|Year : 2018 | Volume
| Issue : 3 | Page : 286-288
SWIFT: Sweep imaging with FOURIER transformation
Harshavardhan Talla1, Vijayalakshmi Nimma2, Tejaswi Bathina1
1 Department of Oral Medicine and Radiology, Meghna Institute of Dental Sciences, Mallaram, Nizamabad, Telangana, India
2 Department of Oral Medicine and Radiology, Government Dental College, Mumbai, Maharashtra, India
|Date of Submission||18-Jan-2018|
|Date of Acceptance||17-Feb-2018|
|Date of Web Publication||18-Oct-2018|
Dr. Harshavardhan Talla
Department of Oral Medicine and Radiology, Meghna Institute of Dental Sciences, Mallaram, Nizamabad, Telangana
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Oral cavity is prone to a variety of diseases starting from simple tooth decay to life-threatening carcinomas. Injudicious use of ionizing radiation may increase the exposure of a patient and this has been overcome with the use of magnetic resonance imaging. A newer modality sweep imaging with Fourier transform is fast, quiet, and identifies tissues with ultra-short relaxation times. A literature search has been done in main databases such as PubMed and Google Scholar and 20 articles have been included in this review.
Keywords: Fourier transform, Magnetic resonance imaging, Sweep imaging
|How to cite this article:|
Talla H, Nimma V, Bathina T. SWIFT: Sweep imaging with FOURIER transformation. J Indian Acad Oral Med Radiol 2018;30:286-8
| Introduction|| |
It is a well-known fact that the treatment and prognosis of a specific disease are determined by correct diagnosis. Use of X-ray-based imaging techniques is the most commonly used diagnostic imaging modalities in dentistry. However, keeping in mind the principle of exposure of a patient to ionizing radiation at a low level (as low as reasonably achievable), some of its limitations such as inability to detect microcracks, pulpal tissue, and its association with increased risk of cancer led toward tomographic imaging methods of both hard and soft tissues which have a greater importance in the assessment of bone volume, gingiva, and proximation of vital structures thereby helping in the field of implantology.
Magnetic resonance imaging (MRI) has proven its applications as a nonionizing and noninvasive technique in analyzing the anatomy of the teeth. Conventional MRI, however, helps in identifying only soft tissues such as periodontal ligament and pulp tissue. Even though hard tissue components of the teeth and demarcation between the soft and hard tissues can be assessed by using solid-state imaging techniques, those methods are time-consuming and also are not suitable for in vivo application.
The importance of MRI has become more prominent after the development of methods which helps in identifying densely calcified structures of the tooth such as enamel and dentin. Although contrast-enhanced MRI technique can be used to identify mineralized structures of the oral cavity, it is only an indirect method wherein a contrast medium helps in providing a higher signal compared to the surrounding structures. Sweep imaging with Fourier transformation (SWIFT) directly helps in obtaining high-quality images of hard tissues in vivo.
Successful experiments conducted by Bloch and Purcell independently on MRI date back in the year 1946 which led to the development of nuclear magnetic resonance spectroscopy as a powerful tool in biology and chemistry. In 2002, Dr. Garwood had the idea that it might be possible to acquire spatially encoded MR signals while simultaneously applying a frequency-modulated (FM) pulse. He tested the idea using Bloch simulations and, later in the same year, experimentally demonstrated it by acquiring steady-state MR signals during gaps inserted into FM pulses.
Principle of SWIFT
The key innovation of SWIFT is the simultaneous signal acquisition and time-shared excitation which is acquired by inserting gaps into an FM pulse. An MRI image is acquired through the spinning magnetic moments of hydrogen molecules that are present in tissues as water molecules. A radiofrequency is then applied to detect the signals causing the spins to resonate in the strong magnetic field. Standard MRI sequences do not acquire the images of enamel and dentin as they are predominantly made up of mineralized components and only a minute amount of water. Therefore, there is a restricted molecular movement of water signals in these highly mineralized components, leading to quick signal decay.
Transverse relaxation time (T2) is the time constant used to describe the signals' free induction decay (FID). The FID value of enamel and dentin has multiple components with a mean T2 of 200 μs for dentin and 60 μs for enamel which is very short when compared to the time interval needed for the standard MRI to acquire spacial encoding with pulsed magnetic field gradients which is typically more than 1 ms. Hence, the water signal from densely mineralized structures decays before the standard MRI digitalizes the signal resulting in an image of little intensity or no intensity represented as black zones.
Solid-state MRI techniques such as single-point imaging and stray field imaging technique are used in imaging mineralized structures. But there in vivo application is limited due to their longer imaging times of around 5–6 hrs. Rapid acquisition with relaxation enhancement and echo planar imaging are the multi-echo imaging methods used for rapid acquisition of data. However, these methods depend on tissues with relatively long T2 for multiple data acquisition. Ultra-short echo time MRI technique is the first used in vivo imaging technique for delineating pulp, enamel, and dentin. But this method uses high peak amplitude so as to record the tissues.
SWIFT is a recently developed imaging technique which overcomes most of these difficulties in detecting fast relaxing signals by using low peak amplitude and also is feasible for in vivo studies.
The basic principle involved in SWIFT is the simultaneous excitation and signal acquisition in a time-shared mode which is achieved in a field gradient by inserting gaps into an FM. Unlike “adiabetic pulse” in which the carrier frequency during the pulse varies with time,, SWIFT uses a frequency where the power is dropped thereby producing a small tip angle for spin excitation. The transmitter and the receiver are alternatively switched on and off with only 1-2 μs of signal acquisition delay in between them. This allows efficient imaging of tissues with a short T2 value. The repetition time TR is accordingly comparable with the pulse length resulting in a shorter acquisition time. This gapped FM pulse is applied repeatedly in the magnetic field changing its orientation with each TR in a stepwise manner resulting in almost continuous gradients with low stress and relatively quiet operation.,,,
Tooth anatomy and dental caries
Applications of MRI in imaging teeth are broadly classified into hard tissue imaging and soft tissue imaging. Hard tissue imaging in particular remains challenging because of less water content in enamel and dentin and the T2 relaxation times of water in these tubules are very short. Studies have shown that imaging teeth with SWIFT, which helps in acquiring such ultra-short T2 relaxation times, not only gives a well-resolved tooth anatomy, delineating enamel, dentin and pulp but also detects early carious lesions. It clearly demarcates the extent of demineralization which is not acquired by any of the radiographic methods. The relative variation in the intensities of enamel, dentin and pulp is in the order of 10:35:100 owing to the amount of water (8:20:100) in these structures, respectively. SWIFT images also help in identifying the finest details of the tooth like accessory canals which are not visualized by standard radiographic techniques.
Restorations and calcifications in pulp
Composite restorations are generally demarcated as radiopaque or radiolucent as they contain varying amounts of minerals or heavy metals. Hence, conventional radiographic methods have difficulty in detecting recurrent caries when present adjacent to the radiolucent restoration or when present in the gingival margins of the restoration. Fortunately, the composite resin materials exhibit short T2 relaxation times and hence easily be detected on swift images. It also helps in demarcating reparative dentin which is formed as a result of past lesions thereby reducing the misconception of existing recurrent caries and multiple restorations., Moreover, in contrast to CBCT images, the presence of restorative materials does not seem to cause image distortion in SWIFT.,,
Detect microcracks in teeth
Detection of microcracks in teeth is mainly done by visual findings in conjunction with other methods such as transillumination, magnification, or using dyes. These methods lack the ability to determine the exact extent and detect the cracks that are within the roots or are apical to restorations. SWIFT MR image helps in visualizing cracks which are as small as 20 μm as the water content in the cracks results in a positive enhancement of the contrast and also have the advantage of minimum artifacts due to any adjoining restorations.,
Detection of oral cancer
SWIFT MRI aids in three-dimensional assessment of medullary and cortical bones in finest detail and also has the potential to detect mandibular invasion, and the results were found to be in excellent qualitative agreement with histopathological findings.
Taking the advantage of SWIFT MR imaging which helps in imaging ultra-short relaxation times of hard tissues such as bone not only aids in accurately visualizing the density of the bone during initial planning of implant placement but also helps in assessing the success of implant placements.
Advantages of swift over standard MRI
SWIFT has the advantage of not only acquiring the images of tissues with ultra-short T2 relaxation times but also is fast, avoiding associated delays of refocusing pulses and time required for an excitation pulse. It demonstrates little or no motion artifacts because it does not have echo time and less distortion due to dental restorations. It is quiet, uses TR in stepwise manner, and hence can be used in patients with ligyrophobia. This technique is unique in imaging both hard and soft tissues simultaneously with high resolution detecting even minute pathological and anatomical abnormalities.
| Conclusion|| |
SWIFT imaging is a fast and a newer imaging modality that offers simultaneous three-dimensional imaging of soft and hard tissues of the teeth without use of ionizing radiation. This modality has many important applications in the field of dentistry with some powerful features such as its ability to visualize hard tissues such as enamel, dentin, and bone and offer a nearly silent operation reducing the anxiety in claustrophobic patients. But more in vivo trials are to be done to demonstrate its application in regular diagnostic imaging.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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