1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA MS 5-2010Determination of SliceThickness in DiagnosticMagnetic ResonanceImagingNEMA Standards Publication MS 5-2010 Determination of Slice Thickness in Diagnostic Magnetic Resonance Imaging Published by: National Electrical
2、Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, VA 22209 www.nema.org Copyright 2010 by the National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Pro
3、tection of Literary and Artistic Works, and the International and Pan American Copyright Conventions. NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was deve
4、loped. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which the document contained herein is one, are develop
5、ed through a voluntary consensus standards development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fairness in the development
6、 of consensus, it does not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, o
7、r other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, expressed or implied, as to the accuracy or
8、 completeness of any information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by v
9、irtue of this standard or guide. In publishing and making this document available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone else. Anyone using this do
10、cument should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other so
11、urces, which the user may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs, or installations for
12、safety or health purposes. Any certification or other statement of compliance with any health or safetyrelated information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. MS 5-2010 Page i CONTENTS Page Preamble ii Forewo
13、rdiii Rationaleiv Scope.iv Section 1 DEFINITIONS 1.1 Baseline Pixel Offset Value . 1 1.2 Contiguous Slices 1 1.3 Edge Response Function (ERF) 1 1.4 Intrinsic Pixel Size . 1 1.5 Slice Coordinate 1 1.6 Slice Profile 1 1.7 Slice Thickness 1 1.8 Specification Volume . 1 Section 2 METHODS OF MEASUREMENT
14、2.1 Scan Conditions 2 2.2 Wedge Method of Measurement . 2 2.2.1 Phantom Description 2 2.2.2 Measurement Procedure 3 2.3 Slab Method of Measurement . 5 2.3.1 Phantom Description 5 2.3.2 Measurement Procedure 6 Section 3 IMPROVING THE ACCURACY OF RESULTS 8 Section 4 CORRECTING FOR ROTATIONAL ERRORS. 9
15、 Section 5 SOURCES OF ERROR AND REPORTING OF RESULTS 5.1 Sources of Error 11 5.2 Reporting of Results 11 Annex A CHANGES TO STANDARD . 12 Figures 2-1 2 2-2 4 2-3 5 2-4 6 2-5 7 4-1 9 Copyright 2010 by the National Electrical Manufacturers Association. MS 5-2010 Page ii Preamble This is one of a serie
16、s of test standards developed by the medical diagnostic imaging industry for the measurement of performance parameters governing image quality of magnetic resonance (MR) imaging (MRI) systems. These test standards are intended for the use of equipment manufacturers, prospective purchasers, and users
17、 alike. Manufacturers are permitted to use these standards for the determination of system performance specifications. This standardization of performance specifications is of benefit to the prospective equipment purchaser, and the parameters supplied with each NEMA measurement serve as a guide to t
18、hose factors that can influence the measurement. These standards can also serve as reference procedures for acceptance testing and periodic quality assurance. It must be recognized, however, that not all test standards lend themselves to measurement at the installation site. Some test standards requ
19、ire instrumentation better suited to factory measurements, while others require the facilities of an instrumentation laboratory to ensure stable test conditions necessary for reliable measurements. The NEMA test procedures are carried out using the normal clinical operating mode of the system. For e
20、xample, standard calibration procedures, standard clinical sequences, and standard reconstruction processes shall be used. No modifications to alter test results shall be used unless otherwise specified in these standards. The NEMA Magnetic Resonance Section has identified a set of key magnetic reso
21、nance image quality parameters. This standards publication describes the measurement of one of these parameters. Equivalence It is intended and expected that manufacturers or others who claim compliance with these NEMA standard test procedures for the determination of image quality parameters shall
22、have carried out the tests in accordance with the procedures specified in the published standards. In those cases where it is impossible or impractical to follow the literal prescription of a NEMA test procedure, a complete description of any deviation from the published procedure must be included w
23、ith any measurement claimed equivalent to the NEMA standard. The validity or equivalence of the modified procedure will be determined by the reader. Uncertainty of the Measurements The measurement uncertainty of the image quality parameter determined using this standards publication is to be reporte
24、d, together with the value of the parameter. Justification for the claimed uncertainty limits shall also be provided by a listing and discussion of sources and magnitudes of error. Copyright 2010 by the National Electrical Manufacturers Association. MS 5-2010 Page iii Foreword This standards publica
25、tion is classified as a NEMA standard unless otherwise noted. It describes two methods for determining slice thickness in diagnostic magnetic resonance imaging. The methods presented are essentially numerical in character and, consequently, will require the preparation and use of supplementary dedic
26、ated computer software to perform the computations. The methods are based upon the determination of the slice profile, from which the slice thickness is obtained as the full width at half maximum (FWHM). The slice profile is obtained either by direct measurement with a thin inclined slab of signal-p
27、roducing material, or by numerical differentiation of the measured edge response function (ERF) from an inclined surface of a wedge immersed in signal- producing material. A correction technique is provided to compensate for errors caused by tilt of the phantom. With the inclined slab approach, bett
28、er signal-to-noise ratio (SNR) can be realized through the use of direct measurement1. However, the extremely thin slabs required for measurement of very thin slices are not practical to fabricate. Differentiation of the ERF degrades the SNR that is obtained for the slice profile and usually require
29、s the averaging of several measurements, but does permit measurement of thinner slices, since fabrication is not limiting. Slices of any thickness, which can provide adequate signal, may be evaluated with the wedge procedure; the slab method is suitable for thicker slices. This standards publication
30、 is intended for use by MRI system manufacturers, manufacturers of accessory equipment (including special purpose radio-frequency coils), and MRI end users. This standards publication has been developed by the Magnetic Resonance Section of the National Electrical Manufacturers Association. User need
31、s have been considered throughout the development of this publication. Proposed or recommended revisions should be submitted to: Executive Director, Medical Imaging 2. TR 3 T1in the signal-producing material; 3. Pulse sequence as used for clinical scans; 4. Slice thickness: both the typical and the
32、typically thinnest in routine clinical use; 5. Multi-slice mode, with at least three slices for which the consecutive profile centers are separated by twice the nominal FWHM (not contiguous). 2.2 WEDGE METHOD OF MEASUREMENT 2.2.1 Phantom Description The phantom shall consist of two opposing wedges o
33、f a material that emits no proton signal, immersed in a reservoir of signal-producing material (see Figure 2-1). These wedges shall be used to measure slice profile while providing correction for errors of tilt (rotation about the y axis). The correction procedure is described in Section 4. The incl
34、ined surfaces of both wedges shall form an angle with the slice plane (see Figure 2-1B). A B Figure 2-1 CROSSED WEDGE PHANTOM Copyright 2010 by the National Electrical Manufacturers Association. MS 5-2010 Page 3 2.2.2 Measurement Procedure 1. Select the acquisition and display parameters so that the
35、 resulting spatial resolution in the x direction (see Figure 2-3) is sufficient to include at least six “intrinsic” pixels across the FWHM of the stretched (projected) slice profile. For the purposes of this standard, the “intrinsic” pixel size is defined as the image domain distance d (in meters) s
36、uch that 1/d (in meters-1) is the span of the measured data in the Fourier domain. An example is shown in the Authorized Engineering Information below. This condition is met when: tan5FWHMd Equation 1 where FWHM refers to the correctly scaled (not stretched) slice profile. For a standard two-dimensi
37、onal Fourier transform imaging technique, the “intrinsic” pixel size in the readout direction is given by: GdRO=1Equation 2 where: dROis the “intrinsic” pixel size in meters, is the duration of the sampling in milliseconds (i.e., the difference in time between the first sample and the last sample),
38、is the magnetogyric ratio in MHz/tesla, G is the readout gradient strength in mT/m. In the phase encoding direction, the “intrinsic” pixel size is given by: =GdtdPE21Equation 3 where: dPEis the “intrinsic” pixel size in meters, is the magnetogyric ratio in MHz/tesla, G is the gradient strength in mT
39、/m of the largest phase encoding, The integral is performed over the duration of the gradient pulse and has the units (mT*msec)/m. Authorized Engineering Information 8-28-1991. This example is valid for standard 2D Fourier transform imaging in which the spin echoes are sampled symmetrically, the dat
40、a are reconstructed with a 2D Fourier transform, and symmetric positive and negative phase encoding steps are used. This analysis assumes that the data have not been filtered. Authorized Engineering Information 8-28-1991. 2. Generate a magnetic resonance image from one of the three orthogonal acquis
41、ition planes (transverse, sagittal, or coronal). Only the center slice of the multi-slice acquisition shall be analyzed. Copyright 2010 by the National Electrical Manufacturers Association. MS 5-2010 Page 4 3. Define the direction y in the slice plane, as shown in Figure 2-2. Figure 2-2 DIAGRAM OF R
42、EFERENCED PLANES 4. Define the direction x in the slice plane, which is perpendicular to y. Note that x forms an angle with the inclined surface of the wedge (see Figure 2-2). 5. Plot the image pixel intensities in the x direction to obtain the ERF. 6. Differentiate the ERF with respect to x to obta
43、in a stretched (projected) slice profile D(x) (see Figure 2-3) as shown below: ()( ) ( )11=iiiiixxxIxIxD Equation 4 where I(xi) is the ERF value at each position xi. Copyright 2010 by the National Electrical Manufacturers Association. MS 5-2010 Page 5 Figure 2-3 SCHEMATIC OVERVIEW OF THE WEDGE METHO
44、D 7. Find the maximum of the projected slice profile D(x). 8. Find the width of the projected slice profile at one half of the maximum, using linear interpolation if necessary (see Figure 2-3). 9. Multiply the resulting width of the projected slice profile found in step 8 by tan () to obtain the pro
45、perly scaled slice thickness (FWHM). 10. Correct the results for tilt (rotation about the y axis) as detailed in Section 4, before reporting results in Section 5.2. 2.3 SLAB METHOD OF MEASUREMENT 2.3.1 Phantom Description The slab phantom shall consist of two thin opposing inclined slabs of signal-p
46、roducing material enclosed by inert material (see Figure 2-4). The inclined surfaces of both slabs shall form an angle with the slice plane. The slab thickness shall be less than one-fifth of the selected slice thickness times cos . As an example, for = 30 degrees and a selected slice thickness of 1
47、0 mm, the slab thickness shall be less than 1.73 mm. These thin slabs shall be used to measure slice profile while providing correction for errors of tilt (rotation about the y axis). The correction procedure is described in Section 4. A liquid or other signal-producing material fills the gaps betwe
48、en the phantom inserts to form the crossed slabs. The dashed lines indicate the slice plane. Copyright 2010 by the National Electrical Manufacturers Association. MS 5-2010 Page 6 Figure 2-4 CROSSED SLAB PHANTOM 2.3.2 Measurement Procedure 1. Select the acquisition and display parameters so that the
49、resulting spatial resolution in the x direction (see Figure 2-5) is sufficient to include at least six “intrinsic” pixels across the FWHM of the stretched (projected) slice profile, as described for the wedge method (see Section 2.2.2). 2. Generate a magnetic resonance image from one of the three orthogonal acquisition planes (transverse, sagittal, or coronal). Only the center slice of the multi-slice acquisition shall be analyzed. 3. Define the direction y in the slice plane, as shown in Figure 2-4. 4. Define the direction x in the slice pl
