Abstract
Purpose.:
To investigate the cause of axial eye motion artifacts that occur in optical coherence tomography (OCT) imaging of the retina. Understanding the cause of these motions can lead to improved OCT image quality and therefore better diagnoses.
Methods.:
Twenty-seven measurements were performed on 5 subjects. Spectral domain OCT images at the macula were collected over periods up to 30 seconds. The axial shift of every average A-scan was calculated with respect to the previous average A-scan by calculating the cross-correlation. The frequency spectrum of the calculated shifts versus time was determined. The heart rate was determined from blood pressure measurements at the finger using an optical blood pressure detector. The fundamental frequency and higher order harmonics of the axial OCT shift were compared with the frequency spectrum of blood pressure data. In addition, simultaneous registration of the movement of the cornea and the retina was performed with a dual reference arm OCT setup, and movements of the head were also analyzed.
Results.:
A correlation of 0.90 was found between the fundamental frequency in the axial OCT shift and the heart rate. Cornea and retina move simultaneously in the axial direction. The entire head moves with the same amplitude as the retina.
Conclusions.:
Axial motion artifacts during OCT volume scanning of the retina are caused by movements of the whole head induced by the heartbeat.
Motion artifacts in medical imaging have been a topic of great interest for many years. Motion artifacts during image acquisition of a patient can be caused by muscular, peristaltic, cardiovascular, and/or respiratory activity.
1 Degradation of image quality resulting from motion can lead to ambiguous clinical interpretation and even erroneous diagnoses.
2 Investigating the causes of these involuntary motions can lead to a better understanding of the underlying physiology and improve clinical interpretation. Moreover, information of the origin of the motions can be used for image correction either during or after imaging.
3 Although motion artifacts are mostly considered disturbing, they can also contain valuable functional information on the patients' health state.
Optical coherence tomography (OCT) is a noninvasive, high-resolution optical imaging technique that is increasingly used in clinical practice.
4 OCT generates cross-sectional (B-scan) and 3-dimensional (3D) images by measuring echo time delay of backscattered light using interferometery.
5 Its main application is in ophthalmology; however, OCT is also applied in fields such as cardiovascular imaging,
6 dermatology,
7 gastroenterology,
8 and urology.
9 In ophthalmology, OCT is mainly used for imaging the central retina, where the retina–vitreous interface, sub- and intramacular edema,
10 and the retinal thickness
11 can be monitored. Around the optic nerve, the thickness of the nerve fiber layer (NFL) is especially important for diagnosis and follow-up of glaucoma patients.
11 In addition to morphologic imaging, OCT also can provide information about (retinal) blood flow using the Doppler shift of the backscattered light. Moving red blood cells cause a Doppler frequency shift of the backscattered light from which the speed of the moving particle can be derived.
12
For OCT imaging of the retina, motion artifacts can be easily removed by using image alignment in image postprocessing. For Doppler OCT, bulk motions by the sample need to be corrected to accurately determine the absolute blood flow velocity.
13 With the higher imaging speeds (of >100,000 A-lines per second)
14 that are reached in Fourier domain OCT,
15 –17 some of these motion artifacts have been ameliorated. Even in these systems, however, movements by a patient can still cause significant image quality degradation
18,19 and can be clearly visible in 3D OCT volume scans of the entire retina.
When OCT is used to visualize the retina, movements toward and away from the laser beam (axial movements) are often visible during acquisition of subsequent cross-sectional images, B-scans, in the so-called slow scanning direction in a 3D or volume scan.
Figure 1 shows a typical 3D scan of the macula (3D OCT-1000 Mark I; Topcon, Capelle aan den IJssel, The Netherlands; 3.6 seconds acquisition time) made in our clinic. The slow scanning direction is indicated with an X. The axial motion of the retina is clearly visible as an oscillation of the retina–vitreous interface.
In this study, we first investigate the relation between axial motions visible in OCT scans of the retina and the heart rate of the subject by simultaneous OCT measurements and noninvasive optical blood pressure measurements on the finger from which the heart rate is derived. Second, simultaneous registration of the axial movement of the cornea and the retina is performed using a dual reference arm OCT setup. Third, we investigate axial head movements with OCT scanning of the teeth. In addition, the axial position of the retina is recorded when a subject is lying down to decrease the influence of head movements.
To determine the heart rate, the blood pressure was measured simultaneously and continuously during OCT imaging using a blood pressure monitor (Nexfin; BMEYE; Amsterdam, The Netherlands). This device used a volume clamp method with a pressurized cuff around the finger of the subject to obtain continuous noninvasive blood pressure. The blood pressure waveform (sample frequency, 200 Hz) and derived parameters (e.g., heart rate) were stored to file after the measurement. At both the start and at the end of the OCT scanning, an electric trigger signal was send to the monitor, and the trigger signal was stored in the blood pressure dataset. In postprocessing, the data of these triggers were used to synchronize the blood pressure measurement with the OCT measurement.
To investigate the influence of the heartbeat on the axial motions, a total of 27 measurements was performed on 5 healthy subjects. The mean age of the subjects was 31 ± 8 years. All subjects were healthy without any known ocular pathology and with clear ocular media. The subjects were asked to fixate and relax while they placed their head on a chin rest. Furthermore, they were asked not to blink their eyes and to avoid large head movements. In the frequency spectrum of the axial OCT shift, peaks that were clearly visible above noise level were located manually and were analyzed. Peaks with frequencies lower than 0.5 Hz were excluded from analysis. The uncertainty in the central frequency of the peak is determined by the resolution of the frequency spectrum. From the blood pressure measurement, the corresponding part was selected using the start and stop triggers as a reference. A frequency spectrum was made of the blood pressure signal, and the fundamental frequency (i.e., the heart rate) was selected and used in further analysis.
Second, the movement of the retina and the cornea were measured simultaneously where the signal from the cornea was displayed in the upper part and the signal from the retina was displayed in the lower part of the B-scan image. Seventeen measurements were performed on 2 subjects. During these measurements, the scan mirror was not rotating. The average A-scan was segmented in a retinal image and a corneal image. The position analysis was performed on both images to determine the shift of the retina and the cornea.
Third, to register axial head motions, 10 measurements were performed on the teeth of 1 subject. The same cross-correlation method was used to calculate the axial OCT shift and to determine the frequency of the motion.
Finally, the effect of the heart rate on head motions was further investigated on 6 subjects who were asked to lie down, resting with the back side of the head on the floor during OCT imaging of the retina. In this way, head movements toward and away from the illuminating beam were restricted because gravity kept the head position constant. Five measurements per subject were performed in this position, and the same method was used in the analysis of the OCT images.
All research adhered to the tenets of the Declaration of Helsinki. We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during this research.