Medical Imaging and Contrast
The goal of medical imaging is to reveal information about structures within the body to aid in diagnosis and sometimes treatment. X-ray imaging and computed tomography (CT) are classic imaging modalities that provide contrast in tissues which have differing attenuation coefficients. X-rays have good spatial resolution where contrast in high (such as between bone and soft tissue). CT produces 3 dimensional images and have better soft tissue contrast than X-ray imaging. Better contrast in both X-ray and CT comes at a cost of using lower energy photons which is correlated with larger radiation dose. Magnetic resonance imaging (MRI) uses the spins of excited protons, and the relaxation times of these protons is the source of contrast in MR images. MRI has a significant advantage over CT or X-rays in that no ionizing radiation is involved. (3)
Although the imaging modalities mentioned above provide sufficient information in certain situations, there are many instances where conventional imaging falls short. Mechanical stiffness in the human body is a wide range and has been used in diagnosis of disease for years. (Palpation can be traced back thousands of years in ancient Egypt). The ultrasound community was the first to take advantage of tissue stiffness contrast. In 1991 Jonathan Ophir and group were the first to produce images of tissue stiffness by correlating pre and post compression ultrasound images to estimate strain (2). Kevin Parker and Jonathan Ophir coined the term elastography as a the mapping of elastic properties of soft tissue. This method was based on Hooke’s Law which states that stress is directly proportional to strain. Stress was directly applied to an area of interest and the resulting deformation was used to calculate the strain distribution. The proportionality constant relating stress and strain, the elastic modulus, which is the resistance to deformation.
Below is graph that compares the different ranges of contrast in various imaging modalities. (4) Notice the range of orders of magnitude obtained from elastography (102 to 107).
Magnetic Resonance Elastography (MRE)
Following the success of ultrasound elastography, in 1995 Raja Muthupillai and Robert Ehman combined elastography with MR. (1) MRE estimates the distribution of mechanical properties using MR measurements of harmonic motion fields generated using an external mechanical actuator. (see Methods for more about mechanical actuators). Pathological processes result in changes in tissue structure which is associated with changes in mechanical properties. Most diseases change the cellular structure of affected cells, predominantly the cytoskeleton which is comprised of a network of filaments and tubules. The changes on the cellular level results in changes of the macroscopic mechanical properties of the overall tissue. Changes in mechanical properties enables elastography to detect characteristics of the disease.
Cancer is a well known example of a disease process that leads to changes in mechanical properties. Breast tumors have been detected by manual palpation to identify stiff regions. A study concluded that 30% of breast cancer for women under the age of 36 were first identified purely by palpation. (5) MRE was driven by the potential to image with palpation for regions that were not accessible and regions that were too small to detect. (6)
MRE works on three steps:
- Mechanical actuation (vibration) of tissue
- Measuring the resulting tissue displacement
- Estimation of tissue mechanical properties from displacement data
Click here learn more about the methods.