Functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) is a diagnostic imaging technique used to obtain functional information within the body. fMRI utilizes the same technology as magnetic resonance imaging (MRI). It is a non-invasive test that uses a strong magnetic field and radio waves to create detailed images of the soft tissue within the body. Instead of looking at the structural anatomy as with MRI, fMRI looks at the blood flow and therefore the functional activity within the body. fMRI detects alterations in blood flow in response to stimuli or actions commonly visualizing cortical activity. (Warson, 2000) It is typically used in clinical practice in pre-surgical patients in neurosurgery or research aimed at neural networks.
Magnetic resonance was developed in the early 1930’s by a Columbia University physicist Isidor Isaac Rabi who experimented with the magnetic properties of atoms. He discovered that a magnetic field combined with radio wave signals caused the nuclei of atoms to flip. (Warson, 2000) The diagnostic technique using this theory called magnetic resonance imaging was later developed in the 1970’s and the first commercial MRI scanner was produced in the 1980’s. Functional MRI was developed in 1991 and is one of the latest technologies of diagnostic imaging. In the early 1990’s, the physicist named Seiji Ogawa discovered that oxygen-poor hemoglobin was affected differently by a magnetic field than oxygen-rich hemoglobin. The generation of the first fMRI scanner is accredited to Seiji Ogawa and Ken Kwong. The theory utilized the combination of strong magnetic field and radio frequency pulses with blood oxygen response to map images of brain activity on a normal MRI scan. (Warson, 2000)
Functional MRI functions similar to an MRI scanner. The cylindrical tube houses a very powerful electromagnet. A typical scanner has a field strength of 1.5 teslas(T) and a research scanner requires a field strength of 3 teslas(T). The magnetic field inside the scanner affects the magnetic nuclei of the atoms. Normal atomic nuclei are randomly oriented but under the influence of the magnetic field, the nuclei become aligned with the direction of the field. When the nuclei point in the same direction the magnetic signals from the individual nuclei add up to a resulting signal large enough to be measured. Neurons within our nervous system are too small to be imaged with current technology. However active brain cells need oxygenated blood. In 1992 the blood oxygen level dependent (BOLD) method was invented by Ogawa. BOLD utilized different magnetic properties of oxygenated and deoxygenated blood to detect changes in regional blood without the need to inject MRI contrast agents like gadolinium. In fMRI, the magnetic signal from the hydrogen nuclei in water is detected. The signal from hydrogen nuclei varies in strength depending on its surroundings. This is the basis of discriminating the signal from different tissues, specifically those of similar densities like grey or white matter. Oxygen is delivered to neurons by hemoglobin in capillary red blood cells. When neuronal activity increases there is an increased demand for oxygen and the response is an increase in blood flow to regions of increased activity. Hemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated. This difference is magnetic properties leads to small differences in the MR signal of blood depending on the degree of oxygenation. Blood oxygenation to active areas proceeds via the hemodynamic response. There is an initial decrease in blood oxygenation with an influx of neural activity, followed by a period where blood flow increases overcompensating for the increase in demand. Normally this is also followed by a “post-stimulus undershoots”. Using this method fMRI can be utilized to produce a map of brain activity, showing which parts of the brain are involved in specific mental processes. The signals processed from fMRI scans are mapped in voxels. Each voxel represents thousands of neurons. Colour is added to the image to create a map of the most active areas of the brain.
As an imaging technique fMRI has significant advantages. The primary benefit of fMRI technology is no ionizing radiation required which allows for repeat and follow up procedures to monitor changes without the risk of additional harm. No chemical contrast agent or a radiopharmaceutical (biosensor) is required. It can also be used for imaging a number of body systems with excellent spatial and temporal resolution. It is non-invasive and functions similar to a traditional MRI. Traditional MRI is used to look at anatomy and structure whereas fMRI is used to see the processes and function in certain areas. fMRI allows the cerebral activity to be imaged with high resolution in comparison to a PET scan without the dangers of radiopharmaceuticals. Its ease of use has made it a prevalent method of imaging normal brain function, specifically for psychologists. It has provided new insight into the investigation of memories, language, pain, learning, and emotion. fMRI can benefit many patients for both diagnosis and treatment as well. The most common use is in brain imaging including the monitoring of brain tumor growth, studying function post trans ischemic attacks (TIA) and cerebrovascular accidents (CVA), diagnosis of Alzheimer’s disease, and determining the origin of seizures. Some other areas currently being studied include ventilation defects, assessments of treatments, detecting and staging cancer non-invasively, and detecting or treating pulmonary embolisms. Researchers are also investigating several other potential applications for fMRI including brain mapping, planning surgery, analyzing emotions, and even market research.
fMRI has its own disadvantages as well. A large disadvantage is the high cost of MRI machines and maintenance. The scans are long and require the patient to be completely still for a clear image. A negative from a research perspective is that fMRI can only look at blood flow in the brain. Activities within the brain are composed of individual neurons which are too small to be imaged. In this sense, the results of a fMRI can be difficult to interpret. In the future research hopes to improve the accuracy of fMRI by focussing on individual neurons. MRI machines also use helium in a liquid state in order to cool the superconducting magnets and efficiently conduct current. It is projected that in the next 25-30 years the earth’s helium deposits will be depleted which poses a threat to the advancement of MRI and fMRI research.
The future of MRI consists of many advancements. MRI small portable machines are being explored with faster scan times and higher resolution. The advancement of fMRI technology moves MRI from the imaging of structures to the diagnose of diseases of the brain. It can help analyze mental processes to determine thoughts and feelings. fMRI may be part of future technology moving towards reading thoughts and emotions. This can be an alternative to lie detection or feeling and intentions and its molecular imaging advancements such as biosensors can help avoid biopsies. Its emergence in technology in recent years has been seen specifically in Thunder Bay with the research of hyperpolarized noble gas technology (HP fMRI). Early Alzheimer’s disease testing is being developed by Dr. Mitchell Albert of the Thunder Bay Regional Health Research Institute and Lakehead University in order to detect and understand Alzheimer’s disease. The research is investigating a new method for detecting subtle changes in the brain using hyperpolarized xenon gas since February of 2017. This advanced technology uses a high-powered diode laser to produce polarized light that aligns the nuclei of the noble gas atoms. These gases can be easily inhaled and dissolved in the bloodstream. When inhaled the hyperpolarized xenon129 gas travels through the bloodstream and “lights up” areas of the brain during a fMRI scan. The hyperpolarized xenon gas provides up to ten times more cerebral activity signal enhancement than traditional fMRI. One of the most difficult challenges when observing brain activity is to detect the low-end signal in contrast with the background signal. Measuring the “afterglow” of voxels can help determine differences. However, the noble gases being used have no natural background signal in the tissue as opposed to hydrogen making it easier to distinguish between background noise. The “afterglow” is measured in highly stimulated areas which helps researchers to see the difference between low brain activity and background activity. This method helps create a detailed anatomical and functional image simultaneously. If the research is successful, HP xenon fMRI can be used for Alzheimer’s disease diagnosis and treatment monitoring. Early detection is important because although there is currently no cure for Alzheimer’s disease, there are treatments that are more effective the earlier the disease is diagnosed. Being able to detect the stage of the disease will help narrow down treatment options.
fMRI research is also looking at using hyperpolarized He3 gas to provide detailed information regarding the alveoli function in the lungs. The hyperpolarized helium gas can be inhaled directly by the patient giving a detailed structure within the lungs along with the process of gas exchange functions. This allows for the studying of treatment options for COPD (ventilation or bronchodilators), assessing drug treatments for cystic fibrosis, ventilation defects in asthma patients, or staging lung cancers.