within few hours after the seizure is over. this technique is called ictal spect and relies on the increased cbf in areas of seizure onset during the seizure. interictal spect is a scan done using the same tracers but during a time when the patient is not having a seizure. in between seizures, a reduction in cbf is seen in areas of seizure onset and is not as pronounced as the blood flow increase during the seizure. cranial ultrasound cranial ultrasound is usually only used in babies, whose open fontanelles provide acoustic windows allowing ultrasound imaging of the brain. advantages include the absence of ionising radiation and the possibility of bedside scanning, but the lack of soft-tissue detail means mri is preferred for some conditions. functional magnetic resonance imaging axial mri slice at the level of the basal ganglia, showing fmri bold signal changes overlaid in red (increase) and blue (decrease) tones functional magnetic resonance imaging (fmri) and arterial spin labeling (asl) relies on the paramagnetic properties of oxygenated and deoxygenated hemoglobin to see images of changing blood flow in the brain associated with neural activity. this allows images to be generated that reflect which brain structures are activated (and how) during the performance of different tasks or at resting state. according to the oxygenation hypothesis, changes in oxygen usage in regional cerebral blood flow during cognitive or behavioral activity can be associated with the regional neurons as being directly related to the cognitive or behavioral tasks being attended. most fmri scanners allow subjects to be presented with different visual images, sounds and touch stimuli, and to make different actions such as pressing a button or moving a joystick. consequently, fmri can be used to reveal brain structures and processes associated with perception, thought and action. the resolution of fmri is about 2-3 millimeters at present, limited by the spatial spread of the hemodynamic response to neural activity. it has largely superseded pet for the study of brain activation patterns. pet, however, retains the significant advantage of being able to identify specific brain receptors (or transporters) associated with particular neurotransmitters through its ability to image radiolabeled receptor "ligands" (receptor ligands are any chemicals that stick to receptors). there is also significant concern regarding the validity of some of the statistics used in fmri analyses; hence, the validity of conclusions drawn from many fmri studies. with between 72% and 90% accuracy where chance would achieve 0.8%, fmri techniques can decide which of a set of known images the subject is viewing. recent studies on machine learning in psychiatry have used fmri to build machine learning models that can discriminate between individuals with or without suicidal behaviour. imaging studies in conjunction with machine learning algorithms may help identify new markers in neuroimaging that could allow stratification based on patients' suicide risk and help develop the best therapies and treatments for individual patients. diffuse optical imaging diffuse optical imaging (doi) or diffuse optical tomography (dot) is a medical imaging modality which uses near infrared light to generate images of the body. the technique measures the optical absorption of haemoglobin, and relies on the absorption spectrum of haemoglobin varying with its oxygenation status. high-density diffuse optical tomography (hd-dot) has been compared directly to fmri using response to visual stimulation in subjects studied with both techniques, with reassuringly similar results. hd-dot has also been compared to fmri in terms of language tasks and resting state functional connectivity. event-related optical signal event-related optical signal (eros) is a brain-scanning technique which uses infrared light through optical fibers to measure changes in optical properties of active areas of the cerebral cortex. whereas techniques such as diffuse optical imaging (dot) and near-infrared spectroscopy (nirs) measure optical absorption of haemoglobin, and thus are based on blood flow, eros takes advantage of the scattering properties of the neurons themselves and thus provides a much more direct measure of cellular activity. eros can pinpoint activity in the brain within millimeters (spatially) and within milliseconds (temporally). its biggest downside is the inability to detect activity more than a few centimeters deep. eros is a new, relatively inexpensive technique that is non-invasive to the test subject. it was developed at the university of illinois at urbana-champaign where it is now used in the cognitive neuroimaging laboratory of dr. gabriele gratton and dr. monica fabiani. magnetoencephalography magnetoencephalography (meg) is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as superconducting quantum interference devices (squids) or spin exchange relaxation-free (serf) magnetometers. meg offers a very direct measurement of neural electrical activity (compared to fmri for example) with very high temporal resolution but relatively low spatial resolution. the advantage of measuring the magnetic fields produced by neural activity is that they are likely to be less distorted by surrounding tissue (particularly the skull and scalp) compared to the electric fields measured by electroencephalography (eeg). specifically, it can be shown that magnetic fields produced by electrical activity are not affected by the surrounding head tissue, when the head is modeled as a set of concentric spherical shells, each being an isotropic homogeneous conductor. real heads are non-spherical and have largely anisotropic conductivities (particularly white matter and skull). while skull anisotropy has a negligible effect on meg (unlike eeg), white matter anisotropy strongly affects meg measurements for radial and deep sources. note, however, that the skull was assumed to be uniformly anisotropic in this study, which is not true for a real head: the absolute and relative thicknesses of diploë and tables layers vary among and within the skull bones. this makes it likely that meg is also affected by the skull anisotropy, although probably not to the same degree as eeg. there are many uses for meg, including assisting surgeons in localizing a pathology, assisting researchers in determining the function of various parts of the brain, neurofeedback, and others. functional ultrasound imaging functional ultrasound imaging (fus) is a medical ultrasound imaging technique of detecting or measuring changes in neural activities or metabolism, for example, the loci of brain activity, typically through measuring blood flow or hemodynamic changes. functional ultrasound relies on ultrasensitive doppler and ultrafast ultrasound imaging which allows high sensitivity blood flow imaging. quantum optically-pumped magnetometer in june 2021, researchers reported the development of the first modular quantum brain scanner which uses magnetic imaging and could become a novel whole-brain scanning approach. advantages and concerns of neuroimaging techniques functional magnetic resonance imaging (fmri) fmri is commonly classified as a minimally-to-moderate risk due to its non-invasiveness compared to other imaging methods. fmri uses blood oxygenation level dependent (bold)-contrast in order to produce its form of imaging. bold-contrast is a naturally occurring process in the body so fmri is often preferred over imaging methods that require radioactive markers to produce similar imaging. a concern in the use of fmri is its use in individuals with medical implants or devices and metallic items in the body. the magnetic resonance (mr) emitted from the equipment can cause failure of medical devices and attract metallic objects in the body if not properly screened for. currently, the fda classifies medical implants and devices into three categories, depending on mr-compatibility: mr-safe (safe in all mr environments), mr-unsafe (unsafe in any mr environment), and mr-conditional (mr-compatible in certain environments, requiring further information). fda mr safety labels for implants and devices mr safe mr conditional mr unsafe computed tomography (ct) scan the ct scan was introduced in the 1970s and quickly became one of the most widely used methods of imaging. a ct scan can be performed in under a second and produce rapid results for clinicians, with its ease of use leading to an increase in ct scans performed in the united states from 3 million in 1980 to 62 million in 2007. clinicians oftentimes take multiple scans, with 30% of individuals undergoing at least 3 scans in one study of ct scan usage. ct scans can expose patients to levels of radiation 100-500 times higher than traditional x-rays, with higher radiation doses producing better resolution imaging. while easy to use, increases in ct scan use, especially in asymptomatic patients, is a topic of concern since patients are exposed to significantly high levels of radiation. positron emission tomography (pet) in pet scans, imaging does not rely on intrinsic biological processes, but relies on a foreign substance injected into the bloodstream traveling to the brain. patients are injected with radioisotopes that are metabolized in the brain and emit positrons to produce a visualization of brain activity. the amount of radiation a patient is exposed to in a pet scan is relatively small, comparable to the amount of environmental radiation an individual is exposed to across a year. pet radioisotopes have limited exposure time in the body as they commonly have very short half-lives (~2 hours) and decay rapidly. currently, fmri is a preferred method of imaging brain activity compared to pet, since it does not involve radiation, has a higher temporal resolution than pet, and is more readily available in most medical settings. magnetoencephalography (meg) and electroencephalography (eeg) the high temporal resolution of meg and eeg allow these methods to measure brain activity down to the millisecond. both meg and eeg do not require exposure of the patient to radiation to function. eeg electrodes detect electrical signals produced by neurons to measure brain activity and meg uses oscillations in the magnetic field produced by these electrical currents to measure activity. a barrier in the widespread usage of meg is due to pricing, as meg systems can cost millions of dollars. eeg is a much more widely used method to achieve such temporal resolution as eeg systems cost much less than meg systems. a disadvantage of eeg and meg is that both methods have poor spatial resolution when compared to fmri.