Negative Study Results

The discipline of neuro-ophthalmology has on occasion been called “the reinterpretation of previously negative imaging studies.” When an imaging study fails to demonstrate expected pathology or answer the clinical question, the first step is to reexamine the study parameters, ideally with a neuroradiologist. Several questions to keep in mind are as follows:

Were the appropriate studies performed, including required sequences and orientations? Was the area of interest adequately imaged (Fig 2-16)?

Are the study results really negative (Fig 2-17)?

Figure 2-16 Delayed diagnosis of a right third nerve schwannoma. A, An 11-year-old patient was noted to have a right third nerve palsy that began at age 5 years and became complete by age 7 years. B, Initial study results were negative, but fine cuts through the cavernous sinus demonstrated asymmetry, with a slight nodule in the superior portion of the cavernous sinus on the right. C, This area became bright with administration of gadolinium contrast, which indicated the presence of a right third nerve schwannoma (arrow). (Courtesy of Steven A. Newman, MD.)

Figure 2-17 A 41-year-old woman was referred for progressive vision loss in the right eye. She had been told she had a swollen optic nerve on the right; her condition was diagnosed as a “mild form of multiple sclerosis.” Visual acuity was 2/200 OD and 20/20 OS, with a right afferent pupillary defect. She had reportedly had 2 previous MRI scans, with negative results. A, Fundus photograph of the right optic disc demonstrated temporal pallor with optociliary shunt vessels. The patient was referred for a third MRI scan, but this study was misdirected for workup of “microvascular brainstem disease” and revealed no abnormalities. B, Sagittal MRI scan taken through the orbit shows abnormal optic nerve sheath appearance consistent with optic nerve sheath meningioma (arrow). (Part A courtesy of Steven A. Newman, MD; part B courtesy of

Eric Eggenb erger, DO.)

Even if the ophthalmologist cannot personally review the studies, speaking directly with the radiologist may prevent certain lesions from being overlooked and can provide the clinical information required to enhance the radiographic report’s accuracy and usefulness.

Glossary

ADC (apparent diffusion coefficient) See DWI (diffusion-weighted imaging).

BOLD (blood oxygenation level–dependent) MRI functional imaging technique that can demarcate areas of high activity during a specific task.

CTA (computed tomography angiography) CT technique used to image blood vessels.

DWI (diffusion-weighted imaging) and ADC (apparent diffusion coefficient) MRI techniques especially useful for detecting acute and subacute stroke and for differentiating vasogenic from cytotoxic edema.

FLAIR (fluid-attenuated inversion recovery) MRI technique that highlights T2-hyperintense abnormalities adjacent to CSF (cerebrospinal fluid)-containing spaces, such as the ventricles, by

suppressing CSF signal intensity.

fMRI (functional magnetic resonance imaging) MRI technique that allows visualization of more active brain areas during a specific task, such as reading.

Gadolinium Paramagnetic contrast agent administered intravenously to enhance lesions.

IR (inversion recovery) MRI pulse sequence that nulls the bright signal of fat or water to create a FLAIR or STIR (short tau [or TI] inversion recovery) image. During MRI, initial 180° radiofrequency pulses are followed by a 90° pulse and immediate acquisition of the signal; in IR sequences, the interpulse time is given by TI.

MRA (magnetic resonance angiography) MRI technique for imaging blood vessels.

MRS (magnetic resonance spectroscopy) MRI technique that can further characterize the tissue composition of part of the brain, which helps differentiate tumor, demyelination, and necrosis.

Pixel Picture unit; any of the small, discrete units that together constitute an image (as on a computer screen); increasing the number of pixels that comprise an image increases image resolution.

Relaxation Process by which an element releases (re-emits) energy that has been absorbed from the radiofrequency pulses during an MRI sequence.

SE (spin echo) In the most commonly employed spin-echo sequence, a 180° pulse follows a 90° pulse. For T2-weighted images, the 90° pulse is followed by 2 180° pulses. The first 180° pulse is administered at one-half the TE (time to echo), and the second 180° pulse is administered one full TE later. The “first echo” image is referred to as proton density, and the “second echo” is T2-weighted.

SR (saturation recovery) With SR, the radiofrequency signal is recorded after a series of 90° pulses, with an interpulse interval less than or equal to an average tissue T1 (0.1–1.5 sec).

T1 Time required for 63% of protons to return to the longitudinal plane after cessation of a 90° radiofrequency pulse. This is also referred to as the longitudinal, or spin-lattice, relaxation time.

T2 Time required for 63% of the magnetic field in the transverse plane created by the radiofrequency pulse to dissipate. This dispersion of the magnetic vector corresponds to the exchange of spin among protons and is referred to as spin-spin relaxation; it is completed much more rapidly than is T1 relaxation.

TE (time to echo) Time following the radiofrequency pulse in which the signal is assessed.

Tesla The unit of measure of magnetic field strength.

TI (interpulse time) See IR (inversion recovery).

TR Time to repetition of radiofrequency pulse.

Voxel Three-dimensional cube determined by the product of the pixel size and the slice thickness.