Propeller

PROPELLER is the PeRiodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction (PROPELLER) MRI, which is a recent method proposed to reduce motion artefacts. In fact, patient motion is a significant problem in MRI of the brain, leading to a reduction in image quality and loss of diagnostic information. Motion artefacts, loss of resolution, and signal reduction combine to reduce anatomic detail and limit the detection of pathological findings within the brain [18, 59]. The amount of motion artefacts during brain imaging depends on patient cooperation. The patients are often suffering from intracranial pathologies causing confusion, agitation or pain, and 14% of subjects require sedation to obtain satisfactory images [43]. Pipe [49] reported two kinds of artefacts: the type I artefact that arises as a result of tissue displacement during the quiescent period between each data sampling and the following excitation rf; and the type II artefact that arises as a result of spin phase induced by motion through magnetic field gradients between an excitation rf pulse and the subsequent data sampling period. Many methods have been used to resolve these problems. Glover, Pauly and Ahn proposed centre-out imaging methods such as projection-reconstruction [22] and spirals [1] to reduce motion artefacts. These are attributable in part to oversampling of central fc-space, which reduces artefact in a manner similar to multiple averaging in conventional imaging. In addition, when data collection begins at the centre of fc-space, in-plane gradient moments are greatly reduced in the central region of fc-space, minimizing type II artefact. Another method is by use of navigator echoes that collect extra data of measurement of motion or motion-related phase to reduce type I artefact [46]. Navigator echoes may also be used to throw out data collected when there is a significant motion, trading longer data collection times for reduced motion artefacts [58]. Sarty [60] describes a method called single trajectory radial (STAR) imaging. It is a single trajectory fc-space sampling method that can be made truly radial over most of its trajectory. It is different from previous methods where the acquisition of radial data in fc-space had to be done line by line with the appropriate TR elapsing between each line acquisition. And it is different from echo planar imaging (EPI) that is capable acquiring complete fc-space data on a Cartesian grid in a single acquisition along a single trajectory. The basic STAR trajectory consists of radial line segments tangentially joined by circles. The gradients required for the linear part of the trajectory are the obvious combinations of constant amplitude gradients. Data acquired on the STAR trajectory may be reconstructed by convolution regridding or by a non-uniform Fast Fourier transform. Reconstruction of the mathematical phantom STAR data produced high quality images free from artefact for all cases as was expected.

PROPELLER MRI also permits correction for inplane displacement and rotation (type I artefact), image phase due to motion (type II artefact), and through-plane motion [49]. PROPELLER MRI acquires data in a series of concentric rectangular strips or blades, each of which rotates through the centre of fc-space. The central region of fc-space is sampled for every blade (Fig. 11.6a, b). This allows the correction of spatial inconsistency in position, rotation and phase between strips, and it allows also the rejection of data based on a correlation measure indicating through-plane motion and decreasing motion artefacts through an averaging effect for low spatial frequencies. The method offers major benefits because the central region of fc-space is sampled multiple times, offering improved artefact suppression, and data within this central region can be compared between each blade. If motion has occurred between the acquisition of each blade, then data can be transposed to its estimated sta tionary position, prior to final image reconstruction. Every PROPELLER-correct image is derived using data that has been corrected for in-plane rotation and translation [50]. Central fc-space data from the first blade was chosen as the stationary data set and compared to corresponding data from each PROPELLER blade. Data

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Fig. 11.6 a, b. Propeller blade comparison. For each TR, one fast spin echo train collects all the phase-encoded lines for a blade. Every blade has a low-resolution image. The blades can be compared, and those that do not match can be discarded or, in the case of T2 andT2 FLAIR, a correction can be attempted. (Courtesy of GE Healthcare Technologies)

Fig. 11.6 a, b. Propeller blade comparison. For each TR, one fast spin echo train collects all the phase-encoded lines for a blade. Every blade has a low-resolution image. The blades can be compared, and those that do not match can be discarded or, in the case of T2 andT2 FLAIR, a correction can be attempted. (Courtesy of GE Healthcare Technologies)

were correlated with the stationary data set, first by rotation of the blade, using data magnitude in fc-space, and then by translation using the complex data in fc-space (Fig. 11.7a, b). The process was then repeated using the average of all corrected blades as the stationary data set with improving image quality (Fig. 11.4c). The amount of motion correction (mm per degrees) was calculated for each PROPELLER blade and the standard deviation was calculated for each image slice [20]. According to Pipe [49], PROPELLER MRI maybe implemented with any method that collects data along parallel lines, including standard spin and gradient echo, echo-planar, grase, and magnetization prepared turbo-spin-echo and turbo gradient echo sequences. It reduces motion artefacts in cooperative patients too. Forbes et al. [20] reported that minimal head motion worsened image quality by more than 5%. The major disadvantage of PROPELLER MRI is an increase in image reconstruction time of approximately 2-3 min per scan. This is due to redundant sampling of fc-space.

The advanced use of PROPELLER MRI is that proposed by Pipe et al. [51] in multishot diffusion-weighted FSE. The clinical utility of diffusion-weighted imaging (DWI) is well established and most DWI data are obtained by single-shot EPI imaging. The extreme variability in phase between applications of diffusion gradients creates significant motion artefacts in the presence of magnetic field inhomogeneities. The authors propose the use of a PROPELLER method which has inherent 2D navigator information in each FSE echo train. FSE data collection is used, which provides greater immunity against geometric distortion than that obtained with EPI sequences and reduces the signal instability present in all diffusion-weighted FSE methods by varying the phase of refocusing pulses. Navigator information is also used to correct data between shots and by a radial sequence, where uncorrected errors are expressed in a rather benign fashion. The authors have found that in the regions of homogeneous B0, the PROPELLER image has a similar contrast to that of EPI. In regions near significant susceptibility-induced field gradients (e.g. skull base, temporal lobes), PROPELLER DWI exhibits a far superior image quality, lacking the geometric warping and bright-signal artefacts common to EPI. PROPELLER reduces artefacts due to metallic implants and non-removable dental fittings because it is an FSE-based sequence. One other benefit of PROPELLER is its immunity to image warping from eddy currents, which is useful for PROPELLER DTI. There is a 50 % increase in minimum image time over conventional FSE due to the oversampling in the centre of fc-space.

PROPELLER MRI therefore offers the possibility of a diagnostic study in patients with significant head movement, where a repeat examination with sedation or general anaesthesia would be required, and an im provement of image quality in the presence of inhomo-geneity of magnetic field in DWI and DTI sequences. These sequences could easily be added to routine imaging.

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