MRI and fMRI in a Nutshell

MRI (AKA nuclear magnetic resonance imaging (NMRI) or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualise detailed internal structures. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body. It is a particularly useful clinical and research tool for imaging soft tissue such as the brain and has been harnessed in clinical research to support anatomical changes in syndromes such as autism, schizophrenia and severe depression to name but a few. One almost cliche study utilised MRI to establish anatomically changes in the anterior and posterior hippocampal structures in London taxi drivers (Gur et al. 200). It is routinely used in clinical practice in the detection of acquired brain injuries, such as tumour, stroke and TBI.

In order to understand how it works, it is necessary to summarise the atom. On its smallest scale the world is made up of atoms. Atoms consist of protons, neutrons and electrons that orbit or ‘spin’. Atoms with an uneven number of protons and neutrons have a 'net spin'. When atoms are outside of an MRI scanner the alignment of these net spins is randomly ordered. Under a magnetic field, they tend to align in parallel or anti-parallel. The MRI is a very powerful magnet, 30,000 times more power than the Earth’s natural polar magnetic field. Another magnetic field, the gradient field, is then applied to kick the nuclei to higher magnetization levels, with the effect depending on where they are located. When the gradient field is removed, the nuclei go slowly back to their original states (AKA relaxation), and the energy they emit is measured with a coil to recreate the positions of the nuclei. MRI thus provides a static structural view of brain matter. Magnetic field gradients cause nuclei at different locations to rotate at different speeds. By using gradients in different directions 2D images or 3D volumes can be obtained in any arbitrary orientation. Sagittal, coronal or horizontal slices can be obtained. Black areas on the scan represent no signal and white areas represent a signal (positive charge).

The brain is 70% water and water is a compound substance made up of hydrogen and oxygen elements. Hydrogen molecules are intrinsically magnetic and water concentration differentiates through different types of brain matter. MRI uses this to construct a grey scale image. For instance cerebrospinal fluid has very high water content and tumour and bone is very dense and low in water content. Images can be T1 or T2 weighted by changing the parameters of 'relaxation' time. White matter appears in a light grey in T1 and a dark grey in T2. Grey matter appears grey in both and cerebrospinal fluid (CSF) appears black in T1 and white in T2. T1 and T2 weighting is a helpful option in clinical scenarios where one type of matter is particularly important to identify (the extent of a tumour for example).
Exciting new techniques involving the introduction of high contrast agents are currently in development. They have some contraindications to health but they have now been approved for specific uses in clinical research and practice.

Functional magnetic resonance imaging or functional MRI (fMRI) is an MRI procedure that measures brain activity by detecting associated changes in blood flow. The primary form of fMRI uses the blood-oxygen-level-dependent (BOLD) contrast, discovered by Seiji Ogawa. The central thrust behind fMRI was to extend MRI to capture functional changes in the brain caused by neuronal activity. Differences in magnetic properties between arterial (oxygen-rich) and venous (oxygen-poor) blood provided this link. When neurons become active, local blood flow to those brain regions increases, and oxygen-rich (oxygenated) blood displaces oxygen-depleted (deoxygenated) blood. In a nutshell deoxygenated hemoglobin is more magnetic than oxygenated hemoglobin, which is virtually nonmagnetic. This difference leads to an improved MR signal since the nonmagnetic blood interferes with the magnetic MR signal less. This improvement can be mapped to show which neurons are active at a time.

fMRI is used to often used in avoiding key functional areas of the brain in prepartion for surgical or other invasive intervnetion. It is also often used to anatomically map the brain and detect the effects of tumors, stroke, head and brain injury, or degenerative diseases such as dementia. Research use is ahead of clinical use, mainly becuase clinical populations present logistical or ethical complications to scanning (scanning a child with ASD, with communication difficulties who require informed consented is an illustration of a collection of these difficulties). Despite this, in addition to the uses of MRI, fMRI has been used clinically to map functional areas, check left-right hemispherical asymmetry in language and memory regions, check the neural correlates of a seizure and test how well a drug works.
Like any technique, fMRI has advantages and disadvantages, and in order to be useful, the experiments that employ it must be carefully designed and conducted to maximize its strengths and minimize its weaknesses.

-Advantages-
-It can noninvasively record brain signals without risks of ionising radiation inherent in other scanning methods, such as CT or PET scans.
-It has satisfactory spatial resolution, particularly in collaboration with a normal MRi scan.
-It can record signal from all regions of the brain, unlike EEG/MEG, which are biased toward the cortical surface.
-It has led to major new understandings of human function, particular in light of real-time intervnetion studies.

-Disadvantages-
-The BOLD signal is only an indirect measure of neural activity and, as described before, could be influenced by elements other than the experimental manipulation (disease, sedation, anxiety, medications that dilate blood vessels, and attention (neuromodulation).
-BOLD signals reveal input rather than output and one isn't necessarily the other.
-The technique has been criticised for its poor temporal resolution. The BOLD response peaks approximately 5 seconds after neuronal firing begins in an area. While interleaved stimulus presentation can increase temporal resolution, it correspondingly reduces the data points collected.
-While the static magnetic field has no known long-term harmful effect on biological tissue, it can cause damage by pulling in nearby heavy metal objects converting them to projectiles.
-The most common risk to participants in an fMRI study is claustrophobia. But people with pacemakers are catastrophically at risk on entering a scanner.
-The scanner is very expensive.
-The costs mean that clinical practice has lagged behind privately funded research.
-fMRI research statisitcal methods have recently fallen under strutiny, thereby questionning past findings.