Assessing TBI with 3T MRI
Traumatic Brain Injury with 3T Magnetic Resonance Imaging
6 December 2023 | Gyrus Group
Traumatic brain injuries (TBIs) can be difficult to assess based on clinical presentation alone. Imaging with computed tomography (CT) is used as a first line test following significant head injury. CT imaging is accurate in detection of skull fracture and to establish the site and extent of bleeding in the head but is less sensitive in detection of microhaemorrhage (minor focal bleeds) and to non-haemorrhagic brain injury. CT exposes the patient to radiation. The development of magnetic resonance imaging (MRI) technology in the 1970s led to dramatically improved imaging of the brain structures, and now plays a pivotal role in assessing TBI both in the acute phase and at delayed follow up.
MRI scanners use a combination of high strength magnetic fields and precise high frequency radio signals to obtain images of body structures. The strength of the magnetic field is measured in Teslas (T). The Earth’s magnetic field is only 0.00005T but modern clinical scanners use superconducting electromagnets to produce 1.5T – 3T fields. 1.5T imaging remains the most common field strength used in hospital settings where scanners are needed for imaging of other body structures. There are significant benefits to use of higher field strength 3T imaging for evaluation of brain structures and these machines are more suited to dedicated brain imaging work. The increased field strength improves the image resolution and gives better delineation of subtly different tissues (e.g. cerebral cortex and white matter of the brain). 3T imaging enables neuroradiologists to more readily detect microscopic brain haemorrhage and subtle changes in the fluid content of the brain which can occur following high velocity injury. These changes indicate the presence of a diffuse axonal injury: well-recognised as cause of disproportionate neurological impairment in patients with relatively subtle CT scan findings.
MR technology can provide additional information beyond simple structural imaging. Diffusion tensor imaging (DTI), for example, is a specialised MRI technique that focuses on the brain’s white matter tracts, revealing the connections between different brain regions. This advanced imaging method maps the path of preferential diffusion of water molecules within brain tissues, providing an indication of the integrity of neural pathways. In the context of TBI, DTI is thought to detect changes in white matter caused by diffuse axonal injuries that may not be seen with CT imaging. DTI remains a research tool but may point to subtle disruptions, helping specialists in brain injury to understand how the TBI may cause cognitive and behavioural disruption and physical disability. DTI imaging protocols can be integrated with structural MR imaging, meaning the patient only needs to attend one scanning session to obtain both DTI and structural images.
Even higher field strengths with 7T MRI can provide incredible detail on the microvasculature of the brain but there are significant challenges to use in humans, and these machines are not in routine clinical use. Other new developments, such as fast-field cycling MRI, improved magnetic field homogeneity, improved transmit and receive coil technology, higher strength gradient coils, improved RF generation technology, and low noise imaging continue to return better image quality and improve patient tolerance for MR imaging.
All of these advances improve our ability to understand and evaluate TBI. If we can identify ever more subtle brain abnormality, we will be better able to understand the effects of a TBI, target treatment and provide a more confident prognosis.