Indian Journal of Radiology and Imaging Indian Journal of Radiology and Imaging

MRI PHYSICS
Year
: 2005  |  Volume : 15  |  Issue : 1  |  Page : 103--106

Evaluation of slice thickness and inter slice distance in MR scanning using designed test tool


P Narayan, S Suri, SR Choudhary, N Kalra 
 Department of Radiodiagnosis and Imaging postgraduate institute of medical educational and research, chandigarh, India

Correspondence Address:
P Narayan
Department of Radiodiagnosis and Imaging postgraduate institute of medical educational and research, chandigarh
India

Abstract

Materials & Methods : A Test Tool Phantom was designed and kept in Heat RF coil to be studied under sequences to evaluate the concerned parameter. An imaging sequence with specified slice thickness and inter slice distance was programmed for imaging purpose. After acquisition of the sequence, the obtained image was evaluated for FW (Full width at half maxima) using the image intensity profile measurement. The evaluated FWHM was correlated with the slice thickness and compared it with set value. To evaluate the interslice distance same test tool was used with three more image slices and the slice position with respect to the origin was evaluated with distance measurement protocol in the system. The difference between tow consecutive slice positions corresponds the interslice distance. The whole measurement was performed on Siemens magnetom Vision Plus MRI System using the designed test tool. Results : Evaluated Slice Thickness and interslice distance were compare with the set valued in the programmed sequences and the variation found were under 1 mm. Conclusion : The designed tool can serve the purpose of routine Quality Assurance in MRI and the slice thickness parameter can be evaluated very easily. The designed test toll fulfills the criteria of task group of AAPM.



How to cite this article:
Narayan P, Suri S, Choudhary S R, Kalra N. Evaluation of slice thickness and inter slice distance in MR scanning using designed test tool.Indian J Radiol Imaging 2005;15:103-106


How to cite this URL:
Narayan P, Suri S, Choudhary S R, Kalra N. Evaluation of slice thickness and inter slice distance in MR scanning using designed test tool. Indian J Radiol Imaging [serial online] 2005 [cited 2019 Nov 15 ];15:103-106
Available from: http://www.ijri.org/text.asp?2005/15/1/103/28758


Full Text

 INTRODUCTION



In any clinical imaging study, it is very important to have accurate confirmation of slice thickness and inter slice distance to that shown by the scanning system. Slice selection attempts to achieve the ideal reception of signal solely from a rectangular slab of material perpendicular to the slice select direction. It is an important parameter, not only because of image signal to noise ration varies linearly with slice thickness, but also because clinical image resolution strongly affected by partial volume effect, which worsen at larger slice thickness. The slice thickness indirectly averages the signal from the full width, of its thickness and project it as two dimensional image. This two dimensional image should expected to have all anatomical parameter is not accurate, it may affect the clinical diagnosis. A test tool was designed to evaluate the above parameters and their performance were evaluated. These two parameters should be evaluated periodically under coil and its acquisition system.

The same can be utilized for all coils supplied with the systems (viz. body, spine, cp-neck etc.)

 MATERIALS AND METHODS



A parallelepiped shaped test object was designed using non signal generating material (Perspex) of thickness 5mm. The dimension of two side wall of the Perspex sheet was (200x25x5) mm3 identical to each other. The two identical Perspex sheet were attached parallel to each other with their inter wall distance of 1mm to reduce the uncertainty and random fluctuations in the signal measurement. The inner space of the object was filled with signal generating solution (MRI active solution), NiSo4.6H2o + Nacl. 2H2o and then attached with a cylindrical phantom supplied with the system at approximately 45 degree from the horizontal due to easier evaluation of the image, however it can be kept at any acute angle. The signal generating solution can be of any material provided their T1/T2 ratio lie in the range of (3-6) and this value should be independent of any minute temperature variations. The standard cylindrical phantom in conjunction with the test object was used to avoid the low signal problem from the object. The tool along with the cylindrical phantom were kept in the standard head RF coil and a scout image cuts taken. A transverse cut of the image is shown in the [Figure 1]. A T1 weighted imaging sequence of coronal plane was programmed on the transverse scout image with covering minimum area of phantom and maximum possible area of designed test object with repetition time 600 ms, echo time 14 ms, flip angle 70 degree, pixel size 1.09x0.98 mm2, matrix size 144x256, no of acquisition 2 as given in standard protocol of the system. The set sequence was run and image found adjusted for contrast and brightness for its better visibility. The image portion of the test object was zoomed properly to avoid the error in measurement of distance in image profile. The intensity profile of the object image was then evaluated in horizontal direction of image and its FWHM was correlated with FWHM of slice profile and shown in [Figure 2]. The slice thickness i.e. FWHM of slice is related with the FWHM of evaluate image in the following fashion which can be very easily derived and understood.

Slice Thickness = (FWHM) image x Tan?, where ? is the angle between horizontal and tool as coronal cuts were taken.

Inter Slice distance was evaluated with slight modification in this designed test tool. A Perspex/plastic tube of diameter 1mm filled with signal generating solution and fixed horizontally at the tip of the tool. This tube served the purpose of localization of relative position image of the scanning under study. A saggital scan for four slices was taken with identical scanning parameter as in the case of slice thickness evaluation with T1 weighted and shown in [Figure 3]. After imaging the distances of different scanned image from the localizer origin were evaluated using distance measuring protocol. An evaluated image along with measured distance is shown in [Figure 4]. This distance was found to be related with distance offset from the origin as follows

OO' = OD' / Tan xx, where xx is the angle which localizer make with the ramp. The difference between two consecutive offset found to be the inter slice distance, i.e., O' O" = 1/ Tan xx (OD' - OD") The schematic diagram of tool and its positioning for evaluation of slice thickness and inter slice distance are shown in [Figure 5][Figure 6][Figure 7] respectively.

The above mentioned method were repeated for different slice thickness and different inter slice distance and their corresponding values evaluated accordingly.

 RESULTS



The evaluated data for slice thickness and inter slice distance was compared with their corresponding set values in the sequence parameter and given in [Table 1] and [Table 2] respectively. The results were found to be under 1mm of set value of slice thickness and 0.9 mm of inter slice distance.

 CONCLUSION



This test tool can serve the purpose of evaluation the inter slice distance and slice thickness and used as routine quality assurance test tool in any MRI. Better results can be expected using two ramp systems with 90 degree angle to each other. If the signal generating sample thickness is reduced further from 1mm, the tolerance level of the tool can still be reduced and better accuracy in the measurement can be achieved. Differences in slice width would be caused by a failure of accurately adjust of the tool can still be reduced and better accuracy in the measurement can be achieved. Differences in slice width would be caused by a failure of accurately adjust of the field gradient or RF pulse. Differences in slice width would be caused by a failure of accurately adjust of the field gradient or RF pulse. Differences in slice thickness between several sequences used for relaxation measurement lead to inaccuracy if uncorrected for.[5]

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