Detection of Relative Afferent Pupillary Defects Using Eye Tracking and a VR Headset.

Purpose: The purpose of this study was to assess the feasibility of detecting relative afferent pupillary defects (RAPDs) using a commercial virtual reality headset equipped with an eye tracker. Methods: This is a cross-sectional study in which we compare the new computerized RAPD test with the traditional clinical standard using the swinging flashlight test. Eighty-two participants including 20 healthy volunteers aged 10 to 88 years were enrolled in this study. We present a bright/dark stimulus alternating between the eyes every 3 seconds using a virtual reality headset, and we simultaneously record changes in pupil size. To determine the presence of an RAPD, we developed an algorithm analyzing the pupil size differences. For the assessment of the performance of the automated and the manual measurement a post hoc impression based on all available data is created. The accuracy of the manual clinical evaluation and the computerized method is compared using confusion matrices and the gold standard of the post hoc impression. The latter is based on all available clinical information. Results: We found that the computerized method detected RAPD with a sensitivity of 90.2% and an accuracy of 84.4%, as compared to the post hoc impression. This was not significantly different from the clinical evaluation with a sensitivity of 89.1% and an accuracy of 88.3%. Conclusions: The presented method offers an accurate, easy to use, and fast method to measure an RAPD. In contrast to today's clinical practice, the measures are quantitative and objective. Translational Relevance: Computerized testing of Relative Afferent Pupillary Defects (RAPD) using a VR-headset and eye-tracking reaches non-inferior performance compared with senior neuro-ophthalmologists.

Fluorescence lifetime distribution in phakic and pseudophakic healthy eyes.

PURPOSE: To investigate the influence of the lens status and to describe fundus autofluorescence lifetimes (FLT) in a large cohort of healthy eyes across a wide age range. MATERIALS AND METHODS: FLT data were acquired from healthy phakic and pseudophakic eyes using fluorescence lifetime imaging ophthalmoscopy (FLIO). Retinal autofluorescence was excited with a 473 nm laser and emitted autofluorescence was detected in a short and a long spectral channel (SSC: 498-560 nm; LSC: 560-720 nm). RESULTS: 141 healthy eyes from 141 participants (56 ± 18 years) were included. The shortest mean FLTs were measured within the macular center, followed by the temporal inner and outer ETDRS (Early Treatment Diabetic Retinopathy Study) grid segments, and the remaining areas of the inner and the outer ETDRS ring. In phakic participants (81%), mean, short and long FLTs correlated with the age (SSC: r2 = 0.54; LSC: r2 = 0.7; both p<0.0001) with an increase of about 33 ps in the SSC resp. 28 ps in the LSC per decade. In pseudophakic subjects (19%), mean FLTs only correlated with age in the long spectral channel (r2 = 0.44; p = 0.0002) but not in the short spectral channel (r2 = 0.066; p = 0.2). CONCLUSIONS: Fundus autofluorescence lifetimes are age dependent. FLTs in the SSC are more susceptible to lens opacities but less dependent on age changes, whereas FLTs in the LSC are largely independent of the lens status but display a higher degree of age dependency. STUDY REGISTRY: ClinicalTrials.gov NCT01981148.

Unilateral corneal arcus and conjunctival vessel alterations in cranial autonomic dysregulation: A case report.

BACKGROUND: Cranial autonomic dysregulation is a common symptom of patients suffering from cluster headache or migraine. The peripheral vascular dysfunction may increase the risk for ischemic or hemorrhagic strokes, myocardial infarction, retinal vasculopathy, cardiovascular mortality, and peripheral artery diseases. Furthermore, it may also manifest with ocular symptoms, e.g., increased lacrimation, conjunctival injection, and facial swelling. CASE PRESENTATION: We here report a case of a patient with migraine and ocular signs of a vascular dysregulation that have led to persisting changes of conjunctival vessels and to a corneal arcus. CONCLUSIONS: Autonomic vascular dysregulation may not only cause headaches but also persisting changes of ocular tissues, e.g., conjunctival vessel alterations and a corneal arcus.

Evaluation of the Reddesa Chart, a New Red Desaturation Testing Method, for Optic Neuritis Screening and Grading in Clinical Routine.

Background: Optic neuritis usually leads to reduced color sensitivity. Most often, the change of red color, the so-called red desaturation, is tested in clinical routine. The aim of this study was to test the feasibility of the Reddesa chart, a new red desaturation test based on polarization, as a screening method for optic neuropathy. Methods: A total of 20 patients with unilateral optic neuritis and 20 healthy controls were included in this prospective pilot study. Ophthalmological examination included assessment of best corrected visual acuity (BCVA), slit lamp examination, fundoscopy, testing of relative afferent pupillary defect (RAPD) and red desaturation with the red cup test and the Reddesa chart. Results: The mean BCVA in the optic neuritis group was 0.76 ± 0.36 in the affected eye (95% of eyes with RAPD, 75% of eyes with difference in the Reddesa test) and 1.28 ± 0.24 in the healthy eye, whereas in the control group, BCVA was 1.14 ± 0.11 in the right eye and 1.15 ± 0.14 in the left eye (none of the eyes with RAPD or abnormal Reddesa test). In our study, the Reddesa test showed a positive predictive value of 100% and a negative predictive value of 80% for detecting optic neuritis. Conclusion: The Reddesa chart allows to quantify red desaturation and has the potential to be implemented as a screening test in clinical routine.

IMAGING ARTIFACTS IN FLUORESCENCE LIFETIME IMAGING OPHTHALMOSCOPY.

PURPOSE: To investigate and quantify the influence of imaging artifacts on retinal fluorescence lifetime (FLIO) values and to provide helpful hints and tricks to avoid imaging artifacts and to improve FLIO image acquisition quality. METHODS: A systematic analysis of potential parameters influencing FLIO quality and/or fluorescence lifetime values was performed in a prospective systematic experimental imaging study in five eyes of five healthy subjects. For image acquisition, a fluorescence lifetime imaging ophthalmoscope (Heidelberg Engineering) was used. Quantitative analysis of FLIO lifetime changes due to imaging artifacts was performed. RESULTS: Imaging artifacts with significant influence on fluorescence lifetimes included too short image acquisition time, insufficient illumination, ocular surface problems, and image defocus. Prior use of systemic or topical fluorescein makes analysis of retinal fluorescence lifetimes impossible. CONCLUSION: Awareness of possible sources of imaging artifacts is important for FLIO image acquisition and analysis. Therefore, standardized imaging and analysis procedure in FLIO is crucial for high-quality image acquisition and the possibility for systematic quantitative fluorescence lifetime analysis.

Divergence bias in Hess compared to Harms screen strabismus testing.

The Hess and the Harms screen test each have different testing distances. While the Harms screen test is usually performed at 2.5 m, the Hess screen test is performed at 0.5 m. The geometry of the closer testing distance of the Hess screen test requires an increase of the convergence angle by 6°. This study investigates the quantitative differences between the two frequently employed screen tests. Ocular deviation of 18 normal subjects and 36 patients with congenital or acquired paralytic or concomitant strabismus were assessed with a complete orthoptic examination including alternate prism cover testing at near (nPCT) and far (fPCT), as well as Hess and Harms screen testing. One-way ANOVA was used for statistical analysis. The Hess test recorded more overall exodeviation compared to the Harms test for patients (mean difference -3.50°, 95% limits of agreement (CI) = [-4.79, -2.21], p < .001), and controls (mean difference -1.78°, CI = [-2.99, -0.56], p = .004). For vertical deviations, there was no statistically significant difference between the two tests for patients (mean difference +0.75°, CI = [-0.41, +1.91], p = .251), and controls (mean difference -0.28°, CI = [-0.68, -0.11], p = 0.231). This study emphasizes the importance to consider the divergence bias when comparing the Hess to the Harms screen test, which is likely explained by the greater vergence demand dependent on the closer testing distance. The exodeviation shift tended to be more pronounced in patients than controls, which may imply that patients with strabismus have an impaired convergence drive.

Differentiation of Underlying Pathologies of Macular Edema Using Spectral Domain Optical Coherence Tomography (SD-OCT).

Purpose: To describe the morphological characteristics of macular edema (ME) of different origins using spectral domain optical coherence tomography (SD-OCT). Methods: This article summarizes and highlights key morphologic findings, based on published articles, describing the characteristic presentations of ME of different origins using SD-OCT. The following pathologies were included: uveitic macular edema, pseudophakic cystoid macular edema (PCME), diabetic macular edema (DME), macular edema secondary to central or branch retinal vein occlusion (CRVO/BRVO), microcystic macular edema (MME), ME associated with epiretinal membrane (ERM), and retinitis pigmentosa (RP). Conclusions: Macular edema of different origins show characteristic patterns that are often indicative of the underlying cause and pathology. Thus, trained algorithms may in the future be able to automatically differentiate underlying causes and support clinical diagnosis. Knowledge of different appearances support the clinical diagnosis and can lead to improved and more targeted treatment of ME.

Comparison of Choroidal Thickness Measurements Using Spectral Domain Optical Coherence Tomography in Six Different Settings and With Customized Automated Segmentation Software.

PURPOSE: We investigate which spectral domain-optical coherence tomography (SD-OCT) setting is superior when measuring subfoveal choroidal thickness (CT) and compared results to an automated segmentation software. METHODS: Thirty patients underwent enhanced depth imaging (EDI)-OCT. B-scans were extracted in six different settings (W+N = white background/normal contrast 9; W+H = white background/maximum contrast 16; B+N = black background/normal contrast 12; B+H = black background/maximum contrast 16; C+N = Color-encoded image on black background at predefined contrast of 9, and C+H = Color-encoded image on black background at high/maximal contrast of 16), resulting in 180 images. Subfoveal CT was manually measured by nine graders and by automated segmentation software. Intraclass correlation (ICC) was assessed. RESULTS: ICC was higher in normal than in high contrast images, and better for achromatic black than for white background images. Achromatic images were better than color images. Highest ICC was achieved in B+N (ICC = 0.64), followed by B+H (ICC = 0.54), W+N, and W+H (ICC = 0.5 each). Weakest ICC was obtained with Spectral-color (ICC = 0.47). Mean manual CT versus mean computer estimated CT showed a correlation of r = 0.6 (P = 0.001). CONCLUSION: Black background with white image at normal contrast (B+N) seems the best setting to manually assess subfoveal CT. Automated assessment of CT seems to be a reliable tool for CT assessment. TRANSLATIONAL RELEVANCE: To define optimized OCT analysis settings to improve the evaluation of in vivo imaging.

PATIENTS WITH EPIRETINAL MEMBRANES DISPLAY RETROGRADE MACULOPATHY AFTER SURGICAL PEELING OF THE INTERNAL LIMITING MEMBRANE.

PURPOSE: Intraretinal cystoid spaces are commonly found after surgical peeling of epiretinal membranes. In this study, we explored whether these cysts were associated with ganglion cell loss and thus might be a manifestation of retrograde maculopathy. The latter is a nonvascular edema with a characteristic morphology that is often found in the inner nuclear layer (INL) of patients with optic neuropathy. METHODS: In this retrospective case series, we identified consecutive patients who underwent surgical epiretinal membrane peeling. We determined the frequency of microcystic macular edema (MME), defined by vertical cystoid spaces in the INL, and we measured the thickness of individual macular layers before and after surgery. RESULTS: Epiretinal membrane peeling resulted in an improvement of visual acuity and a reduction of retinal thickness by about 15%. In total, 35% of patients with MME before surgery showed no sign of MME postoperatively, whereas edema persisted after surgery in 65% of patients. Interestingly, 29% of the patients without MME before surgery developed MME after surgery. Overall, we found MME in 35% of patients before peeling and in 42% after peeling. After surgery, the mean ganglion cell layer thickness was reduced compared with healthy control eyes. Ganglion cell layer thickness correlated inversely with thickness of the INL. Compared with patients without MME, individuals with MME had a thinner ganglion cell layer and a thicker INL in the affected eye. CONCLUSION: Our findings indicate that peeling of epiretinal membranes and internal limiting membranes is associated with atrophy of ganglion cells and thickening of the INL. The latter is associated with the presence of MME. Altogether, we assume that surgical treatment of epiretinal membranes induces a variant of a retrograde maculopathy.

Inter-Rater Reliability of Cyclotorsion Measurements Using Fundus Photography.

BACKGROUND: The foveo-papillary angle (FPA) on fundus photographs is the accepted standard for the measurement of ocular cyclotorsion. We assessed the inter-rater reliability of this method in healthy subjects and in patients with trochlear nerve palsies. PATIENTS AND METHODS: In this methodological study, fundus photographs of healthy subjects and of patients with trochlear nerve palsies were made with a fundus camera (Zeiss Fundus Camera FF 450 plus, Jena, Germany). Three independent observers measured the FPA on the fundus photographs of all subjects in synedra View (synedra View 16, Version 16.0.0.11, Innsbruck, Austria). RESULTS: One hundred and four eyes of 52 subjects (26 healthy controls and 26 patients) were assessed. The mean FPA of the healthy controls was 5.80 degrees (°) [± 0.44 standard error of the mean (SEM)] compared to 11.55° (± 0.80 SEM) for patients with trochlear nerve palsies. The inter-rater reliability of all measured FPAs showed an intraclass correlation coefficient (ICC) of 0.98 (95% CI 0.97 - 0.98). CONCLUSIONS: The inter-rater reliability of objective cyclotorsion measurements using fundus photographs was very high.

Acetazolamide Reduces Retinal Inner Nuclear Layer Thickness in Microcystic Macular Edema Secondary to Optic Neuropathy.

Optic neuropathy (ON) is commonly complicated by microcystic macular edema (MME), that is, small vertical cystoid spaces in the inner nuclear layer (INL) of the macula. We performed a retrospective consecutive case series of 14 eyes from 11 patients with ON and MME that were treated with oral acetazolamide, acting on cellular water transport. Contralateral eyes without MME were used as controls. Segmentation of images obtained with OCT was used to determine changes of individual retinal layer thickness during treatment. Retinal INL thickness consistently decreased in all eyes after 2-3 weeks of treatment. Recurrence of MME was observed after treatment cessation. No significant change of retinal thickness was found in contralateral unaffected eyes. Visual function did not change with treatment. Acetazolamide significantly improved the MME in eyes with ON. However, visual function did not. Acetazolamide is a treatment option for MME associated with ON but without an impact on the visual function.

Strabismus Measurements with Novel Video Goggles.

PURPOSE: To assess the validity of a novel, simplified, noninvasive test for strabismus using video goggles. DESIGN: Cross-sectional method comparison study in which the new test, the strabismus video goggles, is compared with the existing reference standard, the Hess screen test. PARTICIPANTS: We studied 41 adult and child patients aged ≥6 years with ocular misalignment owing to congenital or acquired paralytic or comitant strabismus and 17 healthy volunteers. METHODS: All participants were tested with binocular infrared video goggles with built-in laser target projection and liquid crystal display shutters for alternate occlusion of the eyes and the conventional Hess screen test. In both tests, ocular deviations were measured on a 9-point target grid located at 0±15° horizontal and vertical eccentricity. MAIN OUTCOME MEASURES: Horizontal and vertical ocular deviations at 9 different gaze positions of each eye were measured by the strabismus video goggles and the Hess screen test. Agreement was quantified as the intraclass correlation coefficient. Secondary outcomes were the utility of the goggles in patients with visual suppression and in children. RESULTS: There was good agreement between the strabismus video goggles and the Hess screen test in the measurements of horizontal and vertical deviation (intraclass correlation coefficient horizontal 0.83, 95% confidence interval [0.77, 0.88], vertical 0.76, 95% confidence interval [0.68, 0.82]). Both methods reproduced the characteristic strabismus patterns in the 9-point grid. In contrast to Hess screen testing, strabismus video goggle measurements were even possible in patients with comitant strabismus and visual suppression. CONCLUSIONS: The new device is simple and is fast and accurate in measuring ocular deviations, and the results are closely correlated with those obtained using the conventional Hess screen test. It can even be used in patients with visual suppression who are not suitable for the Hess screen test. The device can be applied in children as young as 6 years of age.

Effect of pharmacologically induced retinal degeneration on retinal autofluorescence lifetimes in mice.

Fluorescence lifetime imaging ophthalmoscopy (FLIO) was used to investigate retinal autofluorescence lifetimes in mouse models of pharmacologically induced retinal degeneration over time. Sodium iodate (NaIO3, 35 mg/kg intravenously) was used to induce retinal pigment epithelium (RPE) degeneration with subsequent loss of photoreceptors (PR) whereas N-methyl-N-nitrosourea (MNU, 45 mg/kg intraperitoneally) was employed for degeneration of the photoreceptor cell layer alone. All mice were measured at day 3, 7, 14, and 28 after the respective injection of NaIO3, MNU or NaCl (control). Fluorescence lifetime imaging was performed using a fluorescence lifetime imaging ophthalmoscope (Heidelberg Engineering, Heidelberg, Germany). Fluorescence was excited at 473 nm and fluorescence lifetimes were measured in a short and a long spectral channel (498-560 nm and 560-720 nm). Corresponding optical coherence tomography (OCT) images were consecutively acquired and histology was performed at the end of the experiments. Segmentation of OCT images and histology verified the cell type-specific degeneration process over time. Retinal autofluorescence lifetimes increased from day 3 to day 28 in mice after NaIO3 treatment. Finally, at day 28, fluorescence lifetimes were prolonged by 8% in the short and 61% in the long spectral channel compared to control animals (p = 0.21 and p = 0.004, respectively). In mice after MNU treatment, the mean retinal autofluorescence lifetimes were already decreased at day 3 and retinal lifetimes were finally shortened by 27% in the short and 51% in the long spectral channel at day 28 (p = 0.0028). In conclusion, degeneration of the RPE with subsequent photoreceptor degeneration by NaIO3 lead to longer mean fluorescence lifetimes of the retina compared to control mice, whereas during specific degeneration of the photoreceptor layer induced by MNU shorter lifetimes were measured. Therefore, short retinal fluorescence lifetimes may originate from the RPE and may be modified by the overlaying retinal layers.

Nystagmus Does Not Limit Reading Ability in Albinism.

PURPOSE: Subjects with albinism usually suffer from nystagmus and reduced visual acuity, which may impair reading performance. The contribution of nystagmus to decreased reading ability is not known. Low vision and nystagmus may have an additive effect. We aimed to address this question by motion compensation of the nystagmus in affected subjects and by simulating nystagmus in healthy controls. METHODS: Reading speed and eye movements were assessed in 9 subjects with nystagmus associated with albinism and in 12 healthy controls. We compared the reading ability with steady word presentation and with words presented on a gaze contingent display where words move in parallel to the nystagmus and thus correct for the nystagmus. As the control, healthy subjects were asked to read words and texts in steady reading conditions as well as text passages that moved in a pattern similar to nystagmus. RESULTS: Correcting nystagmus with a gaze contingent display neither improved nor reduced the reading speed for single words. Subjects with nystagmus and healthy participants achieved comparable reading speed when reading steady texts. However, movement of text in healthy controls caused a significantly reduced reading speed and more regressive saccades. CONCLUSIONS: Our results argue against nystagmus as the rate limiting factor for reading speed when words were presented in high enough magnification and support the notion that other sensory visual impairments associated with albinism (for example reduced visual acuity) might be the primary causes for reading impairment.

Gaze-dependent phoria and vergence adaptation.

Incomitance is a condition with gaze-dependent deviations of ocular alignment and is common in strabismus patients. The physiological mechanisms that maintain equal horizontal ocular alignment in all gaze directions (concomitance) in healthy individuals are poorly explored. We investigate adaptive processes in the vergence system that are induced by horizontal incomitant vergence stimuli (stimuli that require a gaze-dependent vergence response in order to re-establish binocular single vision). We measured horizontal vergence responses elicited after healthy subjects shifted their gaze from a position that required no vergence to a position that required convergence. Repetitive saccades into a position with a convergence stimulus rapidly decreased phoria (defined as the deviation of ocular alignment in the absence of a binocular stimulus). This change of phoria was present in all viewing directions (from 0° to 0.86° ± 0.40°, p < 0.001) but was more pronounced in the gaze direction with a convergence stimulus (from 0.26° ± 0.13° to 1.39° ± 0.33°, p < 0.001). We also found that vergence velocity rapidly increased (p = 0.015) and vergence latency promptly decreased (p < 0.001). We found gaze-dependent modulation of phoria in combined saccade-vergence eye movements and also in pursuit-vergence eye movements. Thus, acute horizontal, gaze-dependent changes of vergence, such as may be encountered in new onset strabismus due to paralysis, can rapidly increase vergence velocity and decrease latency. Gaze-specific (concomitant) and gaze-independent (incomitant) phoria levels will adapt. These early adaptive processes increase the efficacy of binocular vision and maintain good ocular alignment in all directions of gaze.

Enhanced-depth optical coherence tomography for imaging horizontal rectus muscles in Graves' orbitopathy.

PURPOSE: Graves' orbitopathy (GO) is an extraocular eye disease with symptoms ranging from minor discomfort from dry eyes to strabismus and visual loss. One of the hallmarks of active GO is visible hyperemia at the insertion of the extraocular muscles. The aim of the present study was to evaluate the use of enhanced-depth imaging spectral domain anterior segment optical coherence tomography (EDI SD AS-OCT) for detecting pathological changes in horizontal recti muscles of patients with GO. METHODS: Prospective cross sectional study of 27 eyes. Only women were included. EDI AS-OCT was used to measure the thickness of the tendons of the horizontal recti muscles in a predefined area in patients with GO and healthy controls. RESULTS: EDI AS-OCT was able to image the tendons of the horizontal recti muscles in both healthy controls and patients suffering from GO. The mean thickness of the medial rectus muscle (MR) tendon was 256.4 µm [±17.13 µm standard deviation (SD)] in the GO group and, therefore, significantly thicker (p = 0.046) than in the healthy group which had a mean thickness of 214.7 µm (±5.516 µm SD). There was no significant difference in the mean thickness of the tendon of the lateral recti muscles (LRs) between these groups. CONCLUSION: This is the first report showing that EDI AS-OCT is suitable to detect swelling at the insertion site of the MR muscle in GO. MR tendon thickness may be a useful parameter to monitor activity in these patients.

Acute onset incomitant image disparity modifies saccadic and vergence eye movements.

New-onset impairment of ocular motility will cause incomitant strabismus, i.e., a gaze-dependent ocular misalignment. This ocular misalignment will cause retinal disparity, that is, a deviation of the spatial position of an image on the retina of both eyes, which is a trigger for a vergence eye movement that results in ocular realignment. If the vergence movement fails, the eyes remain misaligned, resulting in double vision. Adaptive processes to such incomitant vergence stimuli are poorly understood. In this study, we have investigated the physiological oculomotor response of saccadic and vergence eye movements in healthy individuals after shifting gaze from a viewing position without image disparity into a field of view with increased image disparity, thus in conditions mimicking incomitance. Repetitive saccadic eye movements into a visual field with increased stimulus disparity lead to a rapid modification of the oculomotor response: (a) Saccades showed immediate disconjugacy (p < 0.001) resulting in decreased retinal image disparity at the end of a saccade. (b) Vergence kinetics improved over time (p < 0.001). This modified oculomotor response enables a more prompt restoration of ocular alignment in new-onset incomitance.

Fluorescence lifetime imaging of the ocular fundus in mice.

PURPOSE: Fundus autofluorescence (AF) is characterized not only by its intensity or excitation and emission spectra but also by the lifetimes of the fluorophores. Fluorescence lifetime is influenced by the fluorophore's microenvironment and may provide information about the metabolic tissue state. We report quantitative and qualitative autofluorescence lifetime imaging of the ocular fundus in mice. METHODS: A fluorescence lifetime imaging ophthalmoscope (FLIO) was used to measure fluorescence lifetimes of endogenous fluorophores in the murine retina. FLIO imaging was performed in 1-month-old C57BL/6, BALB/c, and C3A.Cg-Pde6b(+)Prph2(Rd2)/J mice. Measurements were repeated at monthly intervals over the course of 6 months. For correlation with structural changes, an optical coherence tomogram was acquired. RESULTS: Fundus autofluorescence lifetime images were readily obtained in all mice. In the short spectral channel (498-560 nm), mean ± SEM AF lifetimes were 956 ± 15 picoseconds (ps) in C57BL/6; 801 ± 35 ps in BALB/c mice; and 882 ± 37 ps in C3A.Cg-Pde6b(+)Prph2(Rd2)/J mice. In the long spectral channel (560-720 nm), mean ± SEM AF lifetimes were 298 ± 14 ps in C57BL/6 mice, 241 ± 10 ps in BALB/c mice, and 288 ± 8 ps in C3A.Cg-Pde6b(+)Prph2(Rd2)/J mice. There was a general decrease in mean AF lifetimes with age. CONCLUSIONS: Although fluorescence lifetime values differ among mouse strains, we found little variance within the groups. Fundus autofluorescence lifetime imaging in mice may provide additional information for understanding retinal disease processes and may facilitate monitoring of therapeutic effects in preclinical studies.