Elastography is a non-invasive medical imaging technology which ultilises the principle of the difference in the mechanical property of elasticity between normal and abnormal tissues in an organ.
Basic Principle of Elastography
The term elasticity refers to the physical property of the tissue to change shape when a mechanical force is applied and to regain the original shape when the force is withdrawn. The imaging technique is based on the basic principle that there is difference in the elastic properties in between different tissues (for example muscle is stiffer than fat) and also elasticity can change with disease (for example tumor tissue is stiffer than normal tissue and hence a more compressible lesion is mostly not malignant).
Most often, it is combined with ultrasonography. First a conventional ultrasonogram is taken during which the transmit high frequency sound waves through the transducer, which hits the tissue to be imaged and the image is produced based on the reflected sound waves or echos from the tissue. After this, an external mechanical compressive force is exerted over the tissue to be imaged to produce strain and the imaging of tissue is done under this strain. The image produced is called an elastogram.
Applications of Ultrasound Elastography
1. Detection of breast tumors: Can be used as next-line imaging technique when mammography detects a suspicious lesion.
2. Distinguishing benign and malignant breast lesions: It increases the specificity of ultrasound by differentiating benign and breast tumors. Benign tumors reduce their size on mechanical compression while malignant tumors being stiff stays the same size or a bit larger. Thus it reduces the number of needle biopsies.
3. Detection of dermatological malignancies.
4. Can be used with endoscopic ultrasound for differentiating between benign and malignant lymph nodes, targeting suspicious lymph nodes for fine needle aspiration (FNA) biopsy from multiple lymphadenopathy, differentiating suspicious pancreatic masses and taking biopsy.
5. Accurately locate prostate tumor foci, determine extent and take targeted biopsies.
6. Studies have shown that it is a potential adjunctive tool in identifying thyroid cancer especially in cytologically indeterminate nodules.
7. Cardiological applications are also under review especially relating to the function of the cardiac muscles.
8. Identification of musculoskeletal masses.
CT and MR Elastography
Using elastography in advanced imaging technologies like magnetic resonance imaging (MRI) and computed tomography (CT) is also studied. Researchers have already found that MRE can be used in diagnosing various liver pathologies such as fibrosis, cirhosis, portal hypertension, tumors etc. It can also be used to image the kidney, spleen, stomach, bowel etc. Lung strain images using CT elastography is used in diagnosing pulmonary nodules.