Neuroimaging

Neuroimaging is the use of quantitative techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Increasingly it is also being used for quantitative research studies of brain disease and psychiatric illness. Neuroimaging is highly multidisciplinary involving neuroscience, computer science, psychology and statistics, and is not a medical specialty. Neuroimaging is sometimes confused with neuroradiology.
Neuroradiology is a medical specialty that uses non-statistical brain imaging in a clinical setting, practiced by radiologists who are medical practitioners. Neuroradiology primarily focuses on recognizing brain lesions, such as vascular diseases, strokes, tumors, and inflammatory diseases. In contrast to neuroimaging, neuroradiology is qualitative but sometimes uses basic quantitative methods. Functional brain imaging techniques, such as functional magnetic resonance imaging, are common in neuroimaging but rarely used in neuroradiology. Neuroimaging falls into two broad categories:

Structural imaging, which is used to quantify brain structure using e.g., voxel-based morphometry.
Functional imaging, which is used to study brain function, often using fMRI and other techniques such as PET and MEG.
History

The first chapter of the history of neuroimaging traces back to the Italian neuroscientist Angelo Mosso who invented the 'human circulation balance', which could non-invasively measure the redistribution of blood during emotional and intellectual activity.
In 1918, the American neurosurgeon Walter Dandy introduced the technique of ventriculography. X-ray images of the ventricular system within the brain were obtained by injection of filtered air directly into one or both lateral ventricles of the brain. Dandy also observed that air introduced into the subarachnoid space via lumbar spinal puncture could enter the cerebral ventricles and also demonstrate the cerebrospinal fluid compartments around the base of the brain and over its surface. This technique was called pneumoencephalography.
In 1927, Egas Moniz introduced cerebral angiography, whereby both normal and abnormal blood vessels in and around the brain could be visualized with great precision.
In the early 1970s, Allan McLeod Cormack and Godfrey Newbold Hounsfield introduced computerized axial tomography, and ever more detailed anatomic images of the brain became available for diagnostic and research purposes. Cormack and Hounsfield won the 1979 Nobel Prize for Physiology or Medicine for their work. Soon after the introduction of CAT in the early 1980s, the development of radioligands allowed single-photon emission computed tomography and positron emission tomography of the brain.
More or less concurrently, magnetic resonance imaging was developed by researchers including Peter Mansfield and Paul Lauterbur, who were awarded the Nobel Prize for Physiology or Medicine in 2003. In the early 1980s MRI was introduced clinically, and during the 1980s a veritable explosion of technical refinements and diagnostic MR applications took place. Scientists soon learned that the large blood flow changes measured by PET could also be imaged by the correct type of MRI. Functional magnetic resonance imaging was born, and
since the 1990s, fMRI has come to dominate the brain mapping field due to its low invasiveness, lack of radiation exposure, and relatively wide availability.
In the early 2000s, the field of neuroimaging reached the stage where limited practical applications of functional brain imaging have become feasible. The main application area is crude forms of brain–computer interface.
The world record for the spatial resolution of a whole-brain MRI image was a 100-micrometer volume achieved in 2019. The sample acquisition took about 100 hours. The spatial world record of a whole human brain of any method was an
X-ray tomography scan performing at the ESRF, which had a resolution of about 25 microns and requiring about 22 hours. This scan was part of the human organ atlas which has X-ray tomography scans of other organs in the human body with the same resolution.
A crucial idea for magnetic resonance imaging is that the net magnetization vector can be moved by exposing the spin system to energy of a frequency equal to the energy difference between the spin states. If enough energy is delivered to the system, it is possible to make the net magnetization vector orthogonal to that of the external magnetic field.

Indications

Neuroradiology often follows a neurological examination in which a physician has found cause to more deeply investigate a patient who has or may have a neurological disorder.
Common clinical indications for neuroimaging include head trauma, stroke like symptoms e.g.: sudden weakness/numbness in one half of body, difficulty talking or walking; seizures, sudden onset severe headache, sudden change in level of consciousness for unclear reasons.
Another indication for neuroradiology is CT-, MRI- and PET-guided stereotactic surgery or radiosurgery for treatment of intracranial tumors, arteriovenous malformations and other surgically treatable conditions.
One of the more common neurological problems which a person may experience is simple syncope. In cases of simple syncope in which the patient's history does not suggest other neurological symptoms, the diagnosis includes a neurological examination but routine neurological imaging is not indicated because the likelihood of finding a cause in the central nervous system is extremely low and the patient is unlikely to benefit from the procedure.
Neuroradiology is not indicated for patients with stable headaches which are diagnosed as migraine. Studies indicate that presence of migraine does not increase a patient's risk for intracranial disease. A diagnosis of migraine which notes the absence of other problems, such as papilledema, would not indicate a need for radiological investigations. In the course of conducting a careful diagnosis, the physician should consider whether the headache has a cause other than the migraine and might require radiological investigations.

Brain-imaging techniques

Computed axial tomography

or Computed Axial Tomography scanning uses a series of x-rays of the head taken from many different directions. Typically used for quickly viewing brain injuries, CT scanning uses a computer program that performs a numerical integral calculation on the measured x-ray series to estimate how much of an x-ray beam is absorbed in a small volume of the brain. Typically the information is presented as cross-sections of the brain.

Magnetic resonance imaging

uses magnetic fields and radio waves to produce high quality two- or three-dimensional images of brain structures without the use of ionizing radiation or radioactive tracers.
The record for the highest spatial resolution of a whole intact brain is 100 microns, from Massachusetts General Hospital. The data was published in Scientific Data on 30 October 2019.

Positron emission tomography

and brain positron emission tomography, measure emissions from radioactively labeled metabolically active chemicals that have been injected into the bloodstream. The emission data are computer-processed to produce 2- or 3-dimensional images of the distribution of the chemicals throughout the brain. The positron emitting radioisotopes used are produced by a cyclotron, and chemicals are labeled with these radioactive atoms. The labeled compound, called a radiotracer, is injected into the bloodstream and eventually makes its way to the brain. Sensors in the PET scanner detect the radioactivity as the compound accumulates in various regions of the brain. A computer uses the data gathered by the sensors to create multicolored 2- or 3-dimensional images that show where the compound acts in the brain. Especially useful are a wide array of ligands used to map different aspects of neurotransmitter activity, with by far the most commonly used PET tracer being a labeled form of glucose ).
The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen and glucose metabolism in the tissues of the working brain. These measurements reflect the amount of brain activity in the various regions of the brain and allow to learn more about how the brain works. PET scans were superior to all other metabolic imaging methods in terms of resolution and speed of completion when they first became available. The improved resolution permitted better study to be made as to the area of the brain activated by a particular task. The biggest drawback of PET scanning is that because the radioactivity decays rapidly, it is limited to monitoring short tasks. Before fMRI technology came online, PET scanning was the preferred method of functional brain imaging, and it continues to make large contributions to neuroscience.
PET scanning is also used for diagnosis of brain disease, most notably brain tumors, epilepsy, and neuron-damaging diseases which cause dementia all cause great changes in brain metabolism, which in turn causes easily detectable changes in PET scans. PET is probably most useful in early cases of certain dementias where the early damage is too diffuse and makes too little difference in brain volume and gross structure to change CT and standard MRI images enough to be able to reliably differentiate it from the "normal" range of cortical atrophy which occurs with aging persons, and which does not cause clinical dementia.
FDG-PET scanning is also often used in assessment of patients with epilepsy who continue to have seizures despite adequate medical treatment. In focal epilepsy, where seizures begin in a small part of the brain before spreading elsewhere, it is one of the many modalities used to identify the region of brain responsible for seizure onset. Typically, the area of brain where seizures begin is dysfunctional even when patient is not having a seizure and uptakes less glucose, hence less FDG compared to healthy brain regions. This information can help plan for epilepsy surgery as a treatment for drug resistant epilepsy.
Other radiotracers have also been used to identify areas of seizure onset though they are not available commercially for clinical use. These include ¹¹C-flumazenil, 1¹C-alpha-methyl-L-tryptophan, ¹¹C-methionine, ¹¹C-cerfentanil.