Emerging technologies for ultrasonic breast cancer diagnosis: ultrasound elastography

The estimation of tissue hardness and masses by palpation during a physical examination is one of the oldest methods used in medical diagnosis. Palpation was practiced by Egyptian physicians as early as 2600 BCE. An organ or mass becomes palpable when its stiffness or hardness is greater than (or rarely less than) the tissues surrounding it.

The physics of palpation have recently been discussed by Hall.[1] Organs commonly palpated during an examination include the liver, spleen, thyroid, breast, prostate, aorta, uterus, ovaries and scrotum. The extremities are also palpated. Although palpation is incredibly useful, it is subjective and prone to errors in technique and interpretation. It is also dependent on the skill and experience of the examiner.

Until recently, it has not been possible to create qualitative or quantitative images based on tissue hardness. Hardness could be indirectly estimated by compressing tissues and watching the amount they compress relative to nearby structures. The technique of sonopalpation can be used to gain an estimate of tissue hardness by watching it deform under local pressure from a finger or small metal compressor such as a paper clip (Figure 1, video). This method allows the observer to correlate observed deformability with his or her own sensation of the hardness of the mass or structure.

FIGURE 1. Examiner slides paper clip over palpable mass while observing the margins and compressibility of mass with ultrasound transducer.

Ultrasound elastography is an outgrowth of work in tissue motion tracking techniques developed in Australia and the U.K. in the early 1980s.[2] From this work emerged Doppler-based tissue motion tracking in 1987 followed by sonoelasticity imaging in 1990 at the University of Rochester[3] and static ultrasound elastography in 1991 at the University of Texas.[2]

FIGURE 2. Pre- and postcompression waveforms are windowed, and at each depth the matching segment of the waveform postcompression is found. Displacement between pre- and postcompression segments is local tissue displacement at that depth.

Ultrasound elastography is performed by acquiring ultrasound images before and after a slight compression of the breast tissue. The amount of compression is very slight, only about 0.5 mm or less between images. Raw radiofrequency data for each scan line of the image are compared using cross-correlation techniques or other methods, and from this the amount of displacement of each small portion of tissue at various distances from the transducer is obtained (Figure 2). The strain, the change in displacement as a function of distance from the transducer, is used to create an image where large strains are displayed as bright and small strains as dark.

A typical elastogram of a fibroadenoma is shown in Figure 3. Note that an elastogram is always shown along with the corresponding sonogram so that the observer may closely correlate the two.

FIGURE 3. Sonogram (left) and elastogram (right) of fibroadenoma (arrows). In elastogram, softer areas are brighter.

The fibroadenoma (FA) is somewhat darker (harder) than the surrounding tissue, but not by a large margin. Dark areas on an elastogram may correlate with cancer but must always be correlated with the corresponding sonogram since many dark areas are related to normal tissues. The dark areas at the bottom of the elastogram, for example, are seen to be related to chest wall muscle on the sonogram and not to either the FA or a cancer. Note also that the size of the FA of the elastogram is the same as the FA on the sonogram. This is characteristic of benign lesions.

FIGURE 4. Invasive ductal carcinoma. Sonogram is on left and elastogram on right. Arrows mark the lesion lateral margins on each image.

Figure 4 shows an invasive ductal carcinoma. The lesion appears as a hypoechoic mass on the sonogram with posterior shadowing. On the elastogram, it appears as a dark (hard) lesion with an even darker center. The size of the lesion on the elastogram is slightly larger than on the sonogram. The hardness (darkness) and larger size on the elastogram versus the sonogram are characteristic of most breast cancers.

FIGURE 5. Invasive ductal carcinoma. Sonogram is on left and corresponding elastogram on right. Arrows mark lesion boundaries.

Figure 5 shows another typical appearance of breast cancer, an irregular hard mass much larger than the corresponding sonographic lesion.

The above elastograms were obtained by exporting raw RF data from an ultrasound scanner, then processing those data in a separate workstation. Newer experimental systems and commercial systems produce elastograms in real-time as the user gently compresses the breast with the ultrasound transducer. An example of a real-time elastogram is shown in Figure 6.

FIGURE 6. Real-time elastogram of fibroadenoma. (Video courtesy of Timothy Hall)

Using size difference alone, or in conjunction with a subjective hardness estimate based on the elastogram, results have varied. A recent report showed 100% sensitivity and specificity for distinguishing benign from malignant breast masses, but most studies have shown relatively high specificity (around 0.9).[5,6] Studies have suggested that the number of benign biopsies may be significantly reduced by using elastography.[7]

Elastography has been successful enough in clinical testing that most manufacturers are testing or have released high-end commercial ultrasound systems with elastography. The table shows the commercial systems that currently offer or will offer elastography.

Image quality in elastography has improved considerably since its inception in the early 1990s, but there is still considerable room for improvement. Improvements in image quality will likely aid in the characterization of smaller breast lesions and improve the ability of elastography to predict the benignity of lesions, thus reducing unnecessary breast biopsies.

Elastography has also shown promise in the evaluation of lymph nodes[8] and may therefore be useful in detecting occult nodal metastatic disease. Variants of elastography may also be useful in monitoring lymphedema,[9] a common sequela of breast cancer therapy.

REFERENCES

• Hall, TJ. Beyond the basics: elasticity imaging with US. Radiographics 2003;23:1657-1671.

• Wilson LS, Robinson DE. Ultrasonic measurement of small displacements and deformations of tissue. Ultrasonic Imaging 1982;4:71-82.

• Lerner RM, Huang SR, Parker KJ. Sonoelasticity images derived from ultrasound signals in mechanically vibrated tissues. Ultrasound Med Biol 1990;16:231-239.

• Ophir J, Cespedes EI, Ponnekanti H, et al. Elastography: a quantitative method for imaging the elasticity of biological tissues. Ultrasonic Imaging 1991;13:111-134.

• Fischer TA, Frey H, Ohlinger R, et al. Real-time elastography--an advanced method of ultrasound: results in 108 patients with breast lesions. Ultrasound Obstet Gynecol 2006;28(3):335-340.

• Itoh A, Ueno E, Tohno E et al. Breast disease: clinical application of US elastography for diagnosis. Radiology 2006;239:341-350.

• Garra BS, Cespedes EI, Ophir J, et al. Elastography of breast lesions: initial clinical results. Radiology 1997; 202(1):79-86.

• Saftoiu A, Vilmann P, Hassan H, Gorunescu F [Analysis of Endoscopic Ultrasound Elastography Used for Characterisation and Differentiation of Benign and Malignant Lymph Nodes.] Ultraschall Med 2006 Dec; 27(6):535-542.

• Righetti R, Ophir J, Garra BS, et al. A new method for generating poroelastograms in noisy environments. Ultrason Imaging 2005;27:201-s20.