Magnetic Resonance Imaging
Because of its superior contrast resolution, MRI is the imaging modality of choice for evaluation of the central nervous system (see Table 17-1). In addition, the technology does not use ionizing radiation, which is a relative advantage over CT. Instead, MRI uses a strong magnetic field that causes hydrogen atoms found in water and fat to align themselves with the field. Once the atoms are aligned, protons within a selected imaging section/volume are exposed to a series of radiofrequency (RF) and/or magnetic gradient pulses and become excited. As the protons relax again to a steady state, they emit radio waves, which are detected by a receiver coil in the MRI system. The time it takes for the signal to reach the MRI machine following the applied RF (or gradient) pulse is known as the echo time (TE), which varies by type of tissue. The time between RF pulses is known as the repetition time (TR). The TE and TR can be adjusted to modify the contrast between images and thus enhance visualization of different tissues.
The energy given off by the rotating protons is expressed by 2 aspects: the longitudinal relaxation constant, or T1, and the transverse relaxation constant, or T2. T1-weighted images (T1WIs), which are generated with shorter TEs and TRs, are typically used for contrast-enhanced studies. In a T1WI, water appears dark (hypointense) and fat appears bright (hyperintense). Melanin shows an intrinsically elevated T1 signal, which can be helpful in providing a diagnosis in patients with melanoma. Sometimes, however, fat suppression is required in T1WIs to improve contrast enhancement and characterization of tissues, such as the optic nerve and other orbital structures. In comparison, T2-weighted images (T2WIs) use a longer TE to depict differences in water content, thus revealing inflammatory, ischemic, and neoplastic-related edematous changes. On T2WIs, vitreous, cerebrospinal, and other fluids are bright.
On both T1WIs and T2WIs, gray matter is hypointense compared with white matter (Table 17-2, Fig 17-2). In fluid-attenuated inversion recovery (FLAIR) images, the fluid signal is suppressed on T2WIs, facilitating visualization of signal abnormalities associated with changes in the periventricular white matter (eg, as in multiple sclerosis).
Gadolinium-based contrast medium is administered intravenously and used to enhance T1WIs, especially for assessment of inflammatory and neoplastic lesions. Gadolinium may also be administered during high spatial and temporal resolution MRI sequences of large and medium-sized vessels (ie, MR angiography [MRA]), when dynamic contrast enhancement can be assessed more practically than with CTA. The decision to use MRA versus CTA for evaluation of intracranial and orbital blood vessels is often complex and varies depending on the patient and clinical question being asked; consultation with a neuroradiologist may be required in complex cases.
Diffusion-weighted imaging (DWI) is another form of MRI that is relevant to the ophthalmologist, as this sequence is the most sensitive for the detection of acute ischemic changes (eg, cerebrovascular accident). DWI can detect changes within minutes compared with potentially hours with other MRI methods. A quantitative metric of DWI sequences, the apparent diffusion coefficient, can be used to further characterize edema as cytotoxic versus vasogenic (eg, posterior reversible encephalopathy syndrome) (Table 17-3).
Adverse effects are occasionally associated with the gadolinium chelates used for contrast-enhanced imaging in MRI, though at a lower frequency than with iodinated contrast agents in CT. Common symptoms are sweating, pruritus, and rash. Although gadolinium agents do not adversely affect renal function at the doses administered for clinical imaging, certain gadolinium chelates may be restricted in patients with severe end-stage renal disease because of the risk of nephrogenic systemic fibrosis, a rare and potentially fatal multiorgan fibrosing disorder. In addition, gadolinium has been shown to collect in certain neurologic structures after repeated administration; however, no clinical features have been attributed to this deposition. Recommendations for the use of gadolinium-based contrast agents vary by institution; thus, the ophthalmologist is advised to consult with a diagnostic radiologist before ordering such studies in at-risk patients.
Because MRI uses strong magnetic fields to generate pictures, patients with metallic foreign bodies or implants should also be carefully screened before undergoing imaging. Ophthalmologists may be consulted to assess patients for foreign bodies on the ocular surface, within the eye, and/or in the orbit. The incidence of damage from undetected ocular foreign bodies during MRI is low, restricted to a few case reports; however, it is not zero. This is an important consideration when counseling patients before their scans. Patients are also screened at the imaging center before MRI.
The following list highlights general and ophthalmic concerns in patients scheduled to undergo MRI. The reader is also directed to the ACR safety guidelines (see reference list) for further details.
Table 17-2 Signal Characteristics of Normal Ocular Structures in Different Imaging Sequences
Figure 17-2 Brain and orbital magnetic resonance (MR) images showing the anatomy of visual and orbital structures from the chiasm to the anterior orbit. (The left-globe abnormality is not pertinent to the figure’s objective.) A, T1-weighted axial image. B–D, T1-weighted coronal images. E, T2-weighted coronal image with fat saturation. F, T1-weighted coronal image. ACF = anterior cranial fossa; Ant segment = anterior segment; ICA = internal carotid artery; IO = inferior oblique muscle; IR = inferior rectus muscle; LR = lateral rectus muscle; Lev P = levator palpebrae superioris muscle; MCF = middle cranial fossa; MR = medial rectus muscle; Olf fossa = olfactory fossa; SO = superior oblique muscle; Sph sinus = sphenoid sinus; Sph wing = sphenoid wing; SR = superior rectus muscle; Temp lobe = temporal lobe; Vit = vitreous.
(Courtesy of M. Tariq Bhatti, MD.)
Table 17-3 Edema: DWI and ADC
Considerations when ordering an MRI:
Metal in the body, including metallic intraocular or orbital foreign bodies
▪ Screening radiography or CT may be helpful in detecting intraocular and orbital foreign bodies.
▪ Consultation with a diagnostic radiologist is advised regarding the safety of some metals (eg, MRI-compatible aneurysm clips).
▪ Gold weight and titanium mesh orbital floor implants have shown no movement when placed in a magnetic field. Some clinicians prefer to wait for fibrosis to secure the implant before obtaining an MRI.
Cardiac pacemaker or defibrillator
Allergy to gadolinium-based contrast media
Activities 17-1 and 17-2 demonstrate normal structures identified on axial and coronal orbital imaging, respectively, with CT and MRI.
ACTIVITY 17-1 Axial imaging of the normal orbit with computed tomography and magnetic resonance imaging. Developed by Vikram S. Brar, MD. Figures reproduced with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Elsevier/Saunders; 2011: Figs 11-1 to 11-6.
Access all Section 2 activities at www.aao.org/bcscactivity_section02.
ACTIVITY 17-2 Coronal imaging of the normal orbit with computed tomography and magnetic resonance imaging.
Developed by Vikram S. Brar, MD. Figures reproduced with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy.
2nd ed. Elsevier/Saunders; 2011: Figs 11-7 to 11-12.
Expert Panel on MR Safety; Kanal E, Barkovich AJ, Bell C, et al. ACR guidance document on MR safe practices: 2013. J Magn Reson Imaging. 2013;37(3):501–530.
Lawrence DA, Lipman AT, Gupta SK, Nacey NC. Undetected intraocular metallic foreign body causing hyphema in a patient undergoing MRI: a rare occurrence demonstrating the limitations of pre-MRI safety screening. Magn Reson Imaging. 2015;33(3): 358–361.
Marra S, Leonetti JP, Konior RJ, Raslan W. Effect of magnetic resonance imaging on implantable eyelid weights. Ann Otol Rhinol Laryngol. 1995;104(6):448–452.
Modjtahedi BS, Rong A, Bobinski M, McGahan J, Morse L. Imaging characteristics of intraocular foreign bodies: a comparative study of plain film X-ray, computed tomography, ultrasound, and magnetic resonance imaging. Retina. 2015;35(1):95–104.
Seidenwurm DJ, McDonnell CH III, Raghavan N, Breslau J. Cost utility analysis of radiographic screening for an orbital foreign body before MR imaging. Am J Neuroradiol. 2000;21(2):426–433.
Sullivan PK, Smith JF, Rozzelle AA. Cranio-orbital reconstruction: safety and image quality of metallic implants on CT and MRI scanning. Plast Reconstr Surg. 1994;94(5):589–596.
Williamson MR, Espinosa MC, Boutin RD, Orrison WW Jr, Hart BC, Kelsey CA. Metallic foreign bodies in the orbits of patients undergoing MR imaging: prevalence and value of radiography and CT before MR. AJRAm J Roentgenol. 1994;162(4):981–983.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.