Magnetic resonance imaging (MRI)
• Advantage: very high spatial resolution and is very adept at morphological imaging and functional imaging.
• Disadvantage: MRI has a sensitivity of around 10 -3 mol/L to 10 -5 mol/L which, compared to other types of imaging, can be very limiting. This problem stems from the fact that the difference between number of 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. Efforts are underway to increase MR sensitivity.
• There are a number of approaches used for optical imaging. The various methods depend upon fluorescence, bioluminescence, absorption or reflectance as the source of contrast. Unlike radioactive tracers, optical imaging does not have safety issues to consider.
• The downside of optical imaging is the lack of penetration depth, especially when working at visible wavelengths. Depth of penetration is related to the absorption and scattering of light, which is primarily a function of the wavelength of the excitation source.
• Fluorescent probes and labels are an important tool for optical imaging. Several studies have demonstrated the use of infrared dye-labeled probes in optical imaging.
Single photon emission computed tomography (SPECT)
• The main purpose of SPECT when used in brain imaging is to measure the regional cerebral blood flow (rCBF). The development of computed tomography in the 1970s allowed mapping of the distribution of the radioisotopes in the brain, and led to the technique now called SPECT.
• The imaging agent used in SPECT emits non-coincident gamma rays, as opposed to the coincident gamma rays associated with positron emitters (such as F18) used in PET. There are a range of radiotracers (such as Tc99m, In111, I123, Tl201) that can be used in SPECT, depending on the specific application.
• Xenon (Xe133) gas is one such radiotracer. It has been shown to be valuable for diagnostic inhalation studies for the evaluation of pulmonary function and may also be used to assess rCBF.
Positron emission tomography (PET)
• PET imaging employs positron emitting isotope. These positrons annihilate nearby electrons, emitting two 511 keV photons at an angle of180 degrees to each other. These photons are then detected by the scanner based on coincident collisions in the detector.
• PET radioisotopes include C11, N13, O15, F18, Cu64, Cu62, I124, Br76, Rb82 and Ga68. F18 FDG is the most used PET Radiopharmaceutical worldwide. One of the major disadvantages of PET is that the radiotracers must be made with a cyclotron. Since each of these radionuclides typically has a half life measured in minutes or hours, the cyclotron has to be in very close proximity to the imaging facility. This significantly increases the cost of PET radionuclides, compared to that of Tc99m, which is generator-produced.
• PET imaging does have many advantages though, since it often detects disease missed by other modalities. In addition, it operates at molar concentrations as low as picomolar.