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
Common units of tube output are ___?
mGy (air kerma)/100 mAs and mR (exposure)/mAs
This is achieved by the use of fixed-size metal apertures or adjustable shutters. Its application to the full active detector area is the standard of practice.
Collimation of the x-ray beam
This is crucial to ensure that the x-ray beam central axis is perpendicular to the plane of the image receptor and intercepts the center of the chest wall edge of the image receptor. This will protect the patient from unnecessary radiation exposure to the lungs and torso and include as much breast tissue as possible in the image. Tube alignment with respect to the image receptor must be verified during acceptance testing of the system and after x-ray tube replacements.
Alignment of the xray tube and collimator
used for optimizing the oblique projections for subjects with smaller breasts to accommodate positioning of the arm and shoulder at the top edge of the receptor.
Field shifts
What are the differences of dedicated mammography xray generator to a conventional one?
- lower voltages supplied to x-ray tube
- space charge compensaton
- automatic exposure control (AEC)
reduces the breast thickness further over a specific region and redistributes the tissue for improved contrast and reduced anatomic overlap
smaller “spot” compression paddle (~5-cm diameter)
This reduces overlapping anatomy, decreases tissue thickness, and reduces inadvertent motion of the breast.
Compression
achieved witha compression paddle (a Lexan plate)
The full area compression paddle matches the size of the image receptor. While firm compression is not comfortable for the patient, it is often necessary for a clinically acceptable image
has a right-angle edge at the chest wall to produce a flat, uniform breast thickness when an adequate force of 111 to 200 newtons (25 to 44 lb) is applied
Compression paddle
x-rays transmitted through the breast contain primary and scattered radiation. ___ radiation carries information regarding the attenuation characteristics of the breast and delivers the maximum possible subject contrast to the detector. ___ radiation is an additive, gradually varying radiation distribution that degrades subject contrast and adds random noise.
Primary radiation
Scattered radiation
Scattered radiation reaching the image receptor can be greatly reduced by the use of ____
Anti-scatter grid or air gap
True or False
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.
True
Adverse effects of scattered x-ray
Screen film mammo: ___
Digital mammo: ___
SFM: reduction in contrast
DM: decrease signal to noise ratio (SNR)
For contact mammography, an antiscatter grid is located between the ___ and the ___.
breast and detector
This is made of thin copper septa and provides scatter rejection in two dimensions; it provides a better contrast improvement factor than the linear grid
cellular grid
This is the ratio of exposure with the grid compared to the exposure without the grid to achieve the same film optical density.
Bucky factor
For mammography grids, the Bucky factor is ___, so the breast dose is doubled or tripled, but the benefit is improvement of image contrast by up to 40%
2 to 3
This type of magnification is used to improve system resolution, typically for better visualization of microcalcifications
Geometric magnification
is achieved by placing a breast support platform (a magnification stand) at a fixed distance above the detector, selecting the small (0.1 mm) focal spot, removing the antiscatter grid, and using a compression paddle
This reduces scatter by increasing the distance of the breast from the detector, so that a large fraction of scattered radiation misses the detector; consequence of using this is reduced FOV (field of view)
Air gap
The most significant limitation of magnification
geometric blurring
caused by finite focal spot area
Even with a small 0.1-mm focal spot, spatial resolution will be less on the cathode side of the x-ray field (toward the chest wall) where the effective focal spot length is largest
Advantages of magnification
- increased effective resolution of the image receptor by mag factor
2.reduction of image noise relative to the objects being rendered - reduction in scattered radiation
True or False
Digital mammography has been shown to be more accurate for women under the age of 50 years, women with radiographically dense breasts, and premenopausal or perimenopausal women compared to screen-film mammography
True
What comprises a mammographic screen film detector
Design of Mammographic Screen-Film Systems
- cassette
- intensifying screen
- light-sensitive film
Chemical processing in a bath of developer solution renders the invisible latent image into a visible image by reducing the sensitized grains to elemental ___.
silver
Most cassettes are made of low-attenuation carbon fiber and have a single high-definition phosphor screen used in conjunction with a single emulsion film. The most commonly used screen phosphor is?
Terbium-activated gadolinium oxysulfide (Gd2O2S:Tb)
The intensifying screen is positioned in the ___ of the cassette so that x-rays travel through the cassette cover and the film before interacting with the phosphor.
back of the cassette
___ are sensitized by x-ray–induced light from the phosphor screen. Light produces latent image centers on microscopic light-sensitive silver halide grains in the emulsion layered on a film substrate
Mammography films
critical step in the mammographic imaging chain for screenfilm detectors and is performed by automatic film processors
Film processing
true or false
If the developer temperature is too low, the film speed and film contrast will be reduced, requiring a compensatory increase in radiation dose.
If the developer temperature is too high or immersion time too long, an increase in film speed occurs, permitting a lower dose
however, the film contrast is likely to be reduced because film fog and
True
This is also referred to as an H and D curve and represents the relationship between the incident x-ray exposure (proportional to light intensity on the film) and the resulting OD in the processed film
Film characteristic cure
This is a measure of brightness and since mammography films are exposed to high optical densities (OD) to achieve high contrast, a high___ must be provided
luminance
This is measured by ILLUMINANCE and must be reduced to elimitate reflections from film and improve contrast
ambient light intensity
This is the luminous flux incident upon a surface per unit area, measured in lux or lumens/m2
Illuminance
The most important advantage of digital mammography
- ability to overcome exposure latitude limitations of screen film detectors
- produces better image quality at lower doses
- increased productivity
- reduced pt waiting time
- increased quality and conspicuity of lesions
Major drawbacks of screen film detectors
- limited exposure dynamic range
- narrow latitude (which are necessary for high contrast)
HIghly glandular breast:
produce exposure latitude >200:1
causing underexposed ___ and overexposed ____
underexposed glandular tissue
overexposed skin line and thinner parts of breast
major detector technologies used in digital mammography.
Flat panel detector arrays
collects the local signal (electric charge) generated during the x-ray exposure, absorption, and conversion process; stores the charge in a capacitor attached to each detector element;
Thin film transistor (TFT) - an active matrix
Key component of each detector element of flat panel detector array
In some “fast” flat panel designs, readout of the whole array is performed in hundreds of milliseconds, allowing near real-time acquisition of data for applications such as digital tomosynthesis.
- transistor
- charge collection electrode
- storage capacitor
Approved digital mammography systems are based on 4 technologies currently
- indirect x-ray conversion TFT flat panel array receptor
- direct x-ray conversion TFT detector
- cassette-based, dualside readout CR photostimulable storage phosphor (PSP) imaging plate detector and reader system.
- narrow 22 cm long charge-coupled-device (CCD)-detector array, optically coupled to a CsI phosphor x-ray converter,
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
Refers to direct generation of charge by x-rays within the photoconductor and capture by the electrode without intermediate signals
Direct conversion
The laser beam provides energy to free the stored excited electrons from the traps; these electrons fall to a lower energy state with the emission of light, a process called
Stimulated luminescence
Due to high-spatial resolution requirements, digital mammography images are quite ___.
large
FDA regulations require that facilities maintain mammography films, digital images, and reports in a permanent medical record of the patient for a period of not less than ___ years and not less than ___ years if no additional mammograms of the patient are performed at the facility.
5 years and 10 years
It is strongly recommended that diagnostic interpretation of digital mammograms be performed on display monitors approved by the FDA for mammography having a minimum of ___ million pixels (___ MP) and a calibrated, sustained maximal luminance of at least ___ cd/m2 (per ACR guidelines for digital mammography), but preferably at ___ cd/m2
5 million pixels
450 cd/m2
but preferably 600 cd/m2
It is a computer-based set of algorithms that incorporates pattern recognition and uses sophisticated matching and similarity rules to flag possible findings on a digital mammographic image, which may be a digitized film mammogram.
computer-aided detection (CADe)
Provide the capability to localize breast lesions in three dimensions and are used for targeting microcalcifications associated with lesions
whereas localization and biopsy of masses and areas of architectural distortion are performed either with ultrasound guidance or MRI.
Stereotactic breast biopsy
The image receptors of these systems are CCD cameras coupled to x-ray phosphor screens of 5 cm 5-cm FOV by either lens optics or fiber optics. A 1,000 1,000 detector element matrix, with an effective detector element size of 50 mm (0.05 mm), which is equivalent to10 lp/mm, is deemed sufficient to visualize microcalcifications.
Image acquisition is performed by integrating light from x-ray exposure of the phosphor screen on the CCD photodetector, reading the detector element array one line at a time after the exposure, amplifying the electronic signal, and digitizing the output
Stereotactic breast biopsy systems
A method for reducing this superimposition is to acquire multiple low-dose images at several angular positions as the x-ray tube moves in an arc about the breast.
Breast Digital Tomosynthesis
Each image projects the content in the breast volume with different shifts depending on the distance of the object from the detector. With high-speed digital readout, several projection images can be acquired in under 10 s, and each image set is processed with a limited-angle reconstruction algorithm to synthesize a tomogram (in-focus plane) at a given depth in the breast.
Breast digital tomosynthesis
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 ( 7.5 to 50 degrees) in a short time span (4 to 20 s).
technique of choice for detecting nonpalpable breast cancers
Xray mammography
The ___ is the site of carcinogenesis, and thus the preferred dose index is the ___.
glandular tissue; average glandular dose
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.
This is the dose delivered to the plane of tissue in the middle of the breast and is typically lower than the average glandular dose and does not account for variation in breast tissue composition.
Midbreast dose
In Average Glandular Dose, this is used to measure the air kerma for a given kV, mAs and beam quality
air ionization chamber
Average glandular dose (Dg)
Dg = DgN x X ESAK
Xesak = entrance skin air kerma in mGy
D gN = air kerma to average glandular dose conversion factor in mGy dose/mGy incident air kerma
The MQSA regulations limit the average glandular dose for a compressed breast thickness of ___ cm and a breast composition of ___% glandular and ___% adipose tissue (the MQSA-approved mammography phantom) to ___ mGy per film or digital image (___ mGy for two films or images).
4.2 cm
50%
50%
3 mGy per film
6 mGy per 2 films
The average glandular dose using screen-film receptors is typically about ___ mGy per view or ___ mGy for two views. For full-field digital mammography systems, the dose is lower, typically from ___to ___ mGy per view.
2 mGy per view
or 4 mGy for 2 views
1.2 to 1.8 mGy per view
Factors affecting Breast Dose
- speed of screen-film receptor
- film OD
- digital detector SNR level
X-ray absorption is strongly affected by?
- breast thickness
- tissue composition
The thickness of a material required to reduce the air kerma or exposure rate of an x-ray or gamma ray to half its original value
from sources
Half-value layer
When x-rays interact with matter, energy is transferred from the x-rays to electrons in two processes, photoelectric effect (photoelectrons) and Compton scatter (Compton electrons). ___ represents the kinetic energy released per unit mass when an x-ray beam is traveling through air, and it is how the intensity (strength) of the x-ray beam is characterized [6]. ___ can be thought of as the number of individual x-ray photons per unit area, which therefore says nothing about the extent (area) of the x-ray beam. ___ is expressed in grays or milligrays
From internet
Air Kerma
True or false
Digital acquisition with higher effective energy beams is shown to substantially reduce average glandular dose over that of screen-film systems.
True
Higher kV (higher HVL) ___ beam penetrability (lower ESE and therefore ___ average glandular dose) but also ___ subject contrast.
increases; lower;
decreases
