Conventional Breast Imaging

Mammography

Mammography is an x-ray-based technique that makes use of the different absorption rates of fatty and fibroglandular tissue (FGT) in the breast. Because the breast consists only of soft tissues, a low-energy spectrum is used. Typical settings for mammography are between 25 and 35 kV. Whereas original mammography consisted of little more than an x-ray tube placed against the breast with a film on the opposite side, currently mammography is performed with dedicated mammography machines.

In addition to the x-ray tube and detector, these machines also include a compression mechanism that is used to flatten the breast on the detector plate. This leads to a better spread of the FGT and thus better interpretability of the resulting image. Furthermore, it also lowers the dose needed for high-quality images, as that is mainly dependent on the breast thickness. Besides, it prevents motion artifacts that would otherwise obscure the fine details in the image.

A standard mammogram consists of two views per breast. The standard views are mediolateral oblique (MLO) and craniocaudal (CC). As the glandular structures in the breast have a teardrop-like configuration with the tail in the direction of the axilla, the MLO view is usually perpendicular to this and therefore shows most of the breast parenchyma as well as part of the pectoral muscle. The CC view depicts part of the medial breast that is inaccessible in the MLO view and also enables a somewhat higher compression, as it does not include muscles, which is beneficial to discriminate small masses from FGT. Strict quality control is usually employed for these acquisitions. Images are considered adequate when the whole fibroglandular disc is depicted, the nipple shows in profile, and no folds are present. In MLO acquisitions, the pectoral muscle should be visible from the level of the nipple; in CC acquisitions, the pectoral muscle should just be visible on the dorsal side of the breast, although particularly the latter often shows infeasible.

In case the standard views do not sufficiently show the breast tissue, several additional views are commonly performed. These include nipple views to show the retroareolar area, the axillary view (Cleopatra view) that better depicts the axillary tail, and the cleavage view that shows the medial part of both breasts. In women with breast implants, an Eklund view is sometimes performed, in which the prosthesis is pushed to the back and only the FGT of the breast is compressed.

Mammography requires a very high spatial resolution to accurately depict the structures in the breast. Typical voxel sizes are below 100 × 100 microns. Although tumors detected with mammography are on average about 15 to 20 mm in size, the high resolution enables accurate assessment of the lesion margins. Moreover, the high resolution enables the depiction of calcifications in the breast that may be associated with breast cancer. These calcifications commonly have a size of less than 200 microns and are therefore only visible when such high spatial resolutions are achieved.

In the second decade of the 21st century, digital breast tomosynthesis (DBT) emerged as the better mammogram. In DBT, the same machinery is used but the x-ray tube is moved over the breast in a limited angular range (15–50 degrees, depending on the machine used). During this movement, multiple acquisitions are obtained, which are subsequently reconstructed to provide a pseudo-3D stack of images that provides some depth information of the imaged breast. This sometimes enables visualization of lesions that would otherwise be hidden in the normal FGT, and conversely may enable discarding potential lesions that are caused by the projection of overlapping fibroglandular structures. Consequently, it was reported that sensitivity for cancer increases; specificity also increases, albeit especially the latter is strongly dependent on the threshold used for mammographic recall. It is possible to create a synthetic mammogram from the tomographic projections that, when used in conjunction with the tomosynthesis stack, obviates the creation of a regular mammography, which limits the radiation dose applied.

When evaluating a mammogram or DBT, the first objective is to assess whether the obtained views meet the quality criteria for each view. Subsequently, the normal breast configuration is assessed. This includes an assessment of the relative amount of FGT in the breast, which is commonly referred to as density. Density is typically described according to the Breast Imaging Reporting and Data System (BI-RADS) that discerns these four categories: (1) almost entirely fatty tissue, (2) scattered FGT, (3) heterogeneous FGT, and (4) extremely dense FGT ( Fig. 2.1 ). However, there are currently several commercial applications available that automatically determine density from a mammogram and have been shown to be more stable than human assessment. With increasing density, the sensitivity of mammography for breast cancer decreases, as tumors may be masked by overprojecting FGT, and this effect is still present when using DBT. In addition, the risk of developing breast cancer increases with increasing density.

Lesions depicted on mammography and DBT are also reported according to the BI-RADS lexicon. Lesions are being classified as either a mass, calcifications, architectural distortion, or asymmetry. Masses typically have convex margins and are visible in two directions. Assessment is based on their shape, margin, and density. A typical malignant mass is irregular in shape, has a spiculated margin, and is hyperdense compared with the FGT ( Fig. 2.2 ). Typical benign masses are oval, well circumscribed, and hypo- to isodense to the normal breast parenchyma. Calcifications can roughly be divided between calcifications that are certainly benign and those that may be associated with malignant breast lesions. Calcifications that are associated with malignant breast disease in general develop within the ductal structures, either due to impaction of locally trapped fluids or to local necrosis and subsequent calcification. Consequently, these calcifications are tiny and grouped. Single calcifications are hardly ever a cause for worry. Coarse calcifications are likewise virtually always related to benign abnormalities.

Ultrasound

Ultrasound imaging makes apt use of ultrasonic waves that are emitted by piezoelectric crystals that are typically embedded in handheld transducers. Common frequencies for clinical ultrasound range between 4 and 20 MHz. The emitted sound waves reflect on tissue transition boundaries and are received by the emitting transducer. Assuming a fixed speed of sound of ultrasound waves in soft tissues (roughly 1500 m/s), the time between emission and reception of the reflection can be used to calculate the depth of the tissue boundary that is causing the reflection, which is subsequently translated into an image.

The first ultrasound studies of the breast were reported in 1952. Since then, the quality of ultrasound images has dramatically improved. The current conversion of reflected sound waves into an image is really fast, and ultrasound images can therefore be viewed in real time. For breast imaging, typically, linear, high-frequency transducers (14 MHz or more) are used that create high-resolution images of all the tissue that is directly under the probe. The downside of the use of high-frequency probes is that the penetration depth is limited, and in women with very large breasts, it may be required to use lower frequency probes to visualize the deeper parts of the breast.

Breast ultrasound is mainly a targeted technique, which means that it is used to evaluate focal abnormalities primarily detected by other means. This includes both clinical symptoms (palpable lesions) and findings from other imaging tests. Assessment of ultrasound images is performed using the BI-RADS lexicon for breast ultrasound. Most lesions in ultrasound can be characterized as a mass, for which typically shape, margin, orientation, internal echo pattern, and posterior acoustic features are reported ( Fig. 2.3 ). Associated features include architectural distortion, duct changes, and skin abnormalities. Typical malignant lesions present as irregular hypoechoic masses with posterior acoustic shadowing. Benign lesions tend to be oval, with circumscribed margins and a parallel orientation. Posterior acoustic features are commonly absent. Fluid-rich lesions will show posterior acoustic enhancement.

To improve the classification of lesions on ultrasound, the use of Doppler and elastography can be considered. Malignant lesions have a stronger vascularization than benign lesions and a different orientation of feeding vessels (more perpendicular to the lesion), which can be used to strengthen the suspicion. However, the flow in peritumoral vessels is often too slow to be picked up by even state-of-the-art high-resolution Doppler techniques, which implies that absence of Doppler signals is not very informative. Elastography may be used to probe the stiffness of a lesion, as malignant lesions are commonly stiffer than benign lesions; this may be used to avoid biopsy in very soft lesions that are not very suspicious on morphological evaluation but would warrant further evaluation.