Erasmus MC : General MI Knowledge

MI is broadly defined as the characterization and measurement of biological processes in living animals at the cellular and molecular level.

MI is a newly emerging field in which the modern tools of molecular and cell biology is being married in state-of-the art technology for non-invasive imaging. The goals of this field are to develop technologies and assays for imaging molecular/cellular events in living organisms. These approaches should help to lead to better methods for studying biological processes as well as diagnosing and managing diseases.

MI differs from traditional imaging in that probes known biomarkers are used to help image various targets or pathways, particular in vivo. Biomarkers interact chemically with their surroundings and in turn alter the image according to the molecular changes occurring within the area of interest. This is markedly different from previous methods of imaging which primarily imaged differences in qualities such as densities or water content. This ability to image very fine molecular changes opens up an incredible number of exciting possibilities for medical application, including early detection and treatment of disease as well as for basic pharmaceutical development. Furthermore, molecular imaging allows for quantitative tests, which adds a level of objectivity to study of these areas.

There are many different areas of research being conducted in the field of molecular imaging. Much research is currently around detecting what is known as a predisease state or molecular states that occur before typical symptoms of a disease are detected. Other important veins of research are the imaging of gene expression and the development of novel biomarkers.

There are many different modalities that can be used for noninvasive molecular imaging. Each has their different strengths and weaknesses and some are more adept at imaging multiple targets than others. Below is a summary of the AMIE techniques that are available:

OPTICAL IMAGING: Optical imaging’s most valuable attribute is that it doesn’t have strong safety concerns like other MI modalities. The downside of optical imaging is the lack of penetration. 

COMPUTER TOMOGRAPHY (CT): CT makes it possible to reconstruct a film of two-dimensional X-ray pictures into three-dimensional pictures. The introduction of the Charge Coupled Device Detectors (CCD detectors), enlargement of computer mathematics and memory capacity and useful software has made it possible to achieve a resolution of 100 micrometer or higher. CT is also highly suitable to use in combination with SPECT, PET, and MRI to visualize anatomic structures.

POSITRON EMISSION TOMOGRAPHY (PET): A molecule is tagged with a positron emitting isotope. These positrons annihilate with nearby electrons, emitting two 511,000 eV photons, directed 180 degrees apart in opposite directions. These photons are then detected by the scanner which can estimate the density of positron annihilations in a specific area. When enough interactions and annihilations have occurred, the density of the original molecule may be measured in that area. Typical isotopes include 15O, 18F, 16Cu, 62Cu, 124I, 76Br, 82Rb, and 68 Ga. One of the major disadvantages of PET is that most of the probes must be made with a cyclotron. Most of these probes also have a half life measured in hours, forcing the cyclotron to be on site. These factors can make PET prohibitively expensive. PET imaging does have many advantages though. First and foremost is its sensitivity: a typical PET scanner can detect between 10e-11 M to 10e-12 M concentrations.

SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY (SPECT): The imaging agent used in SPECT emits gamma rays, as opposed to the positron emitters used in PET. There are a range of radiotracers that can be used, depending on what is to be measured. For example, Xenon (Xe 133) which can be used for diagnostic inhalation studies for the evaluation of pulmonary function and for imaging the lungs. The radioisotopes used in SPECT have a relatively long half live (a few hours to a few days) making them easy to produce and are relatively cheap (significantly cheaper than PET or fMRI). However, it lacks good spatial or temporal resolution. Additionally, due to the radioactivity of the contrast agent, there are safety aspects concerning the administration of radioisotopes to the subject.

FUNCTIONAL MRI (fMRI) AND MAGNETIC RESONANCE SPECTROSCOPY (MRI): MRI has the advantage of having very spatial resolution and is very adept at morphological imaging and functional imaging. MRI does have several disadvantages though. First, MRI has a sensitivity of around 10^-3 M to 10^-5 M which compared to other types of imaging can be very limiting. This problem stems from the fact that the difference between atoms in the high energy state and the low energy state is very small. For example at 1.5 Tesla the difference between high and low energy states is approximately 9 molecules per 2 million. Although with the use of small animal scanners much higher strength magnets can be used which can detect much lower concentrations then weaker magnets.

RAMAN SPECTROSCOPY: This is a non-destructive optical technique that provides information about the molecular construction of cells and tissue without the use of staining, labeling techniques, etc. Like MRI, the detection limit of a molecule is in the microM till mM range. Raman is unique in research directed on the development of (in vivo) diagnostic tools and fundamental animal research. Changes in overall tissue content due to infection, cancer, elderly or genetic modification can be analyzed using Raman. This technique can be used ex vivo (e.g. Raman microscopy with a resolution of 1 micron) and in vivo but only to spot changes on the surface of the tissue (no deep tissue analysis possible). However, using thin optical fibers (<1 mm diameter) almost every location in the body can be reached. Also this technique can be used in combination with other MI techniques. 

ULTRASOUND (US) TECHNOLOGY: US is a quick, real-time, non-invasive technique. The penetration into the tissue is dependent on the ultrasound frequency. US is a relatively easy real-time method to visualize the internal body of a small animal. In addition, physiological, anatomical, biochemical and functional characteristics of the Tg or treated animal can be studied without intensive and difficult treatment. This technique is very well suited to use in large longitudinal studies. Combination with CT, PET/SPECT and Optical Imaging is possible.