Parallel Imaging in Clinical Applications - Assignment Example

?EPI in MRI College Parallel imaging is a technique that has been used widely in the current clinical applications. Its explorations are at a rise. This is because it has many advantages over the other techniques that are used in magnetic resonance. The concept used in parallel imaging emphasizes on reducing phase encoding steps numbers (Heidemann et al., 2003). It does not compromise spatial resolution neither does it compromise the MR acquisition field of view. Some of the important characteristics that have made parallel imaging essential include faster acquisition, increased spatial resolution, reduced motion artefacts, reduced blurring, shorter echo time and reduced geometric distortion (Jack, 2013, p.29). On the other hand, PI also has some demerits. Parallel imaging requires hardware to like multi-channel receivers and multi-element coil system. Parallel imaging also is restricted to certain SNR since not all protocols are best suited to it. This paper will discuss these advantages and disadvantages of using parallel imaging in the clinical applications. This paper will also discuss how these merits and demerits have affected the way they are used in the clinical applications. Advantages of parallel imaging Parallel imaging encourages increased spatial resolution (Jack, 2013, p.5). When parallel imaging is applied with reduction factor, MR acquisition which is not accelerated is required in reconstructing an image data set that has the same geometry and spatial resolution without using acceleration. When this raw data is reduced, the total scan time is reduced. Furthermore, it leads to an increased spatial resolution. To increase temporal resolution, the acquisition time to be reduced also. This aspect is very important in dynamic MRI. It helps in restricting total scan time by the physiological constraints for example the duration of breath-hold. This is made possible due to the increase in the spatial resolution caused by the parallel imaging (Schoenberg, Dietrich & Reiser, 2007, p.174). The aspect of increased spatial resolution has made parallel imaging to be used in abdominal MRI (Jack, 2013, p.24). Using conventional scans that takes 2 breath-holds of about 10-20s has led to use of motion artefacts by patients who cannot hold their breath for long. To prevent this problem, parallel imaging has been used because it will only use one single breath. This will make liver exams more feasible. This is helps in achieving perfume images that has high resolution (Schoenberg, Dietrich & Reiser, 2007, p.175). Parallel imaging has reduced motion artefacts. PI techniques were developed to increase scanning velocity. It was also realised that the techniques could be used to reduce artefacts. This effect was applied in the diffusion-weighted imaging. This is made possible due to the positive side that shorter scan time causes. Motion artefacts affect MRI that has long scan times. To eliminate this problem, parallel imaging is used to shorten the acquisition times. The advantage of reduced motion artefacts has led to the improved image quality (Schoenberg, Dietrich & Reiser, 2007, p.176). Image artefacts and blurring are caused by the relaxations that occur on the echo-train readout. If the echo-train is shortened, the signal decay will be reduced. This will lead to high image quality. These aspects have been applied in the lung MRI. The lung MRI, HASTE (half-Fourier-acquisition single-shot turbo-spin-echo) is used. The lungs have low proton density. They also have low motion and this has made the lungs very difficult in imaging them. To solve this problem, HASTE has been applied for the lung imaging. Parallel imaging is used in the HASTE and it helps in reducing the in echo spacing. This reduction in inter-echo spacing leads to an increase in resolution and small vessels in the lungs can be detected (Jack, 2013 p.22). Figure 1.0: It shows how parallel imaging has been used to improve image quality in lungs (source: Heidemann et al., 2003). Parallel imaging also encourages reduction in echo times more especially in the single shot sequences that uses linear k-space sampling. This reduced echo times increase signal intensities. This means that SNR will be improved. This improvement in the SNR has been applied in the diffusion-weighted MRI. Diffusion is essential in cerebrovascular accidents. This is because it involves the thermal motion within the molecules. Diffusion imaging therefore is used together with the parallel imaging. Due to the improved SNR, the molecular motion in the brain cannot affect the imaging due to reduced noise (Schoenberg, Dietrich & Reiser, 2007, p.176). Parallel imaging is very important when used with high field strengths. Although at times it is limited to the constraints of the SNR ratio in the low field strengths, it is also important in the high field strengths. When parallel is used with the high field strengths, it leads to many advantages. These advantages are associated with the increased signal-to-noise ratio. The SNR increases with the increase in the static field strengths (Yu & Feng, 2009, p.690). RF energy increases due to the increase of the SNR. This will also increase the specific absorption rate (SAR). This will cause many limitations to the protocols. To reduce these effects, parallel imaging is used to reduce the SAR of MRI (Schoenberg, Dietrich & Reiser, 2007, p.177). Disadvantages of parallel imaging Despite the fact that parallel imaging has many advantages, it also has some limitations. These limitations can be associated with the image quality and the technical preconditions. Parallel imaging requires data acquisition to be combined together with the multi-element coil systems. It also requires sophisticated algorithms to use to understand the multi-channelled data. Therefore, for a MRI system to use parallel imaging, it must have those specifications (Schoenberg, Dietrich & Reiser, 2007, p.178). One of the limitations of the parallel imaging comes in through the use of multi—channel receiver hardware. MRI systems use the parallel acquisition using the many independent channels of RF receiver. However, for a parallel imaging to be used, it requires many receiver channels together with several coil connectors. These requirements of large number of RF receiver channels and connective wires have made the system very expensive and increase in the complexity (Schoenberg, Dietrich & Reiser, 2007, p.178). Another limitation of parallel imaging is through the use of multi-element coil system. Parallel imaging requires appropriate coil system. If the coils are arranged in single direction, parallel imaging will be restricted to some applications only. Parallel imaging is limited to the 2 acceleration factors i.e. R=2. The typical coil configurations as well as the oval shape of both the thorax and the human abdomen are the main reason of these limitations. Coil systems are required for image orientation, acceleration factor and the phase encoding direction. All these aspects are limited to the parallel imaging and these features depend on the coil systems (Schoenberg, Dietrich & Reiser, 2007, p.179). Parallel imaging is also limited to the image reconstructed system. The raw data that has been acquired separately from the coil elements need to be combining to make a single image data set. This is done by ensuring that this data is not aliased to the artefacts. This can be achieved by the use of reconstruction algorithms. These algorithms are very complicated and they require many computations. The data acquired for the parallel imaging is very large. This means it requires a lot of computations during reconstruction. Furthermore, the multiple coil elements including large memory are required for the data. To construct parallel imaging data, fast construction systems are required. Therefore, parallel imaging has many limitations when it comes to the reconstruction of images (Schoenberg, Dietrich & Reiser, 2007, p.179). Another limitation of parallel imaging is the fact that not all protocols are suited for it. This is due to the restrictions that might occur on the SNR and image geometry. Parallel imaging is known to have improved image quality. This is achieved through both SNR and the image artefacts which are localised. In order to acquire reduced data sets that can be sued to shorten scan time, SNR has to be reduced. This causes some limitations to the parallel imaging. This means that a fall in the SNR with affect parallel imaging negatively. SNR decrease is caused by the increase in acceleration. Since parallel imaging has been limited to the high SNR only, some applications will not use SNR but will use the acquisition time rather. Examples of the applications that use the acquisition time include the abdominal breath-hold MRI and the MR angiography. Furthermore, parallel imaging caused losses to the SNR when used in increasing spatial resolution (Schoenberg, Dietrich & Reiser, 2007, p.180). SNR is also lost due to the acceleration and this will lead to the amplification of the image noise. There are many factors that are responsible for the noise amplification. These factors include the coil geometry, acceleration factor and the image orientation. When the noise levels are varied, the SNR might be overestimated and this might lead to some parts of the image having lower SNR (Jack, 2013). This effects as not easily controlled in parallel imaging as compared to the conventional image (Heidemann et al., 2003). Localised image artefacts are also another limitation of parallel imaging. Parallel imaging always occurs when the k-space found in direction of phase-encoding is under sampled. When reconstruction of parallel imaging is imperfect, aliasing of the artefacts occurs. This causes localised image artefacts which compliments noise amplification. This means that the images will contain stochastic noise. This causes a residual aliasing artefact in the FOV (field of view). This occurs when the image object is larger than the reconstructed FOV (Schoenberg, Dietrich & Reiser, 2007, p.181). However, parallel imaging has shown great importance in MRI. Its several advantages have made it widely used in many fields ii medicine. Its advantages have linked it to particular applications. On the other hand, some limitations have prevented parallel imaging from being applied in some sectors of MRI. Therefore, despite the limitations that parallel imaging has, it is still the best choice to use in MRI. Auto-SENSE and GRAPPA There are several techniques that have been developed for parallel imaging. These techniques vary in their philosophies and also in the way they are reconstructed. Each technique has some advantages that has made suitable to be used in some areas. The two techniques that are widely used in clinical applications include the SENSE and the GRAPPA. These two techniques have a couple of similarities and differences between them. These similarities and differences have made them applied in different clinical fields. Sensitive encoding (SENSE) is a reconstruction method used for parallel imaging. Its main feature is its image domain algorithm used for unfolding. To construct a FOV image, SENSE algorithm should be repeated in the reduced FOV pixel location. SENSE provides parallel imaging with coil configurations (Jack, 2013). Both SENSE and GRAPPA support objects to have coil configurations around them. However, the two parallel imaging techniques are not restricted to both linear coil configurations and the localised sensitivities. For SENSE reconstruction technique, g-factor is used for analysing noise enhancement (Jack, 2013). The g-factor also works for GRAPPA because both the SENSE and the GRAPPA have equal requirements when it comes to coil configuration. These two parallel imaging techniques reduce scan time through subsampling the k-space (Blaimer et al., 2004). GRAPPA and SENSE have advantages and disadvantages that have made them used regularly in the clinical applications. Despite the fact that these two techniques are different, they also have some similarities when it comes to the reconstruction quality. SENSE has been used widely by many companies. This means SENSE is available in many companies. SENSE has enhanced image acquisition and many clinics have benefited from this aspect. Another advantage that has made SENSE the most available technique is the reduction in the scan time (Blaimer et al., 2004). This reduction in the scan time has caused requirements of breath-hold to relax. Furthermore, reduction of scan time can be used in improving the spatial revolution. Despite the similarity of reducing the scan time, there are differences that occur when it comes to the noise distributions on the images that have been reconstructed. These differences are due to the reconstruction principles which are different. For SENSE images, noise distribution varies spatially and this can be reconstructed by using geometry factor. To reduce image noise in SENSE, regularisation is used. When regularisation is used, it may lead to image artefacts. For the non-regularised images, image artefacts do not appear. For GRAPPA reconstruction of images, noise distribution has uniform distribution. Spatial variations might occur occasionally (Blaimer et al., 2004). For SENSE reconstruction in parallel imaging, coil calibration is done before the scan. For GRAPPA reconstruction, calibration is embedded with the ACS lines acquired (Jack, 2013). Another difference between SENSE and the GRAPPA is through the way the artefacts appear. SENSE reconstruction operates at pixel-by-pixel basis. This operation causes enhancement of local noise and this noise will be localised in the images which are unfolded. In contrast, GRAPPA reconstruction produces k-spaces with some missing lines. Calculations which are inaccurate are done on the missing lines and this will produce aliasing artefacts in the images that have been reconstructed (Jack, 2013, p.24). When to use SENSE An example where SENSE reconstruction technique is used in clinical applications is the CE-MRA i.e. contrast-enhanced magnetic resonance angiography (Blaimer et al., 2004). The parameter used in this technique is the imaging time. This is because total acquisition should be completed in the first contrast agent. This means that CE-MRA spatial resolution is restricted. SENSE technique encourages higher spatial resolution ad it should have constant scan time. Blaimer et al. (2004) say SENSE reconstruction technique is also applied in the breast imaging. In this application, MR is combined with parallel imaging and this will yield very powerful tool. SENSE also provides very high spatial resolution to be used in the dynamic contrast enhanced breast MRI. However, SENSE is applied in these fields because it has the higher spatial resolution. This high resolution is responsible for the visualisation of high anatomic. This will later produce increased specificity of diagnosis. Use of GRAPPA There are also some situations when GRAPPA is more advantageous that SENSE. This means that GRAPPA will be preferred. Examples of applying GRAPPA in clinical application is the abdominal and lung MRI. GRAPPA is applied in these fields when it is difficult to achieve accurate coil sensitivity. Lungs and the abdomen have low spin density. However, determining spatial coil sensitivity is in the difficult. GRAPPA is preferred because good image quality is achieved because information sensitivity is obtained from the k-space. The lines that are used in the centre of the k-space ensure that good reconstruction quality is achieved (Hoge & Brooks, 2008, p.462). Single-shot EPI also requires accurate coil sensitivity. This accurate coil sensitivity is difficult to achieve. Single-shot EPI is widely used in clinical applications. Some of the problems that single-shot EPI is experiencing are the image distortion, blurring and signal noise. To reduce distortion, pMRI is used. Single-shot EPI cannot be used with SENSE because several problems will be experienced. To solve these problems, GRAPPA is used. This is because reconstruction of missing lines based on the k-space cannot be affected by the distortion of images (Hoge & Brooks, 2008, p.463). However, there are some similarities between SENSE and GRAPPA. These come from the advantages that these two techniques share that make them be used in the same field. The differences between them are the main reasons why one of the techniques is preferred over the other. SENSE has been used widely in the clinical applications. Nevertheless, in some situations, GRAPPA is preferred over the SENSE algorithm (Hoge & Brooks, 2008, p.467). Bibliography Blaimer M, Breuer F, Mueller M, Heidemann Rm, Griswold Ma, & Jakob Pm. (2004). SMASH, SENSE, PILS, GRAPPA: how to choose the optimal method. Topics in Magnetic Resonance Imaging : TMRI. 15, 223-36. Schoenberg, S. O., Dietrich, O., & Reiser, M. (2007). Parallel imaging in clinical MR applications. Berlin, Springer. http://public.eblib.com/EBLPublic/PublicView.do?ptiID=372455. Hoge Ws, & Brooks Dh. (2008). Using GRAPPA to improve autocalibrated coil sensitivity estimation for the SENSE family of parallel imaging reconstruction algorithms. Magnetic Resonance in Medicine: Official Journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 60, 462-7. Jack, M. (2013). Fast Imaging Techniques. 6, 1-31. Schmiedeskamp, H, Newbould, R D, Pisani, L J, Skare, S, Glover, G H, Pruessmann, K P, & Bammer, R. (2010). Improvements in parallel imaging accelerated functional MRI using multiecho echo-planar imaging. Info:Doi/10.1002/Mrm.22222. Wiley-Blackwell. http://dx.doi.org/10.5167/uzh-45306. Yu, L., & Feng, H. (2009). Regionally Optimized Reconstruction for Partially Parallel Imaging in MRI Applications. IEEE TRANSACTIONS ON MEDICAL IMAGING. 28, 687-695. Heidemann Rm, et al. (2003). A brief review of parallel magnetic resonance imaging. European Radiology. 13, 2323-37. Golay X, De Zwart Ja, Ho Yc, & Sitoh Yy. (2004). Parallel imaging techniques in functional MRI. Topics in Magnetic Resonance Imaging : TMRI. 15, 255-65. Read More