5.2.5 Imaging of the nervous system

5.2.5 Imaging of the nervous system

Imaging of the Nervous System

Non-invasive imaging methods have become essential tools for understanding the central nervous system (CNS) in health and disease. The imaging of the nervous system plays a crucial role in psychiatric assessments and can provide important information to help healthcare professionals diagnose and understand mental disorders.

Structural Imaging

Computed Tomography (CT):

CT uses ionising x-ray radiation. A beam of x-rays which is directed through the head rotates around to produce a 2-D picture. The raw data can be reconstructed into different imaging planes i.e. axial, sagittal, and coronal planes. CT scans are shown in varying shades of grey depending on the absorption (or attenuation) patterns that different tissues exhibit when subjected to ionising radiation.

Unlike normal x-rays, which only show 5 densities (air/fat/soft tissue/bone/metal), CT shows a wide range of densities ranging from the air (black) through bone (white). Dense tissues such as bone attenuate (reduce in intensity) x-rays the most and appear white, whereas tissues such as air attenuate the least and appear black.

Psychiatric symptoms are often non-specific and commonly present in patients with medical as well as psychiatric disorders. Often CT head scans are utilised in psychiatry as a screening tool with the goal of detecting suspected and treatable diseases involving the central nervous system.

Functional Imaging

Magnetic Resonance Imaging (MRI):

This takes advantage of the magnetic characteristics of tissues. A substantial magnetic field is first applied to align all hydrogen atoms in the same direction. The atoms are then realigned using a radio frequency (Larmour frequency). When the radio frequency is switched off, the atoms realign and emit a signal. The MRI machine detects this signal and generates an image. T1 and T2 weighting describe how atoms realign after the radio frequency has been eliminated.

The fact that blood with various degrees of oxygenation creates distinct signals is used by functional MRI (fMRI). As a result, it is a measure of oxygen consumption. This is known as blood level-dependent (BOLD) contrast.

Perfusion MRI (pMRI) – evaluates microscopic blood flow within capillaries.

The introduction of MRI has significantly revolutionised the radiological examination of the central nervous system. MRI is a versatile modality that has been used to diagnose practically every type of brain, spinal cord, and surrounding structural problems. However, significant limitations exist, such as limited detectability of subarachnoid haemorrhage or calcifications, difficulties in seeing bone features, and other types of artefacts.

Emission Tomography (PET and SPECT):

Positron emission tomography (PET) is a non-invasive imaging technique used to measure radioactivity in vivo. It entails injecting a positron-emitting radiopharmaceutical intravenously, waiting for systemic distribution, and then scanning for detection and measurement of radiopharmaceutical accumulation patterns within the body.

Single photon emission computed tomography (SPECT) is a 3D nuclear medicine imaging technology that combines scintigraphy with CT.  This permits the radionuclide distribution to be presented in 3D, providing more detail, contrast, and spatial information than planar nuclear imaging alone.

The physiologic information provided by PET and SPECT imaging is priceless, and the data quality is poor/noisy, limiting imaging spatial resolution. As a result, PET and SPECT scans are frequently coupled with CT imaging, allowing for the comparison of functional and anatomical imaging (“hybrid imaging”).

Magnetic Resonance Spectroscopy (MRS):

Magnetic resonance spectroscopy (MR spectroscopy or MRS) is a tool for interrogating tissue for the presence and concentration of different metabolites.

Metabolites that can be measured include GABA, glutamate/glutamine, lactate, lipids, choline etc.

Diffusion Tensor Imaging (DTI):

Diffusion tensor imaging (DTI) is an MRI technique that uses anisotropic diffusion to estimate the axonal (white matter tracts) organisation of the brain.

Some of the many applications include:

  • Assessment of the deformation of white matter by tumours – deviation, infiltration, destruction of white matter
  • Delineation of the anatomy of immature brains
  • Alzheimer’s disease – detection of early disease
  • Schizophrenia
  • Multiple Sclerosis – plaque assessment

References:

 (1) Roarke MC, Nguyen BD, Pockaj BA. Applications of SPECT/CT in nuclear radiology. (2008) AJR. American journal of roentgenology. 191 (3): W135-50. doi:10.2214/AJR.07.3564 – Pubmed

(2) Kapoor V, McCook B, Torok F. An Introduction to PET-CT Imaging. Radiographics. 2004;24(2):523-43. doi:10.1148/rg.242025724 – Pubmed

(3) Al-okaili RN, Krejza J, Wang S et-al. Advanced MR imaging techniques in the diagnosis of intraaxial brain tumors in adults. Radiographics. 2006;26 Suppl 1 : S173-89. doi:10.1148/rg.26si065513 – Pubmed citation

(4) Jellison BJ, Field AS, Medow J et-al. Diffusion tensor imaging of cerebral white matter: a pictorial review of physics, fiber tract anatomy, and tumor imaging patterns. AJNR Am J Neuroradiol. 2004;25 (3): 356-69. Pubmed citation