3.1.2 Neuroimaging

Neuroimaging and its Role in Understanding Brain Function

Neuroimaging techniques have paved the path for neurosciences and psychiatry to understand how the human brain functions. Neuroimaging helps us understand how an individual brain develops from birth to adulthood. Neuroscientists working on development studies have made discoveries regarding the underpinning of cognitive development. Psychologists have used functional neuroimaging to understand the neural mechanisms that are involved in PTSD, panic disorder, and phobias. The uses of neuroimaging in both research and clinical settings are almost limitless (Schrag, 2013).

Computer Tomography:

Computer Tomography (CT) is the most prevalent cranial imaging tool in the clinical setting. A series of cranial X-rays in a 360-degree field. This does limit images to one plane of rotation. Essentially the X-rays show the different densities of brain structures.

The various contrasts (white/black) seen in CT images are due to how tissues absorb X-rays differently. Calcification is visualised better in CT in comparison to MRI.

Challenges with CT imaging:

• Grey and white matter differentiation is poor.
• Poor sensitivity to early ischaemia.
• Poor visualisation of posterior fossa lesions.
• Structures close to bone are often obscured such as the brain stem.

	CT scan:	MRI scan:
Principle:	Multiple X-rays at varying angles to produce cross-sectional images.	Powerful magnetic fields and radiofrequency pulses to produce detailed images.
Radiation:	Minimal.	None.
Purpose:	Observing bone and even better for soft tissues. This is improved when used with intravenous contrast dye.	Excellent for detecting slight differences in soft tissues.
Application:	Produces a general image of an area such as organs, fractures, or head trauma.	Produces detailed images of soft tissues, organs, or ligaments.
Benefits:	Faster.	More detailed images.
Risk:	Harmful for unborn babies. A small dose of radiation. Possibility of reaction to the use of dyes.	Possibility of reaction to metals due to magnetic fields such as artificial joints or eye implants. Loud noises from the machine can be disturbing to certain patients. Increase in body temperature during long MRIs. Claustrophobia.

Magnetic Resonance Imaging:

Magnetic Resonance Imaging (MRI) forms images of the brain by using powerful magnets to create a strong magnetic field that causes protons in the body to align with it. When a radiofrequency current is pulsed through the patient, the protons are excited and spin out of balance, pushing against the magnetic field’s pull. When the radiofrequency field is switched off, the MRI sensors can measure the energy released by protons as they realign with the magnetic field. The quantity of energy released and the time it takes for the protons to realign with the magnetic field vary depending on the environment and the chemical makeup of the molecules. Based on these magnetic properties, doctors can distinguish between different types of tissues (Schrag, 2013).

They are distinct from computed tomography (CT) in that they do not employ the harmful ionising radiation of X-rays.

Functional magnetic resonance imaging (fMRI) is considered the most successful and prominent technology of neuroimaging, it is essential as a proxy measure of issue activity. This technique tracks the changes that occur in the oxygen levels and the blood flow to help identify neural activity. For example, when a brain area is more active than usual then it would consume more oxygen thus resulting in increased blood flow. fMRI is used to map areas associated with cognitive functioning such as areas connected with perception, language, and memory. The technique of Blood Oxygen Level dependent (BOLD) forms the basis of fMRI.

MRI can distinguish between white and grey matter in the brain and can also be used to diagnose aneurysms and malignancies. Because MRI does not use x-rays or other radiation, it is the preferred imaging modality when frequent imaging is necessary for diagnosis or therapy, particularly in the brain. MRI, on the other hand, is more expensive than CT scanning.

MRI pulse sequence:	Summary:
T1 images	Hydrophobic tissues are emphasized. Fat is bright and CSF is dark. T1 imaging is the sequence that allows the image to be enhanced with the contrast agent gadolinium. Gadolinium-enhanced structures appear bright white.
T2 images	Hydrophilic areas are emphasized. Brain tissue is dark and CSF is white. Brain tissues high in water content such as tumours, inflammation, or stroke infarctions appear brighter on T2 images.
FLAIR (Fluid attenuated inversion recovery)	The T1 image is inverted and superimposed onto the T2 image to double the contrast. Beneficial in detecting sclerosis of the hippocampus resulting from temporal lobe epilepsy or localising areas in degenerative disorders.

Structure or pathology:	CT scan	MRI T1 scan:	MRI T2 scan:
Air	Dark	Dark	Dark
Bone	Bright	Bright	Dark
CSF	Dark	Dark	Bright
Fat	Dark	Bright	Bright
Tissue	Shades of grey	Grey matter = grey White matter = white	Shades of grey
Haemorrhage	Bright	Bright	Bright
Infarction	Dark	Dark	Bright
Multiple sclerosis plaques	Dark	Dark	Bright
Tumour	Dark	Dark	Bright

Diffusion Tensor Imaging:

Single Photon Emission Computed Tomography:

Single Photon Emission Computed Tomography (SPECT) studies regional changes in cerebral blood flow within the brain using radioactive substances. This measures the pattern of photon emission from the bloodstream, which changes with the level of perfusion in different areas of the brain. Like fMRI, it does not directly monitor neural metabolism.

Injectable tracers such as technetium-99m-d, and l-hexamethyl propylene amine oxime (HMPAO) are used to assess blood flow to the whole brain. The tracers have single photon-emitting isotopes which help determine the levels of perfusion.

Positron Emission Tomography:

Positron Emission Tomography (PET) uses radioactive tags to determine which part of the brain is most active while doing a task. This imaging technique can use used to study blood flow, receptor distribution as well as metabolic activity of the brain tissue (Davidson, 2003).

Function:	PET ligand:
Blood flow	C15/H215O
Glucose metabolism	F18 deoxyglucose
Dopamine D2 receptors	11C raclopride
GABA-A receptors	11C flumazenil

Magnetic Resonance Spectroscopy:

Magnetic Resonance Spectroscopy (MRS) is used to measure the biochemical changes that occur in the brain and it is done through a noninvasive diagnostic tool. In MRS, tissue can be investigated for the presence and concentration of various metabolites (Parvizi, J. and Kim, G. K., 2010).

There is very limited use of this technique in both psychiatry and radiology.

References:

(1) Davidson, M.C., Thomas, K.M. and Casey, B.J. (2003). Imaging the developing brain with fMRI. Mental Retardation and Developmental Disabilities Research Reviews, 9(3), pp.161–167. doi:10.1002/mrdd.10076.

(2) O’Donnell, L.J. and Westin, C.-F. (2011). An Introduction to Diffusion Tensor Image Analysis. Neurosurgery Clinics of North America, [online] 22(2), pp.185–196. doi:10.1016/j.nec.2010.12.004.

(3) Parvizi, J. and Kim, G. K. (2010) “MRI (magnetic resonance imaging),” in High Yield Orthopaedics. Elsevier, pp. 305–306.

(4) Schrag, A.E., Mehta, A.R., Bhatia, K.P., Brown, R.J., Frackowiak, R.S., Trimble, M.R., Ward, N.S. and Rowe, J.B., 2013. The functional neuroimaging correlates of psychogenic versus organic dystonia. Brain, 136(3), pp.770-781.