RAB: ch.8 Mammography Flashcards
low cost, low radiation dose procedure that has the Sn to detect early stage breast CA.
X-ray mammography
Attempts to identify breast CA in Asymptomatic population
screening mammography
Performed to assess palpable lesions or evaluate suspicious findings identified by screening mammography.
diagnostic mammography
(add’l x-ray projections, magnification views, spot compression views, utz, mri or mammoscintigraphy)
used to differentiate cysts from solid masses; biopsy needle guidance
UTZ
has excellent tissue contrast sensitivity; w/ contrast enhancement can differentiate benign from malignant tumors; used for staging, biopsy guidance, diagnosis, and sometimes screening
MRI
Used 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). Also used in identifying multicentric and miltifocal carcinomas
Mammoscintigraphy (utilizing Tc-99m)
These provide the best differential attenuation between the tissues; Subject contrast is highest here
Low x-ray energies
configured with dual filaments in the focusing cup to produce 0.3- and 0.1-mm focal spot sizes, with the latter used for magnification studies to reduce geometric blurring
Mammography x-ray tube
Two important distinction between mammography and conventional xray tube operation
- low operating voltage, below 40 kV
- filament current is restricted to limit the tube current, typically to 100 mA for the large (0.3 mm) focal spot and 25 mA for the small (0.1 mm) focal spot so as to not overheat the Mo or Rh targets due to the small interaction areas.
Higher filament currents and thus tube currents, up to and beyond 200 mA for the large focal spot and 50 mA for the small focal spot, are possible with tungsten anodes chiefly due to a higher melting point compared to Mo and Rh anodes.
Most common anode target material used in mammography xray tubes.
Molybdenum
but Rh and tungsten (W) are also used as targets; Characteristic x-ray production is the major reason for choosing Mo (K-shell x-ray energies of 17.5 and 19.6 keV) and Rh (20.2 and 22.7 keV) targets, as the numbers of x-rays in the optimal energy range for breast imaging are significantly increased by characteristic x-ray emission. With digital detectors, W is becoming the target of choice. Increased x-ray production efficiency, due to its higher atomic number, and improved heat loading, due to its higher melting point
With digital detectors, ___ is becoming the target of choice. 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.
Tungsten (W)
Higher filament currents and thus tube currents, up to and beyond 200 mA
- have rotating anodes, with anode angles ranging from 16 to 0 degrees, depending on the manufacturer.
- typically positioned at a source-to-image receptor distance (SID) of about 65 cm.
*
In order to achieve adequate field coverage on the anterior side of the
Mammography x-ray tubes
This is the consequence of ___: The intensity of the x-rays emitted from the focal spot varies within the beam, with the greatest intensity on the cathode side of the projected field and the lowest intensity on the anode side.
Positioning the cathode over the chest wall of the patient and the anode over the anterior portion (nipple) achieves better uniformity of the transmitted x-rays through the breast (Fig. 8-5). Orientation of the tube in this way also decreases the equipment bulk near the patient’s head. The anode is kept at ground potential (0 voltage), and the cathode is set to the highest negative voltage to reduce off-focal radiation
Heel effect
All dedicated mammography systems utilize this, achieved by fixed collimation at the xray tube head, in order to avoid exposure of patients’ torsos and to maximize the amt of breast tissue near the chest wall that is imaged
half-field xray beam geometry
What is the tube port window made of?
The tube port and added tube filters play an important role in shaping t
beryllium (Z=4)
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.As we know, low energy xrays provide the best differential attenuation for tissues. ;)
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 energies between 20 and 27 keV. Elements that have these K-shell binding energies include ___ and each can be shaped into thin, uniform sheets to be used as added x-ray tube filters.
Mo
Rh
Ag
Screen-film detectors most often use a ___ 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.
Mo target
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 (
True or False
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
True
The half-value layer (HVL) of a mammography x-ray beam ranges from___.
The HVL of for breast tissues is from. ___?
0.3 to 0.7 mm
1-3 cm
___ 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 8- to 12-keV range (Fig. 8-11A). Therefore, minimum filter thicknesses of 0.05 mm for Rh and Ag are needed to attenuate the L-x-rays to negligible levels
Tungsten (W)
This is a measure of the intensity of the x-ray beam, typically normalized to mAs or 100 mAs, at a specified distsance from the source (Focal spot).
Tube output
Common units of tube output are mGy (air kerma)/100 mAs and mR (exposure)/mAs.
True or false?
Even though W targets are more efficient at producing x-rays, the thicker filters needed to attenuate the L-characteristic x-rays result in lower tube output per mAs compared to the Mo target.
True
However, W spectra have higher HVLs and greater beam penetrability, allow higher tube current, and result in comparable exposure times to a Mo target and filter for a similar breast thickness.
This 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).
Tube output rate
To ensure the ability to deliver a sufficient x-ray beam fluence rate to keep exposure times reasonable, MQSA regulations require that systems be capable of producing an air kerma rate of ___?
at least 7.0 mGy/s, when operating at 28 kV in the standard (Mo/Mo) mammography mode