The importance of an eye tracker to ensure the accuracy of a personalized laser treatment cannot be underestimated. Determining the best eye tracker technology for a personalized treatment may be challenging, given the level of hype and misinformation surrounding these devices. Here, I will explain the role of eye trackers and the key parameters in assessing these devices.
Eye-tracking systems detect positional changes in the eye during treatment. The tracker also follows and then compensates for the movement quickly enough to ensure that that laser pulses hit the intended location. The predictability of refractive outcomes is highly dependent on the eye tracker's accuracy, because even a few misplaced pulses may result in an undercorrection or induced astigmatism.
The Bausch & Lomb (Rochester, New York) Technolas 217 laser systems are equipped with an optimized eye-tracking system that detects positional changes in the eye. The system is capable of following and compensating for eye movement in an appropriate time scale to position the laser pulses at the desired location. The predictability of refractive outcome is affected tremendously by the tracker's accuracy; displacement of single laser pulses—due to eye movement—causes a variation in the ablation pattern with the potential for undercorrection and induced astigmatism.
The Bausch & Lomb Technolas 217 active eye tracker consists of an infrared imaging camera system that captures the geometric center of the undilated pupil, plus a scanning system that responds and compensates for pupil movement found by the camera. The surgeon defines the treatment center according to that reference and, therefore, always has full visual control throughout the treatment. The surgeon may also modify the treatment center at any time. The centration of an eye-tracking device generally provides much higher safety if it is done at the natural unchanged pupil.
More significantly, an active scanner feedback mechanism, which is found only on the Technolas 217 laser, ensures correct scanner mirror position prior to activating a laser pulse. This feature is important with higher-speed eye trackers in combination with high repetition rates to ensure accurate and repeatable ablations.
The B&L 240-Hz detection system sampling rate is 4.2 milliseconds and the fastest scanner mirror reaction time of 2.4 milliseconds, equivalent to to 0.11 milliseconds per 1 mm distance on the treatment plane. The overall reaction time of the B&L eye tracker is fewer than 6.6 milliseconds.
The B&L eye tracker monitors eye movements three-dimensionally. The active x- (lateral direction) and y- (vertical direction) tracking range is defined as a deviation of 1.5 mm from the original fixation. Within this range, the laser adjusts the excimer pulses to the intended location according to the pupil position; outside this range, pulsing is paused.
The B&L system can observe and detect the z dimension of the eye, which uncorrected would result in a defocus of the ablation surface. The eye-tracking system interrupts the laser pulse sequence if the preadjusted treatment plane becomes defocused, due to the patient pressing their head down into the headrest or if the eye position displaces within a passive tracking range of ±0.5 mm. The accuracy of resolving z movements is 0.05 mm.
To account for continuously rapid saccadic eye movements (eg, sudden loss of fixation), the B&L eye tracker has the additional safety feature of a dynamic tracking module that optimizes the laser ablation/minimizes the ablation error. The dynamic eye-tracking module recognizes the speed of eye movements by calculating positional changes (ie, delta-x and delta-y) of the center of the pupil between pictures captured by the eye tracker's video camera system. If the eye speed exceeds 24 µm/milliseconds, whereby the scanner system of the eye tracker cannot safely follow to ensure precise placement of the laser pulse, the eye-tracker software interrupts the pulsing until the eye stabilizes.
Cyclotorsion occurs due to changes in ocular position between standing and recumbent positions. While iris recognition addresses cyclotorsion, intraoperative torsional eye movements also occur. If the eye axis is off by 15º due to intraoperative eye movements, this may result in a significant 50% undercorrection of astigmatism, while a 7º axis change may result in a 25% undercorrection. According to Smith et al, the average axis change from the upright to supine position is 4.3º ±3.5º, while 25% of patients had an axis change of 7º to 16º1. With the active rotational tracker that I use during surgery, I have found that in addition to this positional axis change, there is also an intraoperative rotation amplitude of up to 10º.
Iris recognition on the B&L Zyoptix 100 maps the entire iris and generates a unique Zyoptix identification, providing automatic patient verification and eliminating the risk of mistaken identity. This iris recognition feature eliminates the potential of using the wrong diagnostic (ie, left eye vs right eye or wrong patient file chosen) by checking the digital iris key stored against the treatment file with the actual identified eye at the laser. When mismatched, the treatment will not be allowed to proceed. The Zyoptix 100 also corrects for eye rotation between the Zywave diagnostic measurement (taken in an upright sitting position) and the laser treatment (performed in a horizontal supine position). At the laser, the system can detect overall cyclotorsional angles less than or equal to ±15º. The accuracy of exactly resolving ocular cyclotorsion is ±0.35º between reference images.
The iris recognition eye tracker also compensates for pupil center shift between the dilated and undilated Zywave diagnostic measurements (Figure 1). The dilated pupil is the basis for the treatment calculation to and have obtain enough measured wavefront data in the periphery. With enough data, the optical zone size design is not limited, and higher-order aberration information in dark conditions is obtained. At the laser, it is important to center the ablation pattern based on a functional pupil size in an undilated state, maintaining the visual axis in line through the center of the entrance pupil. For very asymmetric pupil dilations, these center shifts can be up to 500 µm, which—especially when precise positioning of the customized ablation pattern to address higher-order aberrations is applied—has significant impact on correcting the refractive error. Both parameters are expected to positively impact clinical outcomes and represent an important safety feature. The accuracy of exactly resolving pupil center shifts is 0.025 µm.
Bausch & Lomb is the first and currently the only company with an active rotational eye tracker that corrects for intraoperative rotation on a real-time basis by dynamic torsional compensation. It gives an iris-structure–based recognition and continuous recognition with 25 Hz, which is fast enough to follow the rotation of the eye and provide accurate treatment. There is a visual resolution of ±0.35º, and the actual aim of the laser in the ablation process is adjusted on a dynamic basis relative to the iris' position.
The eye tracker consists of static and dynamic rotational components. The static rotational eye tracker compensates for rotation between upright and supine positions using Zywave images as references. The dynamic rotational eye tracker compensates for intraoperative rotation, using reference images of the laser confirming x/y login. The rotational eye tracker compensates for the total torsional error, without a gap beginning at the Zywave until the end of ablation.
B&L's active rotational eye tracker is an important development in the treatment of high astigmatism; ablation of higher-order aberrations, in particular coma and trefoil; and lower-order spherical ablations. Intraoperative cyclotorsional eye tracking provides surgeons with an additional level of safety and can be expected to become the standard of practice in future. The intraoperative active cyclotorsion eye tracking is available only on selected lasers, although it will become a standard feature on B&L lasers in the future.
I have been using the B&L eye tracker with the Technolas 117 laser since 1996. The eye tracker is very easy to use. There are a few things that I do that may be of interest:
1. I do not use a protector, since sometimes if it is brought too close to the pupil, it may confuse the tracker and stop the laser. Instead, I fold the flap on itself (epithelial side up) with the distal flap edge covering the hinge. The tracker works best with this method, and I can concentrate on focus and centration.
2. I check the centration again halfway during the procedure, which occasionally needs fine-tuning.
3. If the laser is not going as smooth as usual, I turn the laser illumination down.n
John So-Min Chang, MD, is the assistant clinical professor at the Chinese University of Shatin, in Happy Valley, Hong Kong. He states that he has no financial interest in the company or products mentioned. Dr. Chang may be reached at johnchang@hksh.hk or +85 22 835 8884.
Up Front | Nov 2006
Active Rotational Eye Tracker Corrects for Intraoperative Rotation
One company currently offers this on a real-time basis by dynamic torsional compensation.
John So-Min Chang, MD
