1195 Nuclear Magnetic Resonance Imaging: Contrast-to-Noise Ratio as a Function of Strength of Magnetic Field Howard R. Hart, Jr.1 Paul A. Bottomley William A. Edelstein Steven G. Karr William M. Leue Otward Mueller Rowland W. Redington John F. Schenck L. Scott Smith Dimitrios Vatis Received July 28, 1 983; accepted after revi- sion August 29, 1983. Presented in part at the joint meeting of the American Roentgen Ray Society and the Society of Photo-Optical Instrumentation Engineers, At- lanta, April 1983. All authors: General Electric Research and Development Center, P.O. Box 8, Schenectady, NV 1 2301 . Address reprint requests to H. A. Hart, Jr. AJR 141:1195-1201, December 1983 0361-803X/83/1416-1 195 © American Roentgen Ray Society The choice of the strength of the magnetic field for an imaging system based on the nuclear magnetic resonance of hydrogen is considered. It is shown by an analysis based on in vitro data that the quality, or contrast-to-noise ratio, of images based on T1 or T2 discrimination increases with field up to 1 .5-2 T. After a brief discussion of potential high-field limitations, results are presented which show that Images of the human head with excellent anatomic detail can be produced at 1 .5 T or 64 MHz. The potential user of an imaging system based on nuclear magnetic resonance (NMR) must choose the strength of the main magnetic field on which the system is based. We shall consider the factors that influence the choice of the main magnetic field, emphasizing the tissue contrast-to-noise ratio. Images of humans have now been obtained at fields ranging from 0.04 T (400 G) [1 ] to 1 .5 T (i 5,000 G) [2], corresponding to frequencies ranging from 1 .7 to 64 MHz. Most of the images published have been made in the field range 0.04- 0.35 T. The quality of the images in this field range has generally been found to increase with the strength of the field [3-6]. Our question is, does this trend continue to higher fields? Although the signal-to-noise ratio is expected to increase with field [2, 7], the inherent contrast between tissues in some cases decreases with increasing field [8]. Since the engineering complexity and cost of the system increase with field, it is important to understand in some detail the dependence of image quality on magnetic field. One would like to know, for example, the magnetic field above which the quality of the image deteriorates, if such a field exists. We first will calculate the field-dependence of the tissue contrast-to-noise ratio using literature values of relaxation times measured in vitro and then will briefly consider some of the complexities involved in moving toward higher field and frequency. Finally, we will present some of our results, which show that excellent images can be obtained at fields as high as 1 .5 T. Calculation of Contrast-to-Noise Ratio The quality of an NMA image is determined by several factors: the inherent contrast between tissues in the NMR parameter being imaged (e.g., density, T1, or T2) [8-i 4], the relative strengths of the NMA signal and the noise, the volume of the resolution element or voxel, the efficiency of the scanning protocol or pulse sequence in use, and the total imaging time. We will introduce the field- dependence of each of these factors and combine the results to obtain a relation giving the image quality, or contrast-to-noise ratio, as a function of field. At this point it is useful to review briefly some of the basics of NMR; Abragam’s text [1 5] contains further details. The frequency of resonance in NMR is governed by the Larmor equation: f = (‘y/2i’) ‘ B (1)