Measurement of recti eye muscle paths by magnetic resonance imaging in highly myopic and normal subjects.

PURPOSE: To analyze the path of extraocular muscles (EOMs) quantitatively in highly myopic subjects with and without restricted eye motility versus control. To elucidate the cause of the acquired motility disorder in patients with high myopia. METHODS: Thirty-three orbits were imaged using a Magnetom or Siemens Vision (Siemens, Erlangen, Germany; both 1.5 Tesla) MRI (magnetic resonance imaging) scanner. Coronal T1-weighted, spin-echo images were obtained with repetition time of 550 msec and echo time of 15 msec. Subjects had to fixate in different positions of gaze. Orbits of three patient groups were analyzed: group 1 (n = 14), patients with high axial myopia and restricted eye motility (average axial length, 31.4 mm; refractive error more than -15 D); group 2 (n = 8), subjects with high axial myopia and normal eye motility (average axial length, 29.2 mm); control group (n = 11), emmetropic subjects with normal eye motility (average axial length, 23.6 mm). RESULTS: Highly myopic patients showed significant displacements of recti EOMs in comparison with control subjects. Mean displacements, measured in the plane 3 mm anterior to the globeoptic nerve junction in primary gaze, were in group 1, lateral rectus (LR) 2.9 mm (2.5 downward, 1.4 medial), medial rectus (MR) 1.3 mm downward and in group 2, LR 1.4 mm (1.3 downward, 0.6 medial) and MR 1.2 mm downward. In groups 1 and 2 the inferior rectus (IR) was displaced 1.3 mm medially and upward. In both groups of myopic patients the superior rectus (SR) was displaced 1.5 mm medially and downward. CONCLUSIONS: In patients with high axial myopia, displacements of all recti EOMs can be detected by MRI. Displacements of SR, MR, and IR were very similar in groups 1 and 2 versus control. LR displacement into the lateral and inferior quadrant of the orbit was greatest in patients with restricted eye motility. Thus, LR displacement is probably the major pathophysiological factor for the restrictive motility disorder in high myopia. EOM dislocations can be explained by myopia-associated alterations in the orbital connective tissues confining EOM positions in relation to the orbital wall.     

Pilocarpine's effects on the blood-aqueous barrier of the human eye as assessed by high-resolution, contrast magnetic resonance imaging

The purpose of this research was to reassess the effects of topical pilocarpine on the integrity of the blood-aqueous barrier, using high resolution, magnetic resonance imaging, and the standard intravenous contrast agent gadolinium dimeglumine. It has long been known that topical pilocarpine gives rise to an increase in protein levels in the anterior chamber of the eye. This protein scatters light and is referred to clinically as 'flare'. Prior studies concluded that pilocarpine-induced flare resulted from disruption of the blood-aqueous barrier. These studies relied upon indirect methods that precluded direct visualization of the posterior chamber of the eye. Five normal, human volunteers (age 22-40) received a single drop of 3% pilocarpine in one eye, following baseline measurements of pupil size and anterior chamber 'flare'. These measurements were repeated every 15 min for 45 min. The subject was then positioned in the magnet and the eye that received pilocarpine was taped closed and covered with a 3 in.-diameter receive-only surface coil. The open contralateral eye focused on a target to maintain fixation of the imaged eye. A baseline image of the eye was obtained and the contrast agent was administered intravenously. A series of additional images was obtained during the following 60 min to track the movement of contrast material from the bloodstream into the tissues and compartments of the eye. Percent enhancement was calculated from selected regions of interest in the images, including the ciliary body, and the anterior and posterior chambers. Within the 45 min after administration of pilocarpine, pupil size in mm decreased from (mean+/-s.d.) 5.7+/-1.5 to 2.5+/-0.5 (p=0.0106). During this period, average flare/s.d. (photons msec-1) increased from 3.7+/-1.1 to 12.5+/-4.7 (p=0.0151). In all cases, MRI images showed rapid enhancement of the ciliary body, followed by a progressive increase in signal in the anterior chamber but not the posterior chamber. These studies confirm that topical pilocarpine gives rise to 'flare' in the anterior chamber. But the lack of enhancement in the posterior chamber strongly suggests that the presence of this added protein in the anterior chamber is not the result of increased permeability of the blood-aqueous barrier of the ciliary body. These studies also introduce the novel concept that not all clinically observed flare is the result of blood-aqueous barrier compromise.     

Three-dimensional modeling of the human eye based on magnetic resonance imaging

PURPOSE: A methodology for noninvasively characterizing the three-dimensional (3-D) shape of the complete human eye is not currently available for research into ocular diseases that have a structural substrate, such as myopia. A novel application of a magnetic resonance imaging (MRI) acquisition and analysis technique is presented that, for the first time, allows the 3-D shape of the eye to be investigated fully. METHODS: The technique involves the acquisition of a T2-weighted MRI, which is optimized to reveal the fluid-filled chambers of the eye. Automatic segmentation and meshing algorithms generate a 3-D surface model, which can be shaded with morphologic parameters such as distance from the posterior corneal pole and deviation from sphericity. Full details of the method are illustrated with data from 14 eyes of seven individuals. The spatial accuracy of the calculated models is demonstrated by comparing the MRI-derived axial lengths with values measured in the same eyes using interferometry. RESULTS: The color-coded eye models showed substantial variation in the absolute size of the 14 eyes. Variations in the sphericity of the eyes were also evident, with some appearing approximately spherical whereas others were clearly oblate and one was slightly prolate. Nasal-temporal asymmetries were noted in some subjects. CONCLUSIONS: The MRI acquisition and analysis technique allows a novel way of examining 3-D ocular shape. The ability to stratify and analyze eye shape, ocular volume, and sphericity will further extend the understanding of which specific biometric parameters predispose emmetropic children subsequently to develop myopia.     

Improved high-resolution ultrasonic imaging of the eye

Currently, virtually all clinical diagnostic ultrasound systems used in ophthalmology are based on fixed-focus, single-element transducers. High-frequency (> or = 20-MHz) transducers introduced to ophthalmology during the last decade have led to improved resolution and diagnostic capabilities for assessment of the anterior segment and the retina. However, single-element transducers are restricted to a small depth of field, limiting their capacity to image the eye as a whole. We fabricated a 20-MHz annular array probe prototype consisting of 5 concentric transducer elements and scanned an ex vivo human eye. Synthetically focused images of the bank eye showed improved depth of field and sensitivity, allowing simultaneous display of the anterior and posterior segments and the full lens contour. This capability may be useful in assessment of vitreoretinal pathologies and investigation of the accommodative mechanism.     

Functional magnetic resonance imaging of eye dominance at 4 tesla

We studied eye dominance in visual cortex and lateral geniculate nucleus (LGN) using functional magnetic resonance imaging (fMRI) at a very high magnetic field (4 tesla). Eight normal volunteers were studied with fMRI at 4 tesla during alternating monocular visual stimulation. The acquisition was repeated twice in 4 subjects to confirm reproducibility. In addition, magnetic resonance signal intensities during three conditions (right eye stimulation, left eye stimulation, and control condition) were compared to determine whether the observed area was truly or relatively monocular in 2 subjects. In both the individual and group analyses, the anterior striate cortex was consistently activated by the contralateral eye more than the ipsilateral eye. Additionally, we found evidence that there were areas in the bilateral LGN which were more active during the stimulation of the contralateral eye than during the stimulation of the ipsilateral eye. The activated areas were reproducible, and the mean ratio of the overlapping area was 0.71 for the repeated scans. The additional experiment revealed that the area in the anterior visual cortex could be divided into two parts, one truly monocular and the other relatively monocular. Our finding confirmed previous fMRI results at 1.5 tesla showing that eye dominance was observed in the contralateral anterior visual cortex. However, the eye dominance in the visual cortex was found not only in the most anterior area corresponding to the monocular temporal crescent but also in the more posterior area, presumably showing the greater sensitivity of the temporal visual field (nasal retina) as compared with the nasal visual field (temporal retina) in the peripheral visual field (peripheral retina). In addition, it is suggested that the nasotemporal asymmetry of the retina and the visual fields is represented in the LGN as well as in the visual cortex.     

应用3D-MRI成像技术重建高度近视眼球形态

目的 利用3D-MRI眼球建模技术探讨高度近视眼的MRI影像表现、形态特征,并与其他影像方法比较,评估其临床价值.方法 横断面研究.65例(124眼)高度近视患者(球镜度高于-6.00 D,且眼轴长度≥26 mm)进行眼部常规临床检查、眼眶磁共振检查,其中40例(80眼)进行眼部B超检查.磁共振图像经处理为眼球模型.按年龄分为＜50岁组和≥50岁组.按球镜度分为:①-6.00～-12.00 D组,②-12.25～-18.00 D组,③-18.25～-24.00 D组.按眼轴分为:①26.00～28.00 mm组,②28.01～30.00 mm组,③30.01～32.00 mm组,④＞32.00 mm组.MRI与B超对于后巩膜葡萄肿的诊断准确率比较采用McNemar检验,不同组别中眼球类型的分布差异采用卡方检验.结果 B超检出后巩膜葡萄肿的诊断准确率为70％,MRI检出后巩膜葡萄肿的诊断准确率为100％,差异有统计学意义(P＜0.05).高度近视眼球形态可分为球型、锥型、碗型、柱型4种类型,碗型、锥型视为后巩膜葡萄肿存在.而4种眼球类型的分布在不同年龄组(x2=57.19,P＜0.05)、不同眼轴组(x2=87.36,P＜0.05)中的差异有统计学意义,在不同球镜度组(x2=67.94,P＞0.05)中的差异没有统计学意义.部分病例于MRI眼球模型上可见不规则隆起与凹陷,但B超不能精确显示.结论 3D-MRI眼球建模能清晰显示高度近视眼球形态与后巩膜葡萄肿的位置及特征,在眼科影像检查手段中诊断准确率高于B超检查,可作为高度近视眼科检查的新手段.     

MRI技术在眼病临床及科学研究中的应用

随着医学影像学的发展,磁共振成像(MRI)技术凭借着无创、成像清晰、对软组织具有高分辨率的特点逐渐运用到眼科疾病及相关动物模型的研究领域中。MRI在眼科最主要应用在眼眶、眼肿瘤疾病,近年来,MRI也逐渐应用于视觉损伤的临床及科学研究中。下面简单介绍MRI在屈光不正、白内障、青光眼、弱视、视神经、脉络膜疾病、视网膜病变等眼病及眼科科研领域中的应用。     

Retrobulbar vasculature using 7-T magnetic resonance imaging with dedicated eye surface coil

Purpose

To determine the resolution and utility of using a dedicated, single-loop eye coil at 7 T to image the posterior ocular structures and vascular anatomy.

Methods

Imaging was performed on eight subjects (age range 26–54 years, four female, four male) with 7 T using a transmit head coil for excitation and a dedicated 5-cm eye surface receive coil. Acquisition parameters at 7 T for 3D spoiled gradient echo (3D-SPGR) sequences were optimized.

Results

It was possible to delineate the retina, sclera, and choroid, and fine details within the anterior and posterior segments of the eye. Retro-orbital and posterior ocular anatomy remained well visualized despite motion and susceptibility artifacts of anterior ocular structures. The ophthalmic arteries and their first-order branches were consistently visualized and improved with registration and summation of repeat scans. Furthermore, the central retinal vessels could be visualized. Intravenous gadolinium contrast reagent did not noticeably improve image quality.

Conclusions

High-resolution 7-T MRI with a dedicated eye coil can provide unique high-resolution noninvasive images of retro-orbital and posterior ocular structural and vascular anatomy and is able to resolve structures as small as the central retina vein.     

Magnetic resonance imaging of the human eye in vivo.

Because of the broad range of T1 (longitudinal relaxation time), T2 (transverse relaxation time), and proton density of the component tissues in the eye, proton magnetic resonance imaging (MRI) can provide ocular images of high contrast. The use of surface coils further enhances the signal-to-noise (S/N) ratio and shortens the image acquisition time to 1 to 6 min. High-resolution MR images of the eye therefore can be acquired readily and routinely. At present, with conventional spin-echo imaging, the in-plane resolution of MR images can be as high as (but not limited to) 0.3 x 0.3 mm with a 3-mm slice thickness. MRI can be used to detect and differentiate ocular lesions and to determine the ocular shape and measure ocular dimensions such as tissue thickness, curvature, and volume.     

Functional magnetic resonance imaging of the visual system.

Functional magnetic resonance imaging (fMRI), which is a technique useful for non-invasive mapping of brain function, is well suited for studying the visual system. This review highlights current clinical applications and research studies involving patients with visual deficits. Relevant reports regarding the investigation of the brain's role in visual processing and some newer fMRI techniques are also reviewed. Functional magnetic resonance imaging has been used for presurgical mapping of visual cortex in patients with brain lesions and for studying patients with amblyopia, optic neuritis, and residual vision in homonymous hemianopia. Retinotopic borders, motion processing, and visual attention have been the topics of several fMRI studies. These reports suggest that fMRI can be useful in clinical and research studies in patients with visual deficits.     

New techniques in magnetic resonance imaging.

Technical developments in magnetic resonance imaging of the brain and orbits provide a challenge to radiologists and practicing clinicians alike. Several new trends in brain imaging are reviewed, including fluid-attenuated inversion recovery (FLAIR), diffusion-weighted imaging (DWI), and perfusion and functional magnetic resonance imaging. These techniques are increasingly used in the clinical environment. Two innovative magnetic resonance techniques for imaging the orbit are also reviewed.     

Orbital magnetic resonance imaging

Magnetic resonance images of the eye and orbit performed with surface coils at 1.5 tesla showed anatomic details superior to those of conventional third- and fourth-generation computed tomography.     

The role of high-resolution computed tomography and magnetic resonance imaging in the evaluation of isolated orbital neurofibromas

PURPOSE: To analyze the imaging findings in patients diagnosed with isolated orbital neurofibromas. DESIGN: Retrospective observational case series. METHODS: Computed tomographic (CT) and magnetic resonance imaging (MRI) findings were reviewed in five patients with orbital neurofibromas in the absence of systemic neurofibromatosis in this multicenter study. The main outcome measures were bony and soft tissue abnormalities involving the orbit and surrounding anatomic spaces. RESULTS: Patient age ranged from 36 to 43 years (mean, 41 years); three patients were men and two were women. Duration of symptoms ranged from three to 20 years. Radiologic findings included intraconal and extraconal soft tissue masses in all patients, bony orbital abnormalities in three patients, and intracranial extension in two patients. In addition, MRI studies in one patient revealed the previously unreported presence of neurofibromas involving extraocular muscle. CONCLUSIONS: Isolated orbital neurofibromas may present with differing radiologic appearances. The combined use of high-resolution CT and MRI may be of benefit in the preoperative diagnosis of these uncommon orbital tumors.