammography x-ray tubes
Molybdenum
With digital detectors, ____ is becoming the target of choice for mammography x-ray tubes
Tungsten
Increased x-ray production efficiency, due to its higher atomic number, and improved heat loading, due to its higher melting point, are major factors in favor of W. Digital detectors have extended exposure latitude, and because postacquisition image processing can enhance contrast, characteristic radiation from Mo or Rh is not as important in digital mammography as it is with screen-film detectors.
Mammography x-ray tubes have rotating anodes, with anode angles ranging from ____ degrees, depending on the manufacturer.
16 to 0
The tubes are typically positioned at a source-to-image receptor distance (SID) of about ____
65 cm
Refers to the actual anode angle plus the physical tube tilt
Effective anode angle
In order to achieve adequate field coverage on the anterior side of the field, the x-ray tube must be physically tilted so that the effective anode angle is at least ____ degrees for coverage of the 24 x 30-cm field area
22
Anode angle: 0 deg, tube tit: 24 deg
EAA: 0 + 24 = 24 deg
Anode angle: 16 deg, tube tilt: 6 deg
EAA: 16 + 6 = 22 deg
T/F: The anode is kept at ground potential (0 voltage), and the cathode is set to the highest negative voltage to reduce off-focal radiation
True
Orientation of the cathode-anode direction of the x-ray tube is from the chest wall side of the breast (over the ____) to the anterior side of the breast (over the ____)
Cathode
Anode
Focal spot nominal sizes of ____ for contact mammography (breast compressed against the grid and image receptor) and ____ for magnification imaging (breast compressed against a magnification stand, which supports the breast at a distance from the image receptor to provide geometric image magnification) reduce geometric blurring so that microcalcifications can be resolved.
0.3 to 0.4 mm
0.10 to 0.15 mm
In order to avoid exposure of the patients’ torsos and to maximize the amount of breast tissue near the chest wall that is imaged, all dedicated mammography systems utilize a ____, which is achieved by fixed collimation at the x-ray tube head
“Half-field” x-ray beam geometry
As a result, the central axis of the x-ray beam is directed at the chest wall edge of the receptor and perpendicular to the plane of the image receptor.
Furthermore, by convention, the nominal focal spot size is measured at the ____, which bisects the x-ray field along the chest wall—anterior direction of the x-ray field
Reference axis
As a consequence of the effective focal spot size variation in the field (the line focus principle), sharper image detail is rendered on the ____
Anode (anterior) side of the field toward the nipple
More evident with magnification examinations
Focal spot resolvability with or without magnification is measured by imaging a high-resolution bar pattern with up to ____
20 line pairs/mm
For conventional breast imaging, the bar pattern is placed ____ above the breast support surface near the chest wall; for magnification imaging, it is placed ____ above the magnification platform.
4.5 cm, 4.5 cm
The resolving capability of the imaging system is limited by the component that causes the most blurring. In magnification mammography, this is generally the ____, whereas in contact mammography, it may be the ____, and at other times ____.
Focal spot
Detector element size
Patient motion
The tube port and added tube filters play an important role in shaping the mammography x-ray energy spectrum. The tube port window is made of ____
Beryllium
The low atomic number (Z=4) of beryllium and the small thickness of the window (0.5 to 1 mm) allow the transmission of all but the lowest energy (less than 5 keV) bremsstrahlung x-rays
Added x-ray tube filtration improves the energy distribution of the mammography output spectrum by selectively removing the lowest and highest energy x-rays from the x-ray beam, while largely transmitting desired x-ray energies. This is accomplished by using elements with K-absorption edge (Chapter 3) energies between 20 and 27 keV. Elements that have these K-shell binding energies include the following, except:
a. Mo
b. Rh
c. Al
d. Ag
c. Al
A Mo filter should never be used with a Rh target, because Rh characteristic x-rays are attenuated significantly as their energies are above the Mo K-absorption edge
Target-Filter combo for thin and thick breast tissues
Thin: Mo-Mo
Thick: Mo-Rh
Screen-film detectors most often use a Mo target and 0.03-mm Mo filtration with a kV of 24 to 25 kV for thin, fatty breasts and up to 30 kV for thick, glandular breasts. For thicker and denser breasts, a Mo target and Rh filter are selected with higher voltage, from 28 to 32 kV, to achieve a higher effective energy and more penetrating beam (Fig. 8-9B, right graph).
W targets are now used for many digital mammography systems because of their higher bremsstrahlung production efficiency and higher tube loadings than Mo and Rh targets. K-edge filters can optimize the output energy spectrum for breast imaging. However, an unfiltered W spectrum contains a huge fraction of unwanted L x-rays in the ____ range
8- to 12-keV
Therefore, minimum filter thicknesses of 0.05 mm for Rh and Ag are needed to attenuate the L-x-rays to negligible levels, as shown in Figure 8-11B.
In some applications, an Al filter is used, but because of its low Z and lack of a useful K-absorption edge, a thickness of 0.7 mm is necessary to attenuate the L x-rays.
The half-value layer (HVL) of a mammography x-ray beam ranges from ____ for the kV range and combinations of target material, filter material, and filter thickness used in mammography
0.3 to 0.7-mm Al
The HVL depends on the following except:
a. Target material (Mo, Rh, W)
b. kV
c. mA
d. Filter material and filter thickness
c. mA
Usually, the HVL for breast tissues is from ____
1 to 3 cm
It is a measure of the intensity of the x-ray beam, typically normalized to mAs or to 100 mAs, at a specified distance from the source (focal spot)
Tube output
Common units of tube output are mGy (air kerma)/100 mAs and mR (exposure)/mAs
Tube output rate is the air kerma rate at a specified distance from the x-ray focal spot and is a function of the tube current achievable for an extended exposure time (typically ~300 mAs for an exposure time greater than 3 s)
The projected x-ray field must extend to the chest wall edge of the image receptor without cutoff, but not beyond the receptor by more than ____ of the SID
2%
Like most contemporary x-ray imaging systems, ____ generators are used for mammography due to low voltage ripple, fast response, easy calibration, long-term stability, and compact size.
High-frequency
Most mammographic techniques use phototiming, and the proper choice of kV is essential for a reasonable exposure time, defined as a range from approximately ____ to achieve an OD of 1.5 to 2.0 for film, or the desired SNR for digital images.
0.5 to 2.0 s
Too short an exposure can cause visible grid lines to appear on the image, whereas too long an exposure can result in breast motion, either of which degrades the quality of the image.
A compression paddle has a right-angle edge at the chest wall to produce a flat, uniform breast thickness when an adequate force of ____ is applied
111 to 200 newtons
(25 to 44 lb)
T/F: X-ray scatter increases with increasing breast thickness and breast area
True
T/F: Scattered radiation is an additive, gradually varying radiation distribution that degrades subject contrast and adds random noise
True
Screen-film: (+) scatter > less contrast
Digital: (+) scatter > more random noise, degrading SNR
In digital mammography, unlike screen-film mammography, the main adverse effect of scattered x-rays is not a reduction of contrast. Contrast can be improved by any desired amount by post-acquisition processing, such as windowing. The main adverse effect of scatter in digital mammography is that scattered photons add random noise, degrading the signal to noise ratio.
Linear focused grids with grid ratios (height of the lead septa divided by the interspace distance) of ____ are common (e.g., 1.5 mm height, 0.30-mm distance between septa, 0.016-mm septa thickness), with carbon fiber interspace materials.
Grid frequencies (number of lead septa per cm) of ____ are typical. To avoid grid-line artifacts, the grid must oscillate over a distance of approximately ____ during the exposure
4:1 to 5:1
30/cm to 45/cm
20 lines
Magnification has several limitations, the most significant being ____ caused by the finite focal spot area
Geometric blurring
It is the most commonly used screen phosphor
Terbium-activated gadolinium oxysulfide (Gd2O2S:Tb)
This scintillator emits green light, requiring a green-sensitive film emulsion
It represents the relationship between the incident x-ray exposure (proportional to light intensity on the film) and the resulting OD in the processed film.
H and D curve
A film characteristic curve is a graph of the OD as a function of the logarithm of the relative exposure and has three major sections: (a) the toe has a low OD that changes very little with incident exposure up to a threshold; (b) the linear section represents increasing OD with increasing incident exposure, with subject contrast mapped as optical densities that render the radiographic contrast; and (c) the shoulder represents saturation of the film, where increases in incident exposure do not significantly increase the OD.
It is a graph of the change of OD per unit change in the incident exposure, as a function of the logarithm of the relative exposure.
Gradient curve
It is a device that emulates a range of incident radiation exposures by using a constant light source and calibrated optical attenuation steps to expose a mammographic film to known relative light intensities.
Sensitometer
Developing the film produces a range of numbered OD steps known as a ____
Film sensitometric strip
It is a device that measures the light transmission through a small area of the film and calculates the OD
Densitometer
Blocking clear portions of the film and areas of the view box that are not covered by film to prevent extremely bright light from degrading the radiologist’s visual adaptation to the range of luminances displayed by the images
Film masking
The film must be fully darkened in background areas around the breast (Fig. 8-23) by collimating the x-ray beam to the full area of the detector
It describes absorption of x-rays in the CsI, the production of secondary light photons directed to a photodiode, and the generation of the charge, which is stored on the storage capacitor in that detector element.
Indirect conversion
It refers to the direct generation of charge by x-rays within the photoconductor and capture by the electrode without intermediate signals.
Direct conversion
Digital tomosynthesis for breast imaging is made possible with fast readout TFT arrays by acquiring many sequential images (11 to 51) over a limited angle ( ____ ) in a short time span ( ____ ).
+/- 7.5 to +/- 50 degrees
4 to 20 s
Preferred dose index in a mammography
Average glandular dose
The glandular tissue is the site of carcinogenesis. Because the glandular tissues receive varying doses depending on their depths from the skin entrance site of the x-ray beam, estimating the dose is not trivial
The midbreast dose, the dose delivered to the plane of tissue in the middle of the breast, was the radiation dosimetry benchmark until the late 1970s. The midbreast dose is typically lower than the average glandular dose and does not account for variation in breast tissue composition.
