FEATURE NeuroimagiNg
An increase in chemical shift with 3T MRI results in better MR spectroscopy (MRS) imaging when compared with MRS at 1.5T
half i.e 0.5 mmol/kg. However, T2 values remain unchanged or show only slight reduction with increase in field strength, so we use T2 to our advantage, whose effects proportionately scale with field strength. The signal changes that we look at with blood oxygenation level dependent contrast at 1.5 tesla is on the order of 1% to 2%. That signal change goes to 3% to 5% at 3 tesla. Hence, 3 tesla studies are more sensitive to deposition of blood products and tissue mineralization.
DIFFUSION TENSOR IMAGING AT 3 TESLA DWI studies at high field are typically acquired using EPI techniques. The greater signal intensity afforded at 3T is particularly enticing for diffusion-weighted imaging (DWI) needs. DTI, also known as tractography, attempts to analyze the magnitude and orientation of random microscopic motion of water molecules in brain tissue. With diffusion tensor MR imaging, diffusion anisotropy can be quantified and subtle white matter changes not normally seen on conventional MRI can be detected. This property of DTI has been exploited in studies of fibre tracking and neurodevelopment. It also has tremendous potential in conditions such as cerebral ischemia and wallerian degeneration, epilepsy, multiple sclerosis, Alzheimer’s disease, schizophrenia, metabolic disorders, infections and brain tumours. Echo planar imaging studies are
inherently prone to susceptibility artifacts that can limit the evaluation of structures in close proximity to the bony skull base and air-filled paranasal sinuses. These artefacts are intensified by the presence of a metal nearby or at higher field strength. Parallel imaging techniques, on modern 3T systems, by decreasing the echo-spacing (ES) and TE of the scan reduces susceptibility artifact
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and ameliorates signal loss due to T2 decay on these long ET acquisitions 3 tesla also unleashes the role of MR imaging in pre-surgical planning to avoid invasive angiography or direct cortical mapping at the time of surgery. Also due to better delineation of anatomic detail, 3 tesla perfusion studies plot a patient’s physiology up to the edge of the lesion, and diffusion tensor images trace white- matter tracts that must be preserved.
MR SPECTROSCOPY An increase in chemical shift with 3T MRI results in better MR spectroscopy (MRS) imaging when compared with MRS at 1.5T. This is due to increase in SNR and improved spatial resolution. It also leads to improved spectral resolution or the ability to visualize changes in peaks in metabolites. The scan-time reductions with 3 tesla may also permit a substantial increase in the use of spectroscopy to characterize lesions and, potentially, to reduce the need for biopsies. A conventional body exam that includes spectroscopy can take up to 80 minutes at 1.5 tesla. The same exam could potentially be completed in just 20 minutes with 3T. 3 tesla MR imaging can perform superfast spectroscopic imaging without sacrificing image quality. With PARALLEL imaging and sensitivity encoding (SENSE) techniques, resolution and speed can be increased with reduced scan acquisition times. At 1.5 tesla, sometimes differentiating
between chemical constituents become difficult. Higher field strength also results in better peak separation, which makes peak differentiation much more straightforward.
FUNCTIONAL MRI Functional MR identifies active regions of the brain, and relies on blood flow changes occurring over several seconds. With given motor activity, primary motor cortex responsible for that particular movement shows increase in blood flow as well as oxygen delivery to the brain tissue. However the increase in oxygen delivery is more than the local tissue oxygen extraction which results in lower deoxyhemoglobin levels at post capillary level. This sequence of events is responsible for signal changes in MRI, also known as BOLD sequence. Due to increase in SNR with 3T, these changes are explicitly displayed.
MR ANGIOGRAPHY As we have already mentioned that higher field strengths results in longer T1 which leads to higher background tissue suppression when compared to 1.5 tesla. This results in higher visibility of contrast in vascular structures. Lower flip angles help to reduce pulsation artifacts, thus reducing acquisition time and higher spatial resolution. The use of PI with both non-contrast and contrast-enhanced techniques allows for shorter scan times with increased resolution. This results in greater image quality, especially in small vascular structures when compared with 1.5T imaging.
OTHER CLINICAL APPLICATIONS OF 3T IN NEUROIMAGING 3 tesla MRI is also catching foothold in monitoring the effects of rehabilitation on patients with cognition loss, however the effects are seen to be more promising in patients after successful treatment of
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