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Investigative Ophthalmology & Visual Science, Vol. 30, No. 7, July 1989 Copyright © Association for Research in Vision and Ophthalmology Nuclear Magnetic Resonance Microscopic Ocular Imaging for the Detection of Early-Stage Cataract Chang B. Ann,* Janet A. Anderson,^ Sung C. Juh,* Inja Kim,t William H. Garner, f and Zang-Hee Cho*§ A nuclear magnetic resonance (NMR) microscopic ocular imaging was performed at 7.0 Tesla to investigate its usefulness in the detection of early-stage cataracts. For this study, galactose cataracts were generated in experimental rabbits through diet (35% galactose), and enucleated eyes were imaged at various times after initiation of the diet. In previous studies using a 0.6 Tesla conventional magnetic resonance imager (MRI), the contrast between normal and cataractous tissues in the lens was not well defined, mainly due to the partial volume effect coming from the limitation of resolution and signalto-noise ratio (SNR). With resolution of 60 X 60 X 80 nm, early localized precataractous tissue changes were clearly observed after 5 days diet. Precataractous tissue changes were seen histologically but no visible evidence of lens change was detected by the conventional slit lamp biomicroscope at this time. Substantially elongated spin-spin relaxation times (T2) in localized cataractous tissues (72.4 ± 8.8 msec) were consistently observed compared with those in normal lens region (16.1 ± 3.2 msec); however, the changes of the spin-lattice relaxation time (Ti) were not significant: Some ocular NMR microscopic images with corresponding histological photographs are demonstrated to show the potential of NMR microscopy. Invest Ophthalmol Vis Sci 30:1612-1617,1989 Magnetic resonance imaging (MRI) has begun to find clinical relevance in ophthalmology.1"8 The noninvasive characteristics plus inherent high resolution and various chemical contrasts made NMR imaging a unique diagnostic modality, and now NMR imaging begins to supercede existing diagnostic technologies, such as x-ray and computerized tomography (CT). Since the early 1980s, clinical and physiological applications of NMR imaging have been largely investigated.9"11 Although several reports have already shown the potential of NMR imaging in the study of eye,1"8 NMR imaging of the eye has been limited, mainly due to the resolution. Previously we reported a study of galactose cataract using a conventional whole-body MRI operated at 0.6 Tesla.12 With this system, however, the detection of early-stage localized cataractous change was limited by the large size of the picture element (pixel) compared to the size of precataractous tissue. Since the image intensity in each digitized pixel is the integrated spin signal within the unit volume, if the pixel size or resolution is not sufficiently fine, a pixel containing both cataractous and normal tissues appears ambiguously, thereby reducing contrast and detectability. The pixel size and the image contrast are also associated with the signal-to-noise ratio (SNR) of the employed system, which is mainly determined by the strength of the main magnetic field. The limitation of the conventional MRI in the reduction of echo time (TE) (about 25 msec) was another factor degrading the accuracy of the measured T2 values, especially in the lens, where T2 is pretty short. In the study reported here, most of these problems were solved by the introduction of a new high-field NMR imaging and microscopy.1314 Galactose cataract in the rabbit was chosen for the animal model. Due to the increased fraction of free water to bound water in the cataractous tissue, the spin-spin relaxation time (T2) increases exceedingly. Using this NMR characteristic of cataract, several imaging pulse schemes were investigated to achieve maximal contrast between normal and cataractous tissues. The application of NMR imaging to the detection of early-stage cataract can be useful for understanding the etiology of cataract as well as for the design of preventive measures. From the Departments of *Radiological Sciences and fOphthalmology, University of California, Irvine, Irvine, California. J Current address: Ophthalmology Research, Sharp Cabrillo Hospital, San Diego, California. § Also at Department of Electrical Science, Korea Advanced Institute of Science, Seoul, Korea. Presented in part at the 1988 meeting of the Association for Research in Vision and Ophthalmology, Sarasota, Florida. Supported in part by the National Eye Institute grant EY-07941-01. Submitted for publication: August 19, 1988; accepted January 9, 1989. Reprint requests: C. B. Ahn, PhD, Department of Radiological Sciences, University of California, Irvine, Irvine, CA 92717. 1612 Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933378/ on 05/08/2017 NMR MICROSCOPIC IMAGING FOR THE DETECTION OF CATARACT / Ahn er ol No. 7 1613 Materials and Methods Experimental System A general concept of NMR microscopy and a detailed description of the employed 7.0 Tesla NMR microscopy system can be found elsewhere.14 In this section, only a brief description specifically related to ocular imaging is given. A specially designed radiofrequency (r.f.) coil and gradient coil set were developed for eye imaging. The employed r.f. coil shown in Figure 1A is a single-turn solenoidal coil made of copper sheet (diameter = 1.5 cm, width = 2 cm), which provides maximal r.f. homogeneity as well as high sensitivity. A modified Golay gradient coil is employed, which can generate gradientfieldsof up to 40 gauss/cm when derived by the gradient amplifier made for the NMR microscopy system. The integrated imaging probe (right part) is shown in Figure 1B and C; it will be placed at the center of the vertical magnet with field strength of 7.0 Tesla (proton resonance frequency = 300 MHz). r.f. Coil for NMR A Ocular Imaging 7.O Tfcftln NMR Microscopic Iroaelng Probe Imaging Methods A conventional spin echo sequence was employed with repetition times (TR) of 1 sec, 2 sec, and 4 sec. The used echo times (TE) were 12 msec, 40 msec, and 80 msec. By considering physical eye size (~ 1.5 cm), image matrix (256 X 256), and measurement time (10-30 min), the pixel size was chosen as 60 X 60 X 80 urn. Among experiments with various TRs and TEs, three imaging methods appeared to be most promising, namely: (1) short-TE and long-TR sequence resulting in image intensity close to proton density; (2) long-TE and long-TR sequence generating T2-weighted image, and (3) calculated T2 map from the above two sequences with same TRs but with different TEs. These imaging methods will be used mainly in further discussion of the experimental results. Animal Models Galactose cataract in the rabbit was chosen for the animal model. Galactose cataracts are formed by osmotic changes in the lens fibers.15 Galactose is converted to dulbicol in the lens fibers. The presence of alcohol in the lens fibers creates hypertonicity that is corrected by an influx of water. The water influx is followed by electrolyte changes, membrane permeability breakdown and eventually a large influx of sodium because of the increased lens hydration. The increased relaxation time in the cataractous tissue results from the increased fraction of free water to bound water due to the loss of water binding sites on the aggregated proteins.8 Fig. 1. NMR microscopy system for ocular imaging. (A) Singleturn solenoidal r.f. coil (diameter = 1.5 cm, width = 2 cm). (B) Integrated imaging probe (right part). (O Top view of the imaging probe. The central object is the r.f. coil, which is surrounded by the gradient coil. In this high resolution NMR imaging and microscopy study of cataract, young (4-week) albino New Zealand rabbits were fed on a diet consisting of 35% galactose to induce cataractogenesis. Control rabbits of the same age were also maintained in the same facility and were fed a normal diet. NMR microscopic imagings of both control and cataractous animals were performed from 1 day to 4 weeks following Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933378/ on 05/08/2017 1614 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / July 1989 Vol. 30 Control / Rabbit Eye Control / Rabbit Eye Proton Density Image ( 2000/12 msec ) T 2 Weighted Image ( 2000/40 msec T2 Map of Rabbit Eye / Control D Fig. 2. NMR microscopic images of the control rabbit eye. (A) TR/TE = 2000/12 msec. (B) TR/TE = 2000/40 msec. (C) T2 map. (D) Histological image of the equatorial section of the lens. the initiation of the diet. The development of lens opacities was also examined by a slit-lamp biomicroscope before the NMR scanning. Each rabbit was sacrificed by intravenous injection of a T-61 euthanasia solution (Hoechst-Roussel, Somerville, NJ) and eyes were immediately enucleated. One fresh enucleated eye was placed in a specially designed eye holder to fit in the r.f. coil of the microscopy system and scanned within one-half hour of the enucleation. The other eye was embedded in paraffin and stained with hemotoxilin/eosin, and then sent to a histology laboratory to examine microscopic structural changes. All investigations described in this paper were carried out in accordance with the ARVO Resolution on the Use of Animals in Research. Results Ten rabbits were placed on the galactose diet and examined 1, 2, 3, 4, 5, 7, 10, 14, 21 and 28 days from the initiation of the diet. Two control rabbits were also examined. Figure 2 shows the NMR microscopic images of the control rabbit eye with (A) TR/TE = 2000/12 msec and (B) TR/TE = 2000/40 msec. The calculated T2 map from Figures 2A and B is shown in C, and the corresponding histological photograph is shown in D. In the T2 map, the displayed range of T2 is from 0 (black) to 128 msec (white). For example, the T2 values in the vitreous or anterior chamber are greater than 128 msec (appearing white), while the average T2 value in cornea is 19.6 msec (appearing dark gray). A better differentiation between lens cortex and nucleus is made in the T2 map (Fig. 2C). The T2 values in lens cortex were reported to be longer than T2 in the lens nucleus,516 which is also consistent with our measurement in the T2 map. The experimentally obtained images after 5 days' diet are shown in Figure 3, (A) TR/TE = 2000/12 msec, (B) TR/TE = 2000/40 msec, (C) T2 map, and (D) histological image. Note the appearance of cataract at the equatorial region of the lens in NMR microscopic image (marked with a black arrow in Fig. Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933378/ on 05/08/2017 No. 7 NMR MICROSCOPIC IMAGING FOR THE DETECTION OF CATARACT / Ahn er ol 1615 Early Detection of Cataract 5 Days Diet / Rabbit Eye 5 Days Diet / Rabbit Eye Proton Density Image ( 2000/12 msec ) T2 Weighted Image ( 2000/40 msec ) T2 Map of Rabbit Eye / 5 Days Diet D Fig. 3. NMR microscopic image of the rabbit eye after 5 days1 diet. (A) TR/TE = 2000/12 msec. <B) TR/TE = 2000/40 msec. Early-stage cataract can be observed at the equatorial region. (C) Tj map. (D) Histological photograph of the lens at the equatorial section. 3B), which had not yet been detected by conventional slit-lamp biomicroscope. Due to the increase of T 2 , the cataractous region appears bright in the T2weighted image, while the normal lens region appears dark due to the fast decay of the signal with short T2 values. The increased T2 values are also demonstrated in the T2 map in Figure 3C. Other experimentally obtained eye images are shown in Figure 4 (after 2 weeks' diet). At this stage, the lens opacity could be seen by both conventional slit-lamp biomicroscope as well as by NMR microscope. With extremely long echo time (80 msec), the signal from the cataractous tissues only remained as shown in Figure 4C. Also, fully formed vacuoles are seen in the histological photograph of Figure 4E. The average T2 value of the cataractous tissues in the lens was 72.4 msec with a standard deviation of 8.8 msec, while the average T2 in normal lens (over both nucleus and cortex regions) was 16.1 msec with a standard deviation of 3.2 msec. A summary of examinations of the 12 rabbits by the NMR microscope, conventional slit-lamp biomicroscope and histology is given in Table 1. Discussion With the introduction of high-field NMR microscopy, it was possible to detect early-stage cataract (5 days after diet) before any other evidence could be observed by slit-lamp biomicroscopy. It has been noted that lens clarity as examined by slit-lamp biomicroscope is not a reliable index in detecting earlystage cataract. The importance of detecting earlystage cataract lies in the fact that the progression of galactose cataract can be reversed by early treatments.17 The role of high-resolution NMR imaging in the detection of cataract is evident. For example, in conventional MRI, imaging resolution is about 1 X 1 X 4 mm with a magnetic field of about 0.5-2.0 Tesla, which is much larger than the size of cataractous tissue (see NMR images of cataract lens in Figs. 3B and 4C). In our experiment, however, it was possible to get a microscopic resolution (60 X 60 X 80 fim) due to the SNR improvements by employing a high magnetic field (7.0 Tesla) and a small r.f. coil (diameter = 1.5 cm). Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933378/ on 05/08/2017 1616 INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / July 1989 2 Weeks Diet / Rabbit Eye Proton Density Image ( 2000/12 msec Vol. 30 2 Weeks Diet / Rabbit Eye T 2 Weighted Image ( 2000/40 msec ) 2 Weeks Diet / Rabbit Eye Heavily T 2 Weighted Image ( 4000/80 msec ) T2 Map of Rabbit Eye / 2 Weeks Diet Fig. 4. NMR microscopic images of the rabbit eye after 2 weeks' diet. (A) TR/TE = 2000/12 msec. (B) TR/TE = 2000/40 msec. (C) TR/TE = 4000/80 msec. Only cataractous regions are seen in the heavily T;-weighted image. (D) T2 map. (E) Htstological photograph of the equatorial section of the lens. From a series of proton density images (Figs. 2A, 3A and 4A), some changes of hydration densities between images are observable; however, the difference between normal and cataractous tissues in each image is not clear. In contrast to the proton density images, the T2-weighted images (Figs. 2B, 3B, 4B and C) show high contrast between normal and cataractous tissues. Thus, the T2-weighted imaging seems the most promising single-acquisition technique in the detection of early-stage cataract. Some contrast enhancement and, more importantly, a quantitative tissue characterization can be achieved if one calculates the T2 map. However, this technique requires at least two acquisition steps with different echo times. Throughout these experiments, no distinct contrast was observed by T[ relaxation, which might be re- Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933378/ on 05/08/2017 No. 7 NMR MICROSCOPIC IMAGING FOR THE DETECTION OF CATARACT / Ahn er ol Table 1. A summary of the detectability of rabbit cataracts by NMR microscopy, slit-lamp biomicroscopy and histology Cataract detectability NMR Rabbit status Control (2) 1 days' diet 2 days' diet 3 days' diet 4 days' diet 5 days' diet 7 days' diet 10 days'diet 14 days' diet 21 days' diet 28 days' diet microscopy Slit lamp Histology N N Q Q N N N N N Q D D D D N Q (small vesicles) Q (small vesicles) D (vacuole) D (vacuole) D (vacuole) D (large vacuole) D (large vacuole) D (large vacuole) D (large vacuole) D D D D D D D—detectable, Q—questionable, and N—nondetectable. lated to the minor changes of hydration density in galactose cataract, as noted in the above proton density images. Similar observations were reported recently.8 Histological examination of the lenses revealed the presence of small vesicles in the equatorial region as early as 2 days after initiation of the galactose diet. These vesicles appeared to coalesce into larger vacuoles, still predominantly in the equatorial region by day 10. By this time (day 10), equatorial vacuoles were seen by slit-lamp biomicroscope. These histological observations agreed well with the images obtained by the NMR microscope. The current work can be applicable in both theoretical and experimental studies of cataractogenesis, cataract pharmacology and cataract animal model. In addition to the high SNR and resolution provided by the use of a high magnetic field, NMR microscopy has an inherent high spectral resolution that may be used for further studies in chemical spectroscopic imaging and localized spectroscopy for the early detection of metabolic changes caused by cataract. Key words: NMR microscopy, NMR imaging, galactose cataract, resolution Acknowledgments The authors thank Karen Mundweiler, Allergan Inc., for the histological preparations, and Arlene Gwon, MD, Allergan Inc., for assistance in the interpretation of the histological preparations. 1617 References 1. Atlas SW, Bilaniuk LT, Zimmerman RA, Hackney DB, Goldberg HI, and Grossman RI: Orbit: Initial experience with surface coil spin-echo MR imaging at 1.5 T. Radiology 164:501, 1987. 2. Mafee MF, Putterman A, Valvassori GE, Capek V, and Campos M: Orbital space-occupying lesions: Role of magnetic resonance imaging and computerized tomography: A review of 145 cases. In The Radiologic Clinics of North America: Imaging in Ophthalmology, Part I, Mafee MF, guest editor. Philadelphia, W. B. Saunders Company, 1987, pp. 529-560. 3. De Keizer RJW, Vielvoye GJ, and De Wolff-Rouendaal D: Nuclear magnetic resonance imaging of intraocular tumors. Am J Ophthalmol 102:438, 1986. 4. Aguayo J, Glaser B, Mildvan A, Cheng H-M, Gonzalez RG, and Brady T: Study of vitreous liquifaction by NMR spectroscopy and imaging. Invest Ophthalmol Vis Sci 26:692, 1985. 5. Neville MC, Paterson CA, Rae JL, and Woessner DE: Nuclear magnetic resonance studies and water "ordering" in the crystalline lens. Science 184:1072, 1974. 6. Racz P, Tompa K, and Pocsik I: The state of water in normal and senile cataractous lenses studied by nuclear magnetic resonance. Exp Eye Res 28:129, 1979. 7. Greiner JV, Kopp SJ, Sanders DR, and Glonek T: Organophosphates of the crystalline lens: A nuclear magnetic resonance spectroscopic study. Invest Ophthalmol Vis Sci 21:700, 1981. 8. Cheng H-M, Yeh LI, Barnett P, Miglior S, Eagon JC, Gonzalez G, and Brady TJ: Proton magnetic resonance imaging of the ocular lens. Exp Eye Res 45:875, 1987. 9. Cho ZH, Oh CH, Kim YS, Mun CW, Nalcioglu O, Lee SJ, and Chung MK: A new nuclear magnetic resonance imaging technique for unambiguous unidirectional measurement of flow velocity. Appl Phys 60:1256, 1986. 10. Ahn CB, Lee SY, Nalcioglu O, and Cho ZH: An improved nuclear magnetic resonance diffusion coefficient imaging method using an optimized pulse sequence. Med Phys 13:789, 1986. 11. Ahn CB, Lee SY, Nalcioglu O, and Cho ZH: The effects of random directional distributed flow in nuclear magnetic resonance imaging. Med Phys 14:43, 1987. 12. Wong EK, Cho ZH, Gardner BP, Ahn CB, Kim I, Jo JM, Juh SC, and Anderson JA: In vivo magnetic resonance imaging studies of galactose cataracts. ARVO Abstracts. Invest Ophthalmol Vis Sci 28(Suppl):80, 1987. 13. Cho ZH, Ahn CB, Juh SC, Anderson JA, Kim I, Wong EK, and Garner WH: High field NMR microscopic imaging of the early stages of galactose cataract formation. ARVO Abstracts. Invest Ophthalmol Vis Sci 29(Suppl):3l8, 1988. 14. Cho ZH, Ahn CB, Juh SC, Lee HK, Jacobs RE, Lee S, Yi JH, and Jo JM: NMR microscopy with 4 ^m resolution: Theoretical study and experimental results. Med Phys 15:815, 1988. 15. Kinoshita JH: Mechanisms initiating cataract formation. Proctor Lecture. Invest Ophthalmol 13:713, 1974. 16. Gomori JM, Grossman RI, Shields JA, Augsburger JJ, Joseph PM, and DeSimeone D: Ocular MR imaging and spectroscopy: An ex vivo study. Radiology 160:201, 1986. 17. Unakar NJ, Genyea C, Reddan JR, and Reddy VN: Ultrastructural changes during the development and reversal of galactose cataracts. Exp Eye Res 26:123, 1978. Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933378/ on 05/08/2017