Enhancement Of MRI Imaging by Combining Ramp Sampling and Parallel Imaging Techniques in Cerebral Tumors

Our aim was to decrease geometric distortion artifact in DWI on brain by using different MR techniques such as ramp sampling and parallel imaging. The study was done on 15 patients, They were subject to routine brain MRI examination for different symptoms, examination was done on 1.5 T scanner, patients were divided to three group five patient in each group, geometric distortion in frontal and occipital lobe measured in the routine acquired DWI and after used the interested strategy compared to anatomical T2-FSE acquired in the routine examination then reduction percentage for each strategy was calculated. Application of parallel technique imaging showed significant artifact reduction in frontal lobe with approximately 54.36 % and about 34.8% in occipital lobe, while ramp sampling showed reduction with approximately 19% in frontal lobe and reduced artifact by 41.58% in the occipital lobe.


Introduction
Magnetic field gradient was used in diffusion MRI to generate brain image and considered as the clinical domain routine when (Echo Planar Imaging) was available on MRI devices [1]. Mapping of water molecules Diffusion MRI non-invasively (magnetic resonance imaging) helps in imaging of brain microstructural characteristics in-vivo [2]. Two sharp intense magnetic fields were used to generate MR signals which are very sensitive to water movements [2]. Diffusion-weighted magnetic resonance imaging uses echo-planner technique as gold standard procedure imaging in MRI of brain [3]. The main disadvantage of EPI is artifact caused by main magnetic field inhomogeneity as investigated by many clinical [4], [5], [6].This artifacts is major problem in air rich tissues or air edges which separates bones and muscles this artifact is known as susceptibility distortion [7], [8]. Misalignment of data can produce change in organs anatomy by several millimeters, which can reduce the accuracy of image analysis in the brain lesions [6], [9], [10]. It was recognized that maximum error happen in phase encoding direction which generated by phase-encoding steps [11].
DWI is the optimum technique in tissues with high gradient strength like nerves and brain due to its signal to noise ratio [12] [13].Signal distortion and bad sharpness in EPI is produced as results of absence of time at the peak of gradient wave front [14].
The aim of this study was to reduce geometric distortion artifact and enhance resolution for EPI-DW images for the brain. We have used two different MRI strategies which are ramp sampling and parallel imaging. Ramp Sampling allowing echo sampling during ramp ups and downs of the frequency encoding gradient therefore ramp sampling reduce echo spacing, allow faster sampling and improve image resolution [14].

Patients
Fifteen patients were selected form fifty patient with normal brain anatomy, in Mansoura advanced radiology center, Mansoura Egypt.

MR examination
Device of magnetic field strength a 1.5 tesla is used type (GEHC) and sigma scanner with high resolution is used. DWI techniques are used added to ramp sampling asset or both. Dicom viewer soft is used to investigate imaged before and after using new strategy and for measurement of geometric distortion.

2-Results and discussion
For calculating the reduction percentage of each strategy, we divided the patients in to three groups, each group include five cases with different ages and sex, for first group we added DWI sequence with ramp sampling and compared the acquired images with that one acquired with conventional parameters, for the second group we added DWI sequence with ASSET and compared the acquired images with that one acquired with conventional parameters, for the last group we acquired DWI in the routine examination with ramp sampling technique then we added DWI sequence with both ramp sampling and ASSET then compared the acquired images with that one acquired with ramp sampling only. Then average percentage was calculated.

Effect of Ramp sampling
In the following first five cases [1, 2, 3, 4 and 5] we added the ramp sampling to the DWI in a separate sequence in addition to the routine MR examination and compered the acquired images with that one acquired without the ramp sampling as following:

Case [1]
In this case we choose two different slices figure

Case [2]
As done in case [A] two different slices for second case were chosen for studying the effect of ramp sampling as Figure (3) & (4) show structure of normal brain with geometric distortion artifact in anterior region in DW image with conventional parameters (B) and the reduction after ramp sampling (C).

Case [3]
Two different slices for third case were chosen for studying the effect of ramp sampling as Figure   As seen in figure (5) DWI acquired without ramp sampling (B) suffer from blurring and low resolution unlike image (C) that acquired with ramp sampling which shows increase in resolution, better contrast between different tissue and incensement in signal strength.

Case [4]
Figure (7) shows normal structure of brain for fourth case. The selected slice shows the voxel displacement in anterior and posterior region, (C) shows the reduction in both regions after using ramp sampling.

Case [5]
Figure (8) shows the last case in this group for studying the effect of ramp sampling on SE/EPI DWI, measurements shows decreasing in geometric distortion artifact in anterior and posterior lobe of normal brain.

Effect of Parallel imaging
In the second group the five cases [6, 7, 8, 9 and 10] were subjected to routine MR examination for brain and a separate DWI sequence acquired with ASSET technique then the resultant images were compared with that one acquired with neither ramp sampling nor ASSET.

Case [6]
In this case one slice have voxel displacement in both anterior and posterior region was choose for measurement of this displacement compared to T2 of the same slice as figure (9) shows.

Case [7]
Figure (10) shows the result for second case after using ASSET (C). it is obvious that in addition to decreasing voxel displacement in anterior and posterior region using ASSET increase resolution and decrease noise when compared with the same slice acquired without ASSET (B), T2-FSE used as reference (A).

Case [8]
Figure (11) shows structure of normal brain for a pediatric patient as a stretching in DWI acquired without ASSET (B) compared with the noticeable decline in this after using ASSET (C) Figure (11): Patient 8 pediatric case.

Case [9]
Another case clearly shows the image defects in DWI and the positive effect of using ASSET with EPI-DWI. Figure (12) shows slice of normal brain for the fourth case in this group, ASSET improve image quality and decrease geometric distortion artifact (C) when compared with those one acquired without ASSET Figure (12): patient 9 ASSET with EPI-DWI.

Case [10]
Figure (13) shows a slice of the last case in this group which shows the frontal sinus and susceptibility artifact, using ASSET decrease susceptibility artifact and increase resolution (C). Table (3) shows reduction percentage for voxel displacement in anterior region in DWI before and after using parallel imaging technique for the examined five cases. Table (4) shows the reduction percentage in posterior region for the first three cases, for the last two cases there is no posterior artifact was detected.

Effect of Ramp sampling and ASSET
in this step we have used both strategies (ramp sampling and ASSET) instead of using only one strategy and compared the acquired images with the same one acquired with only one strategy in these cases is ramp sampling, the following five cases [11, 12, 13, 14 and 15] show the result of applying both strategies on the same DWI sequence:

Case [11]
Figure (14) shows first a slice of the first case in this group, image (B) shows DWI acquire with ramp sampling technique it seem to be blurring and have low resolution when compared with image (C) which acquired with both ramp sampling and ASSET techniques.

Case [12]
Figure (15) shows the second case in this group, although we showed that using ramp sampling decrease geometric distortion artifact in EPI-DWI ( table [1], [2] ), using both techniques together (ramp sampling+ ASSET) (15-C) shows more reduction in this artifact in addition to increase in resolution. Figure (16) shows another slice in the upper brain for the same patient for measurements the artifact in anterior and posterior lobes.

Case [14]
Figure (18) shows geometric distortion artifact in anterior and posterior lobe for normal brain of the forth case, DWI acquired with ramp sampling (B) shows blurring, more geometric distortion in both lobs and more noise when compared with that one acquired with both ramp sampling and ASSET (C).   [6] voxel displacement (mm) in posterior region after using both ramp sampling and ASSET compared with those one with ramp sampling only.

Conclusion
Many parameters can affect on quality of MRI image . We showed that Ramp sampling and ASSET can be used to decrease geometric distortion artifact in EPI-DWI for brain in addition to increasing image resolution and decreasing noise Although ramp sampling shows reduction in geometric distortion artifact and signal loss, parallel imaging technique impact more positively in images artifacts and resolution. Maximum reduction for geometric distortion artifact and susceptibility artifact in air-tissue interface can be achieved by using both strategies together during data acquisition.