Nuclear Associates 76-907 and 76-908 AAPM MRI Phantoms Users Manual March 2005 Manual No. 38616 Rev. 3 ©2003, 2005 Fluke Corporation, All rights reserved. Printed in U.S.A.
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Table of Contents Section 1: 1.1 1.2 1.2.1 1.2.2 General Information................................................................................... 1-1 Introduction .................................................................................................. 1-1 Phantom Description.................................................................................... 1-1 3D Resolution and Slice (3DRAS) Phantom (Model 76-908) ................. 1-2 Uniformity and Linearity (UAL) Phantom (76-907).......
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General Information Introduction 1 Section 1 General Information 1.1 Introduction This user's manual describes a set of MRI phantoms that were manufactured based on the AAPM recommendations and their use in measuring the MRI system performance. These phantoms were designed to measure them conveniently and quickly.
Nuclear Associates 76-908 & 76-907 Operators Manual 1.2.1 3D Resolution and Slice (3DRAS) Phantom (Model 76-908) The 3DRAS phantom has outer dimensions of 6" x 6" x 5" (Figure 1-1). Six resolution inserts and slice thickness 4 ramp sets were placed inside of the rectangular box to allow image acquisition in any one of the three directions (sagital, coronal, and transaxial) without repositioning the phantom. Outer Shape 6" X 6" X 5" Cubical Box Figure 1-1.
General Information Phantom Description 1 #1 Filling Hole ( ¼”) 2 mm Gap Resolution Block W/Horizontal Holes 2 mm Gap #4 1 mm Gap Resolution Blocks w/Holes into the Paper #2 Resolution Blocks w/Vertical Holes Slice Thickness Block Groups (4) 1 mm Gap Box Dimension of the Box 6: x 6: #3 Walls are Made with ¼” Material Figure 1-3. Resolution Sections in Three Imaging Planes The slice-thickness phantom consists of two crossed thin ramps (Figure 1-4). Figure 1-4.
Nuclear Associates 76-908 & 76-907 Operators Manual A ramp-crossing angle of 90° yields an angle of 45° between the ramp and the imaging slice plane. Each slice thickness section consists of four triangular blocks arranged to form (signal producing) hot ramps filling the gaps of 1 mm or 2 mm (Figure 1-5). Slice Thickness Measurement Gaps Gaps are 1 mm or 2 mm Throughout Figure 1-5.
General Information Phantom Description 1 MR Phantom for Spatial Linearity, Signal-To-Noise, and Image Artifact Glue Glue Linearity Grid, Light Cover 13” Glue, Orientation References Glue Glue Not To Scale 13” Figure 1-6. UAL Phantom MRI Phantom for Spatial Linearity, Signal-To-Noise, and Image Artifact Glue Glue Glue 3/16” Wall ¼” Wall Glue 1” 13” Glue Flood Section Two Grids Stacked Not to Scale Glue Glue 4” Figure 1- 7.
Nuclear Associates 76-908 & 76-907 Operators Manual The flood phantom has a uniform solution, the image of which can be used to measure the uniformity in signal intensity in the images. The grid section consists of grids that can be used to assess geometric linearity. A small pancake shaped container holding producing solution is attached on the diagonal surface of the phantom. It is used to detect phase errors.
Operation Phantom Preparation 2 Section 2 Operation 2.1 Phantom Preparation The phantoms can be shipped filled with solutions or solutions can be shipped separately. Each user can also prepare a solution. 2.1.
Nuclear Associates 76-908 & 76-907 Operators Manual 2.2 Preparation for Scanning 2.2.1 Positioning the Phantom The 3DRAS phantom can be placed in a head coil or a body coil. The center of the phantom should coincide approximately with the center of the RF coil. The UAL phantom can be used only with a body coil. 2.2.2 Scanning Parameters For all measurements, scan conditions should be carefully recorded.
Operation Tests with 3D Resolution and Slice (3DRAS) Phantom 2 resonance frequency adjustment and some may be completely automated. Resonance frequency should be recorded daily for trend analysis. Values of resonance frequency should generally not deviate by more than 50 ppm between successive daily measurements. 2.3.2 Signal-To-Noise Ratio The signal is defined as the mean pixel value within the region-of-interest minus any pixel offset. Noise is defined as the random variations in pixel intensity.
Nuclear Associates 76-908 & 76-907 Operators Manual • The high contrast resolution should remain constant for repeated measurements under the same scan conditions and should be equal to the pixel size. For example, for a 25.6 cm field of view with a 256 x 256 acquisitions matrix, the resolution should be 1 mm. 2.3.4 Slice Thickness Slice thickness is defined as the full width at half maximum (FWHM) of a slice profile. The full width at tenth maximum (FWTM) is an additional descriptor of the slice profile.
Operation Tests with 3D Resolution and Slice (3DRAS) Phantom 2 direction) is measured and related to the slice position. For a 45° ramp, the distance from a centered reference pin to the slice profile midpoint will be equal to the point of the ramps located at the isocenter. All measurements should be made along the line made up of the magnet isocenter and the centers of the imaging planes. Comparison of external position marker should generally agree with the actual slice position within +2 mm.
Nuclear Associates 76-908 & 76-907 Operators Manual Figures 2-2 and 2-3 represent two adjacent sagital views of the phantom. The resolution holes are well resolved in vertical and horizontal direction and the two slices are 6 mm apart (SP = 14.5 and 20.5). The slice thickness images of short vertical lines on top and bottom of the phantom image again are from the crossed hot ramps of 2 mm (top) and 1 mm (bottom). Figure 2-2. Figure 2-3.
Operation Tests with 3D Resolution and Slice (3DRAS) Phantom 2 Figures 2-4 and 2-5 show two adjacent coronal views of the phantom. The slice thickness lines of 1 mm ramps on the right and bottom of the images are too faint to be seen. Figure 2-4.
Nuclear Associates 76-908 & 76-907 Operators Manual Figure 2-6 shows a resolution hole image against a background less opaque than in previous images, indicating that the position of the resolution block is such that it is partially covered by the slice. For resolution measure, it is advised to use a slice that covers the central portion of the slice hole section.
Operation Tests with 3D Resolution and Slice (3DRAS) Phantom 2 Figures 2-7 and 2-8 show an example of the slice thickness measurement analysis. A signal intensity profile is drawn vertically through a set of slice thickness ramp Images. The profile is the slice thickness profile. One can also obtain the slice thickness by measuring full width half maximum of the profile. Figure 2-8 represents the same data using a larger zoom factor. Figure 2-7.
Nuclear Associates 76-908 & 76-907 Operators Manual 2.4 Tests With Uniformity and Linearity (UAL) Phantom 2.4.1 Image Uniformity Image uniformity refers to the ability of the MR imaging system to produce a constant signal response throughout the scanned volume when the object being imaged has homogeneous MR characteristics.
Operation Tests with Uniformity and Linearity (UAL) Phantom 2 (2) Non-linear magnetic field gradients Variability is best observed over the largest field-of-view. The phantom should occupy at least 60% of the largest field-of-view. Figure 1-1 provides an illustration of a pattern that is used to evaluate spatial linearity. Consideration should be given to determining the spatial linearity for a typical multi-slice acquisition with the largest available image matrix to maximize spatial resolution.
Nuclear Associates 76-908 & 76-907 Operators Manual DC-Offset Errors DC-offset errors typically appear as a single bright pixel (sometimes as a dark pixel if overflow or processing has occurred) at the center of the image matrix. The existence of this error is assessed visually. Receive Quadrature Errors Receive quadrature ghosts will be evaluated using the central slice of the multi-slice sequence acquired with the phantom at the isocenter.
Operation Tests with Uniformity and Linearity (UAL) Phantom 2 Figure 2-9 2-13
Nuclear Associates 76-908 & 76-907 Operators Manual Figure 2-10 is a flood image showing the uniformity of signal intensity. Field non-uniformity is also shown on the outer edges. Figure 2-10 Figure 2-11 shows a horizontal intensity profile with a narrow window setting. Such information can be used as baseline data.
Operation Tests with Uniformity and Linearity (UAL) Phantom 2 Figure 2-12 is an image of the signal bubble for quadrature error. If there were any quadrature phase error, another ghost circle would be visible in the lower right-hand corner of the image. Quadrature error has become less common in most of the recent MRI systems.
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