Abstract:
A method of measuring the  17  O isotope content and distribution in a body by magnetic resonance imaging (MRI) by collecting first and second nuclear magnetic resonance (NMR) signals of the body under the same conditions of examination except that before one examination, the body is first irradiated with radio waves at the  17  O resonance frequency, whereas before the other examination the body is not so irradiated. The difference in the results of the two examinations is measured to provide an NMR image of the  17  O isotope distribution in the body.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a method of measuring the  17  O isotope content of a body, and particularly to such a method using nuclear magnetic resonance (NMR) spectroscopy examination techniques. Possible applications of the novel method of the invention include measuring the rate of perfusion of a solution administered to a living body, and measuring oxidative metabolism activity in a living body. 
     Oxygen appears in nature in the form of three stable (i.e, non-radioactive) isotopes, one with predominant abundance  16  O (99.76%), and two rare isotopes  17  O (0.037%) and  18  O isotope (0.20%). The isotopes  16  O and  18  O do not possess magnetic moment and therefore they do not have an effect on Nuclear Magnetic Resonance (NMR) spectroscopy, but the isotope  17  O does have a magnetic moment and therefore can be detected by NMR. The  17  O isotope also has an effect on other magnetic nuclei associated with it. 
     There are several applications where the detection of minute amounts of  17  O by NMR is important. One is the measurement of the rate of perfusion by Magnetic Resonance Imaging (MRI): J. Pekar et al., Magn. Reson. Med. 21, 313 (1991); K. K. Kwong et. al., Magn. Reson. Med. 22, 154 (1991). This is accomplished by injecting physiological solution containing H 2   17  O and following the rate of distribution of the  17  O isotope in the body. Another application is metabolic functional imaging: see J. Pekar et al., supra; also G. D. Mateescu et al., in &#34;Synthesis and Applications of Isotopically Labelled Compounds&#34;; T. A. Baillie and J. R. Jones Eds. Elsevier, Amsterdam, P.499 (1989). In this technique, molecular oxygen (O 2 ) labeled with the  17  O isotope is inhaled by the patient. Since O 2  is a paramagnetic molecule, it cannot be observed by NMR. However, in metabolic active respiring tissues, it is converted to H 2  O by oxygen metabolism. 
     There have been several suggestions in the literature (e.g., see above publications) of observing the  17  O isotope directly by NMR. However, because of the low sensitivity of  17  O NMR, and the high cost of  17  O enriched compounds, this approach seems impractical for medical applications. 
     The present invention is directed to a novel magnetic resonance imaging (MRI) method based on indirect detection of the  17  O isotope using  1  H (proton) NMR. Conventional magnetic resonance imaging (MRI) is exclusively based on the detection of the echo signal produced by protons after being stimulated by a sequence of strong radio frequency (RF) pulses generated at the resonance frequency of the protons in a magnetic field. Since most of the hydrogen in living tissues is present in water molecules, the image produced by MRI reflects the distribution of water in the observed object. Because of the high natural abundance (99.985%) of  1  H and its high gyromagnetic ratio, leading to high NMR sensitivity, proton MRI is by far the most commonly used MRI method. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method of measuring the  17  O isotope content and distribution in a body by magnetic resonance imaging (MRI), comprising: collecting first and second NMR signals of the body under the same conditions of examination except that before one examination, the body had first been irradiated with radio waves at the  17  O resonance frequency, whereas before the other examination the body had not been so irradiated; and measuring the difference in the results of the two examinations to provide a nuclear magnetic resonance (NMR) image of the  17  O isotope distribution in the body. 
     According to further features in the preferred embodiment described below, the result of the difference between the two examinations is normalized by the sum of both examinations. 
     We have found that the increase in the transverse relaxation time (T 2 ) following the  17  O decoupling is directly related to the  17  O content of the sample. This finding explains the difference between the proton echo signal with irradiation of the  17  O resonance and without it, and enables the optimization of the echo time in order to maximize the difference between the echo signals, while preserving a high signal-to-noise ratio. 
     Possible applications of the novel method include measuring, by well known MRI techniques, the rate of perfusion of solutions containing water enriched with  17  O in the different parts of a living body, and measuring oxidative metabolism activity in a living body. 
     Further features and advantages of the invention will be apparent from the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIGS. 1 and 2 illustrate a setup used for measuring the  17  O isotope content of water in a series of glass bulbs in accordance with the method of the present invention; 
     FIGS. 3 and 4 illustrate the results produced during one example of practicing the method according to the invention as described below; 
     FIG. 5 illustrates one manner of applying the novel method for the NMR imaging of the body; and 
     FIG. 6 illustrates the results produced during a second example of praticing the method according to the invention as described below. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will be better understood by the following description of two examples of practicing the method in accordance with the present invention: 
     EXAMPLE 1 
     A one dimensional image of a sample was performed containing various concentrations of  17  O enriched water. The sample was a 5 mm NMR tube, filled with nine cylinders of about 4 mm height and 4 mm diameter. Each cylinder contained different amounts of  17  O enriched water, as shown in FIG. 1. The resonance frequency of the protons was 360 MHZ, and that of the  17  O was 48.6 MHz. The Z shim coil was used for the production of a gradient for one dimensional imaging (about 0.1 G·cm -1 ). The irradiation power at the  17  O frequency was about 30 watts, and may be applied either as continuous wave (CW) irradiation or according to a composite pulse coupling scheme, GARP: A. J. Shaka et al., J. Magn. Reson, 64, 547 (1985). The latter was used in the example shown. The pulse sequence of the experiment was a Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence: H. Y. Carr and E. M. Purcell, Phys. Rev., 94, 630 (1954); S. Meiboom and D. Gill, Rev. Sci. Instrum., 29, 688 (1958) performed on the proton channel, in which the decoupling scheme on the  17  O channel took place between the first pulse (90°) and the beginning of the acquisition, as described in FIG. 2. The CPMG sequence was carried out with eight 180° pulses with a total echo time of 320 msec. 
     FIG. 3 shows the two images. One image was obtained without irradiating at the  17  O resonance frequency (solid line), and the other image was obtained under the same conditions except with the irradiation (dotted line). 
     FIG. 4 shows the image obtained by subtraction of the image without irradiation of the  17  O from the one with irradiation. The resulting image was normalized by the sum of the two images in order to compensate for the non-linearity of the Z-gradient, and the different size of the proton signals resulting from r.f field inhomogeneity and uneven sensitivity along the sample. It is seen that the intensity of the image is roughly proportional to the  17  O content, except for the very high concentration of 4.6%, where the decoupling power was not enough to fully decouple the broadening due to the  17  O scalar coupling. However, in the practical implementation of the invention, only the low concentations of  17  O are of interest. 
     Similar results were obtained using an aqueous solution of 7% human serum albumin (HSA) instead of water. This solution had protein concentration similar to that of serum. 
     One way to apply the technique for NMR imaging (MRI) of the body is by using the modification of the spin-echo spin-warp technique as described in: W. A. Edelstein et al., Phys. Med. Biol., 25, 751 (1980), and as illustrated in FIG. 5. The above reference describes a pulse sequence to obtain an NMR signal representing the distribution of proton density in a body. FIG. 5 sets out a similar sequence but with the inclusion of an additional rf signal labelled 17-O RF. A first NMR signal is obtained by using the pulse sequence of FIG. 5, but without the inclusion of the signal 17-O RF. A second NMR signal is obtained by using the pulse sequence as shown in FIG. 5. The difference between the two NMR signals is taken to provide an NMR image of the  17  O isotope distribution in the body. 
     EXAMPLE 2 
     The second example is a demonstration of the potential application of the method as a tool for metabolic functional imaging. In this example, two live newborn mice were introduced into the NMR probe and were let to breath air mixed with 28%  17  O 2 , enriched oxygen. Consecutive direct  17  O measurements and indirect proton measurements using the method described above were taken on the same sample every 10-15 minutes.  17  O spectra were taken accumulating 128 scans. Proton spectra were taken using the CPMG spin echo sequence with TE=600 ms for the injection experiment and TE=1s for the respiration experiment. γ (CPMG) was 10 ms. Only one scan was collected for each proton spectrum: one with irradiation at the  17  O resonance frequency and one without it. Proton T 2  was 50 ms. 
     FIG. 6 depicts the correlation between the direct  17  O measurements of H 2   17  O and the indirect measurements reflected by the increase in proton echo signal following an irradiation of the  17  O resonance. The graph shows that at low H 2   17  O concentrations, a fairly linear correlation exists between the H 2   17  O content as reflected by the direct  17  O measurements and the indirect measurements. The linearity is lost at about H 2   17  O concentration of 0.3%. 
     The invention thus provides a new method of indirect MRI of  17  O, which is particularly useful for functional imaging of brain oxidative metabolic activity. This method is advantageous for several reasons: (a) it is highly sensitive even to minute amounts of H 2   17  O (down to 0.1%) and hence may reduce drastically the price of functional imaging experiments; (b) it reflects the amount of H 2   17  O in the tissue; (c) since  17  O resonance frequency in MRI instruments is relatively low, power deposition is relatively low; and (d) implementation of the pulse sequence to MRI is fairly straightforward, using interleaved sequences with and without  17  O decoupling, hence minimizing the error due to all time dependent effects. 
     While the invention is described with respect to two examples, it will be appreciated that many variations, modifications and other applications of the invention may be made.