Mammography physics and technique
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Mammography is the cornerstone of breast imaging and offers the necessary reliability to diagnose curable breast cancers. It involves using low-dose x-rays of the breast to detect tumors that are too small to feel. Digital mammography offers superior contrast resolution in dense breasts compared to conventional mammography but has lower spatial resolution, potentially missing some lesions. Mammography equipment includes an x-ray tube, compression device, and digital detectors to capture and process images, allowing diagnosis according to the BI-RADS assessment categories.
Mammography physics and technique
- 1. MAMMOGRAPHY- PHYSICS AND TECHNIQUE Dr. Archana Koshy
- 2. There are several breast imaging modalities available such as Ultrasound, CT,Digital Mammography ,MRI and scintimammography . Mammography remains the cornerstone of breast imaging . Only mammography when correctly performed and interpreted offers the necessary reliability to diagnose the curable forms of breast cancers. Ultrasound,MRI , CT are useful adjuncts once a lesion has been detected by physical examination or by radiographic mammography .
- 5. XEROMAMMOGRAPHY -X-rays strike an electronically charged plate of selenium- coated aluminum. The loss of charge at sites of X- ray interactions leads to latent image formation which is converted to visible image by reading the charge pattern. This technique gives a very sharp but low contrast image.
- 6. INDICATIONS Screening of asymptomatic women Screening of high risk women Follow up of patients after mastectomy of same and opposite breast / same breast with implant . Investigations of benign breast diseases with eczematous skin,nipple discharge , skin thickening . Investigation of a breast lump Investigation of occult primary with secondaries . Male breast evaluation .
- 7. PHYSICS
- 8. MAMMOGRAPHY EQUIPMENT Generator Xray tube – fine focus of 0.2-0.5 mm with an additional 0.1 mm focus for magnification . Target – Molybdenum and Rhodium Beryllium window – Minimises absorption of radiation within the tube . Molybdenum filter – By transmitting only characteristic radiation ,absorbs unwanted radiation and forms a monoenergetic beam. Compression device : 1-4 mm thick plastic plate
- 11. TARGET The xray tubes are designed with Tungsten ,molybdenum,Rhodium targets . Tungsten target is operated under 30 kVp with a 0.5 mm Al filter – Brehmsstrahlung and 12 keV L-shell characteristic xrays . Molybdenum target with Mo/Rh filter is more suitable for mammography . Rhodium target filtered with Rh filter gives a similar spectrum as molybdenum . Since the atomic number is higher , the brehmsstrahlung radiation is also higher than molybdenum .
- 12. FILAMENT Positioned within a focussing cup with two focal spot sizes . Filament types are either double wound/flat ribbon/circular filament . Focal spot size is very critical in mammography where high spatial resolution is required . Small focal spot is used with small anode angle , which permits the use of high mA factors . Usually the cathode is positioned towards the chest wall , which makes patient positioning east and takes care of the heel effect .
- 13. FILTERS Thin beryllium window or borosilicate glass window is used to reduce inherent filtration , since it offers low attenuation . For a tungsten target Xray tube , Mo or Rh filter is recommended . In case of Rhodium target Xray tube , Rhodium filter of 0.025 mm is used which gives high quality xrays with higher penetration . Combination is suitable for thicker and dense breast imaging .
- 14. GRIDS Stationary grids or grids placed in between the screen and the film are no longer used as the thin grid lines compromised on the quality of the image . Covered tiny details such as microcalcifications Hence oscillating grids are used Grid ratio of 4:1 or 5:1 The grid lines are eliminated by the motion of the grid . Grids improve the image quality and cause a significant reduction
- 16. HEEL EFFECT Shape of the breast requires higher intensity of radiation near the chest wall, to create uniform exposure to the screen-film . The cathode is positioned toward the chest wall and the anode is toward the nipple . Permits easy positioning of the patient . Increases the intensity of radiation near the chest wall, where greater penetration is needed . The anode is often grounded with zero potential and the cathode is given higher potential .
- 18. COMPRESSION PEDDLE (i) Decreases the thickness of the breast , thus reduces the scattered radiation –improves the contrast . (ii) Decreases the kinetic blur . (iii) Reduces geometric unsharpness by homogenously bringing the object close to the film . (iv) Makes breast thickness uniform in film density. (v) Differentiates the easily compressible cysts and fibro- glandular tissue from the more rigid carcinomas (vi) Separates the super imposed breast lesions . (vii) Reduces radiation dose to the breast tissue .
- 19. -The compression device is parallel to the receptor surface . -Radiolucent plate that is flat and parallel to the support table . - Should match the cassette size
- 20. AUTOMATIC EXPOSURE CONTROL AEC system employs phototimers to measure the Xray intensity and quality . Kept closer to the image receptor , to minimize the object to image distance – improves spatial resolution . TWO TYPES : (i)Ionization chamber type (ii) Solid state diode type
- 21. SCREEN FILM SYSTEM The Xray film should have high resolution and small grain size and used along with single intensifying screen . The emulsion surface of the film must face the screen . Prevents loss of resolution due to light diffusion in the screen . Gadolinium Oxysulfide activated with terbium is used as screen phospor .
- 24. CRANIO-CAUDAL VIEW The casette is placed under the breast at the level of the inframammary fold . The breast is then pulled until the inframammary fold is taut . Compression is applied and Xray beam is directed vertically from above . Postero medial aspect should also be included .
- 26. MEDIOLATERAL OBLIQUE VIEW Best view to image all of the breast tissue and the pectoral muscle . The C-arm of the mammographic unit is rotated to 45 degree so that the cassette is parallel to the pectoral muscle . The film holder is kept high up in the axillary fossa and the patient s arm is abducted at the elbow by 80degrees. The xray beam enters the breast from the medial side –compression is applied to the pectoralis major muscle .
- 28. SUPPLEMENTARY VIEWS 1. Lateral projections (mediolateral/lateromedial) 2. Extended craniocaudal projection 3. Cone down compression view 4. 90 degrees lateral view and angled craniocaudal views 5. Tangential views in palpable masses 6. Spot and magnification views 7. Cleavage view 8. Modified compression technique .
- 29. Adipose tissue comprises a large portion of most breasts and is radiolucent. The radiographically visible densities include: (i)varying degree of ducts (ii)lobular elements (iii) fibrous connective tissues structures. The supportive connective tissue is highly cellular and richly vascularised with significant water density- constitutes the essence of breast opacity in mammography.
- 30. Sexual hormones influence the degree of hydration of the adult breast. The breast is more radiolucent and better contrasted in the first part of the menstrual cycle Pregnancy and lactation make the glandular and connective tissue structures hypertrophic and more hydrated which leads to diffuse increase in breast density.
- 31. DIGITAL MAMMOGRAPHY The stages in digital imaging : 1. Image capture by digital detector 2. Conversion of latent image into digital data set 3. Processing of image data 4. Display of processed image 5. Transmission and archival of data set
- 34. IMAGE ACQUISITION DEVICES CHARGE COUPLE DEVICE TECHNOLOGY Digital detectors consist of a phosphor, which is optically coupled to the CCD. ADVANTAGE- The slit collimation of the X-ray beam results in a significant reduction in scatter. This eliminates the need for a grid, leading to potential dose savings .
- 35. SELENIUM-BASED DETECTORS Selenium based detectors consist of a thin layer of amorphous selenium deposited on the imaging plate. A uniform positive charge is built up on the surface of the selenium. When X-ray photons strike the surface of the selenium, electrons are freed. This leads to a partial discharge of the uniform surface charge pattern. The degree of discharge reflects the local radiation exposure, leading to a latent image in the form of a charge distribution pattern. This charge pattern is then “read” and the resultant electrical signal digitised.
- 36. ADVANTAGES OF DIGITAL MAMMOGRAPHY 1. Acquired in less than a minute as compared to conventional mammography . 2. Spot view digital systems are available to help guide breast biopsy . 3. Superior contrast resolution especially in dense breast tissues 4. Offers the possibility of three dimensional breast tomography using relatively low doses .
- 37. DRAWBACKS Until ways are found to increase the spatial resolution of digital mammography, conventional film mammography will remain better able to detect subtle tissue changes such as microcalcifications. In dense breast tissues even digital mammography may overlook some lesions that are masked by overlying structures
- 39. BI-RADS CATEGORY 0 -Need additional Imaging evaluation CATEGORY I - Negative CATEGORY 2-Benign finding CATEGORY 3-Probably benign findings CATEGORY 4-Suspicious abnormality CATEGORY 5 -Highly suggestive of a malignancy
Editor's Notes
- #3: It is known and verified by careful statistical analysis in multiple studies that early detection of cancer has an excellent prognosis, which allows majority of breast to be preserved and decreases the mortality. Radiographic mammo can detect occult breast cancer with a proven efficacy for the screening of asympotmativ women-Can detect clustered microcalcifications –usually the only indication of an intradcutal malignancy . Although mammo is a poweful tool for detection and follow up of suspicious lesions,it has imp limitations in detecting subtle soft tissue lesions , especially in the presence of dense glandular tissues .
- #4: The breat mainly consists of three tisues , fibrous tissue,glandular tissue , adipose fat ., In pre menopausal women, the fibrous and glandular tissues are characterised by ducts ,glands and connective tissue surrounded by fat layer . Degenration of fibroglandular tissue and increased of adipose tissue is found in post menopausal women , The radiologic image of the breast is composed of juxtaposition of two anatomic units , the mammary gland (connective glandular tissue ) and the fatty sunbcutaneous amd retromammary tissue . The breast lies upon the deep pectoral fascia, which in turn overlies pectoralis major and serratus anterior superiorly and external oblique and its aponeurosis (as the latter forms the anterior wall of the sheath of rectus abdominis) inferiorly . Between the breast and the deep fascia the loose connective tissue in the ‘submammary space’ allows the breast some degree of movement on the deep pectoral fascia. Advanced mammary carcinoma may, by invasion, cause tethering or fi xation of the breast to the underlying musculature.
- #5: On mammogram skin appears at the periphery of breast as a uniform, well-defined opaque rim. Its thickness ranges from 0.5 to 2 mm with an average of 1 mm. Nipple areolar complex is situated at thesummit of mammary cone. Areola is composed of muscular and cutaneous glandular structures and appears as thickening of cutaneous line forming a segment of arc around the nipple.
- #6: It was a very useful technique for breast imaging, which provided high detail breast imaging with wide exposure latitude at considerably lower dose than that required by the film technique of the 1960s.
- #7: High risk – family history of ca breast , atypical hyperplasic of lobule , papillomayosis Mammo is 85-954% sensitive and can be used as a screening technique for breast masses but due to the low spec as compared to USG . MRI it not very useful to charc the lesion . But its ability to guide stereotactic biopsy is an added advantage .
- #9: 1 generator having a high output rate to minimise the exposure time 2. Xray tubes in newer machines have a rotating anode . Molybdenum and rhodium as it gives characteristic spectra of 17.4 kev and 19.6 kev . 17 give optimum soft tissue contrast that is desirable to form a good mammography image . The compression plate is to compress the breast tissue to a uniform thickness for the incident radiation , thereby decreasing geomteric and motion unsharpness ,radiation dose and scattered radiation . Even the nature of the lesion can be assessed to an extent by compressibility . The exposure factors usually used are low kvp and high mAs (with a time less than 2 sec ) \breast is a radiosensitive organ therefore risk of mslignancy increases with increasing exposure .
- #12: Brehmsstrahlung xray is useful for mammogram and the charactertistic xray too low for image formation and contributes only to patient dose . Molybdenum produces strong k shell characteristic xrays . The spectral variation between tungsten and molybdenum is due to their atomic number . Mo and rh have char xrays in the range useful for mammography . Their brehm is smaller than tungsten hence most of the filters that are used are Mo and Rg target with their respective filters or combined .
- #13: focal spot sizes ( 0.3 , 0.1 mm respectively ) actual focal spot size is 0.3 – 0.4 mm and that 0.1 mm focal spot size if for obtaining a magnified image . The xrsay tube in newer systems have a rotating anode . Double wound – to increase electron density Flat ribbon to give more focused unifrm beam Circular fil – to create camel hump profile ( two foval spots ) Smaller focal spot is necessary to image microcalcifications It also minimizes geometric blurring and gives high spatial resolution .,
- #14: Filters are used to remove the unwanted high energy brehmsstrahlung xrays . Total beam filtration should not be lesser than 0.5 mm .. Generally filter material is same as target material. Allows the K characteristic xrays to reach the breast and supress the low and high energy brehmstrahlung xrays l. these filters remove the bresstrahlung photons above the k edge energy . Grids and colimators are also used . Currently high transmission cellular gris is employed . Which is basically a crossed grid ratio of 3:8:1 which reduces radiation in two directions ..cu and air are used as grid strip and interspace material .
- #16: He best way to counter scatter radiation is to use moving lsits that are aligned The pre collimated breast is also protected from irriadiation . so the post collimation breast will receive only the transmitted radiation into the detector . The difficulty in this approach lies in the mechanics between moving the slits together and keeping the alingment properly wit the cray beam
- #17: Heel effect csuses the xray intensity is always higher at the cathode side . Anode side is bulky and away from the patient . Effective focal spot size is hgiher than the chest wall side which reduces the spatial resolution , hence mammo tubes use tilted xray tubes with long source – image distance . The
- #19: Breast compression ,whether manual or motorized should be gentle and progressive . Improves the image on mammo by the foll ways : Helps maintain perfect immobility helps reduce motion u nsharpness May be difficult or almost impossible in heamorrhagic lesions ,acute inflammatory lesions and in hypersensitive breasts . Should be applied in a proper manner , otherwise it can produce artefacts by skin folds .
- #20: Cassette size – 18 x 24 cm or 24 x 30 cm Can be operate by foor from either side of the patient . Spot compression peddle is also used . To provide compression in a particular region of interest . Useful to reduce thickness locally Eliminates super imposed anatomy by spreading the tissue furthe r.
- #21: Matches the cassette size . Each type will have single or multiple detectors along the chest wall- nipple axis . Detectors are filtred differentially to assess the beam quality . Thick breast is best images with Rh rh combo and thin breast with mo mo combo AEC must be accurate and reproducible with lesser radiation dose.
- #22: The cassettes , screens and films are especially made for mammography . The cassette is made of carbon fiber , to have low attenuation are available in 18 x 24 cm and 24 x 30 cm sizes . The INTENSIFYING SCREEN INCREASES THE SPEED,ENHANCES THE CONTRAST and reduces radiation dose to the patient . This facilitates the xray to interact at the surface layer of the phoshor so that the light produced wil travel only a short distance to reach the film . If the screen is placed between the patient and the film , then excess blurringoccurs that will reduce the spatial resolution . Gadolinum is used since it emits green light to match the green senisitve film .
- #23: The thin screens sometimes let some photon pass on completely without recoridng and image ,. Dose reduction is calculated from the fact as to how many photons the screen absorbs A signal that is usually passing throgu the breast carries valuable information but it also vcarries with it unnecessary noise that is pointless It can hide the primary signal . Thertefore the clarity of the image is based on the signal to noise ratio./ A parameter known as the detector quantum efficiency is calculates wherein the signal to noise ratio the whole swuare is calcuylate at the time of striking thr screen and when it leaves the detector .
- #24: A thicker screen would be more efficient in absorbing these photons . However the problem that usually occurs is that the photons is converted to light in the screen and the light is radiated to all points causing more of a blur than a point . Advantage is additional wuantum detection efficiency and dose reduction by providing greater photon absorbing ability Disad : Cross over occurs when th elight from one screen to the other exposing the emulsion with wide edge causing a blur Parallax also reduces the sharpness of the image The image on the tube side will be smaller than that of t he image on the far side and the lack of superimposition will cause a blur . Sp even though double emulsion films reduce patient dose,they highly compromise on image sharpness \ So double emulsion /screen films should have magnification power that is equal to the single emulsion films using vontact technoique .
- #25: A correctly perfomred CC should show the pecs at the posterior edge of the breast indicating that it has been positioned as far forward as possible . The appropriate position is determined by poserior nipple line A line is drawn form the nipple back to the edge of the film and it has to be within a 1 cm meadsurement when the MLO view of the same breast is taken . If not , it indicates that some tissue has been excluded in the cc view .
- #26: On mammogram skin appears at the periphery of breast as a uniform, well-defined opaque rim. Its thickness ranges from 0.5 to 2 mm with an average of 1 mm
- #27: When properly positioned , MLO view should show a good isualisation of posterior breast ,axillary tail and inframammary tissue is obtained .
- #29: Additional view in order to localise the exact position of the lesion . Shows less breast tissue and pectoral muscle than MLO proj necessary for needle localisation of the lesion . Extended CC view , aka cleopatra view , better imaging of lateral part of the breast ,especially the axillary tail . Used to show lesion high in the axillary fold not visible on CC . Spot and magnification views performed to obtain more details of a lesion . Used for better visualisation of microcalcifications . Modified compression technique is applicable for implants .
- #30: The anterior and posterior fat planes and fatty lobules are radiolucent structures. Ductal image is due to collagen and periductal collagenous tissue. These ducts look like serpigineous structures converging at the nipple
- #31: Fifteen days onwards of menstrual cycle, the image becomes denser and connective tissue structures look thick as a result of diffuse hydration.
- #32: In contrast to conventional film mammography, which uses radiographic film to acquire, store and display an image, digital mammography conveniently offers a means of separating these functions, allowing optimisation at each stage Once the digital image is stored in computer memory, it can be displayed with contrast independent of the detector properties and can be changed by the radiologist Unlike the film mammography unit, digital mammography detector size is limited to a small format mammography film (10 × 12 centimeters), which means that a woman with a larger than average size breast would require multiple images to visualise her entire breast in each projection.
- #34: Unlike the film mammography unit, digital mammography detector size is limited to a small format mammography film (10 × 12 centimeters), which means that a woman with a larger than average size breast would require multiple images to visualise her entire breast in each projection.
- #35: Initially, only small field sizes were possible in the order of 5 × 5 cm. Consequently, such systems were first employed in digital spot mammography for stereotactic needle biopsy.7,8 The success of digital spot mammography has led to the development of full-field devices using CCD technology; such devices are capable of imaging the whole breast
- #40: The American College of Radiology appointed a committee in the late 1980s to develop guidelines to standardise reporting of mammographic findings.6 This work was published as the Breast Imaging Reporting and Data System, referred to as BI-RADS. provides a dictionary of terms to use when describing lesions seen on mammography and provides specific final. Category 2 -This is also a normal mammogram,but the interpreter may wish to describe a finding. These include lesions which can be clearly labelled as benign, e.g. involuting, calcified fibroadenomas or fat containing lesions, 3 -Short interval follow-upsuggested. A finding placed in this category should have a very high probability of being benign (< 2 per cent probability of malignancy). These include small (< 8 mm) circumscribed masses, small round clustered microcalcifications and focal asymmetric Densities . Some authors also include multiple rounded densitites seen for the first time and a solitary dilated duct in this category. Category 4 Suspicious abnormality—may be malignant. Biopsy should be considered (Table 22.3). Category 5 Highly suggestive of malignancy.This category includes spiculated lesions and fine,linear, branching microcalcifications with or without secondary signs (appropriate action should be taken).