Tesla scannersIntroductionThe quantity of available signal in conventional magnetic resonance imaging (MRI) is inevitably linked to the static magnetic field strength of the magnetic system. In the recent past clinical MRI strengths operated at fields strengths at or below 1.5 Tesla. However, due to improvements in the magnetic design and shielding to ease the sitting requirements, 3 Tesla clinical scanners have been improvised and implemented and are now widely available (Gaa et al, 2004). The push for high-field MRI is wholly tied to the benefits of potentially higher signal-to-noise ration, contrast and spectral resolution for certain specific applications.
These benefits often facilitate higher spatial and/or temporal resolutions as compared to the previous models. However, there are various challenges that must be resolved in order to achieve the perceived benefits of 3T scanners. This paper is therefore focused to discuss the advantages of going to higher field MRI. This paper will also discuss the challenges of going to higher field. The influence of the 3T magnet on image qualityAdvantages The introduction of 3T scanners in clinical applications has completely changed the way body diagnosis is being done across hospitals the world over.
Faster scans, reduced image degradation, increased contrast between tissues, sub-millimetre resolutions and much more can be captured with a short period. Extensive discussion about the benefits of 3T scanners is as follows. Imaging at 3Tb has been greatly enhanced by various factors. One of the factors that influence imaging at 3T magnets is the radio frequency waves (RF). In MR imaging the localization is limited to spatial resolution lower than the wavelength. As a consequence localization of imaging is encoded in resonance frequency.
The sounds emitted by identical body voxels are indistinguishable (Fischbach et al 2004). However, because of the varying liquid content of various body voxels allows distinction of the frequencies of sounds produced by these body voxels. The RF wavelength (λ) is proportional to the speed of light in the medium C medium, and inversely proportional to RF frequency f: λ αThe varying fluid content of body voxels causes variation in radio frequency which consequently allows users to acquire images with spatial resolutions and greater coverage than in the case of lower field scanners (Fschbach et al, 2004). Owing to the fact that resonance frequency of a specified voxel is directly proportional to the magnetic field strength, one can obtain a spatial variation of resonance frequency by varyiong magnetic field strength through varying the gradient.
Since 3T scanners permits variation of the gradient by taking images from various positions of the patient, one can be able to vary magnetic field strength based on variation of field strength and hence to obtain variation in spatial resonance frequency. This permits up to 181 cm coverage, which makes it more essential in musculoskeletal studies in a single scan without reconfiguring the coils or changing the patient’s position (Fischbach et al 2004).
It is impossible for one to localize signals from various directions at the same time hence in most cases imaging method employed at 3T scanners is 2 dimension Fourier Transform (2DFT) imaging. This method applies three gradients successfully that employs three different localization principles: selective excitation, frequency encoding and phase encoding unlike previous MRI scanners which could not integrate the three gradients.
Thus the image obtained at 3T scanners is of a higher contrast as opposed to those obtained from previous scanners. The image below illustrates how images are obtained from various gradients to obtain a clear image (Lutterbey et al, 2005).