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The Effect of Axial Length on Laser Spot Size and Laser Irradiance
Michael Stur, MD;
Siamak Ansari-Shahrezaei, MD
Arch Ophthalmol. 2001;119:1323-1328.
ABSTRACT
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Objective To determine the effect of the axial length of the eye on laser spot
size and irradiance.
Design Experimental study using a calibrated Gullstrand-type model eye.
Methods The model eye, which was fitted with a scale of half circles in the
center of the artificial fundus, was first examined using 2 different fundus
imaging systems, then with a setup of a slitlamp, 2 indirect condensing laser
lenses, and a laser unit with a spot size of up to 7 mm. The axial length
of the model eye was set to different values ranging from 20 to 31 mm, and
the magnifications of the fundus imaging systems and the laser lenses were
calculated and compared for a treatment spot with a diameter of 4 mm. The
laser irradiance for treating the spot at different axial lengths was also
recorded.
Results Whereas the magnification of a fundus imaging system is inversely related
to the axial length, the laser spot size is directly related to axial length
when using indirect condensing laser lenses. Therefore, the changes of magnification
produced by axial ametropia are mostly compensated, so that the intended size
of the treatment spot is obtained even in eyes with a high axial ametropia.
The laser irradiance, on the other hand, has a significant variation for the
observed range of the axial length.
Conclusion Axial length has a significant effect on laser spot size and laser irradiance.
Clinical Relevance The effect of axial length on laser spot size and laser irradiance may
be ignored when administering photodynamic therapy with verteporfin but has
to be considered for transpupillary thermal treatment of choroidal neovascular
lesions.
INTRODUCTION
THERE ARE several different means to deliver laser power to the ocular
fundus. Most often, a contact lens is placed on the cornea, but laser may
also be applied with the help of a binocular indirect ophthalmoscope, or a
fiberoptic or a noncontact lens. The traditional method of placing transpupillary
laser burns with the help of a contact lens uses small spot sizes with a diameter
ranging from 50 to 500 µm. These spot sizes are produced by means of
the optic of the laser delivery attachment to the slitlamp. The real size
of the spot on the retina is identical to the reading on the laser adapter
only when Goldmann-type contact lenses with a magnification factor of 1 are
used. Popular laser lenses, eg, Mainster Standard and Wide Field (Ocular Instruments,
Bellevue, Wash) or Transequator and Quadraspheric (Volk, Mentor, Ohio), have
magnification factors higher than 1, which alter the spot size accordingly.
These magnification factors are not constant but change according to the axial
length of the eye.1 When using one of these
lenses for laser treatment, the experienced laser surgeon will notice the
actual size of the laser spot and change the energy setting until the desired
intensity of the laser burn is achieved. Thus, there is no need to know the
true size of the laser spot on the retina when performing traditional retinal
laser photocoagulation.
Recently, new treatment modalities using laser application have been
introduced. Photodynamic therapy (PDT) with verteporfin (Visudyne; Novartis
Ophthalmics, Atlanta, Ga) has recently been approved by the US Food and Drug
Administration and other regulatory agencies in Europe and North America for
the treatment of predominantly classic subfoveal choroidal neovascular membranes
(CNV) in age-related macular degeneration (AMD).2
Transpupillary thermotherapy (TTT) for occult and classic CNV is currently
undergoing investigation in several trials regarding its safety and efficacy.3 Both new methods are very different. Photodynamic
therapy is a medical treatment that uses laser activation of a photosensitive
drug, and TTT is a thermal treatment that uses laser-power absorption in the
choroidal pigment. Both methods use large laser-spot sizes to cover the whole
CNV during a long (TTT, 60 seconds; PDT, 83 seconds) laser exposure. Thus,
it is necessary for both new methods to determine the size of the CNV to be
able to cover the whole lesion during treatment. At present, the size of the
lesion is calculated by determining the largest linear dimension of the lesion
with a ruler on the film negative of the angiogram and dividing the obtained
length by the magnification factor of the camera provided by the manufacturer.
This simplified calculation ignores the fact that the magnification of an
object in the ocular fundus by the fundus camera is not constant but changes
according to the refractive status of the eye.4
Several formulas have already been published that allow the exact calculation
of the size of an object in the ocular fundus. They require the knowledge
of exact biometric data of the examined eye (refraction, corneal curvature,
and axial length) and the fundus imaging system, which might have a constant
magnification if it is a true telecentric device, or a magnification changing
according to the refraction of the examined eye if it is not telecentric.5-7
Another problem with PDT and TTT is that the effect of the laser treatment
cannot be visualized as well as the effect of conventional photocoagulation,
which provides burns of mild, moderate, or high intensity. In PDT, there is
no immediate change in the ophthalmoscopic appearance of the lesion. In TTT,
there should be no change or only a minimal whitening of the lesion at the
end of the laser exposure. This raises the question whether it might be useful
to know the exact amount of laser irradiance applied to the CNV, and to investigate
whether the laser-spot size and the laser irradiance are affected by the axial
length of the treated eye, especially when using indirect condensing laser
lenses (eg, Mainster Standard and Wide Field or Volk Transequator). These
lenses produce a real inverted image of the fundus and, therefore, have optical
characteristics similar to those of a fundus camera. To answer these questions,
we performed an experimental study using a calibrated model eye of the Gullstrand
type and investigated the effect of axial length on the treatment variables
currently recommended for verteporfin therapy.
MATERIALS AND METHODS
The objectives of our methods were as follows: (1) to calculate the
correlation between image magnification and axial length when using 2 different
fundus imaging devices, the fundus camera (CF-60 UV; Canon, Tokyo, Japan)
and the scanning laser ophthalmoscope (SLO 101; Rodenstock Instruments, Düsseldorf,
Germany); (2) to compare the results with the requirements of establishing
the correct laser-spot size with when using the 2 Mainster lenses; and (3)
to determine the variation of laser irradiance when using the 2 Mainster lenses
with different axial lengths.
MAGNIFICATION OF THE FUNDUS IMAGING SYSTEMS
A Gullstrand-type model eye8 with a laser-etched
scale of concentric half circles in the center of the artificial fundus (Figure 1) was filled with distilled water
and placed in front of each of the 2 fundus imaging devices. The axial length
was set at different values ranging from 20 to 31 mm, corresponding to a refractive
range of -16.5 to +12.5 diopters (D). At each value, we obtained an
image of the scale with the 40° image-field setting and measured the diameter
of the smallest half circle, which has a true size of 4 mm. By dividing the
measured length by 4, we obtained the actual magnification of the fundus image
at the different axial lengths, and by dividing the measured lengths by the
camera magnification factor, we calculated the laser-spot size according to
the treatment protocol of the Treatment of Age-Related Macular Degeneration
With Photodynamic Therapy (TAP) study group. Similar setups can be used to
calculate the fundus camera correction factor p (degrees
per millimeter) and thus to calculate the exact magnification using the Littmann
or Bennett formulas. Since we only wanted to compare the magnification factors
obtained within our setup, these last 2 calculations will be described in
another report (Siamak Ansari-Shahrezaei, MD, unpublished data, December 1998).
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Figure 1. Image of the scale of half circles
as obtained using the 40° field of the fundus camera (CF-60 UV; Canon,
Tokyo, Japan).
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MAGNIFICATION OF THE MAINSTER STANDARD AND WIDE FIELD LENSES
The model eye was placed in front of the laser (Opal PDT-Laser; Coherent,
Palo Alto, Calif). A Mainster Wide Field lens and a Mainster Standard lens
whose cups were filled with 2% methylcellulose (2% Methocel; Cibavision, Munich,
Germany) were placed on the cornea, the aiming beam was carefully focused
on the scale in the fundus of the model eye, and we tried to fit the spot
of the aiming laser into the half circle with a diameter of 4 mm. After fitting
the aiming-beam spot into the half-circle scale, we recorded the reading from
the laser console, which provided the setting of the spot size required at
each axial length and the laser power setting provided for this spot size.
The actual spot-size magnification of the laser lens was calculated using
the following formula:
,
where Ma is the actual magnification; Ms, the standard magnification
provided by the manufacturer of the lens; S, the actual diameter of the half
circle; and R, the spot-size reading obtained at each axial length from the
laser console.
STATISTICS
For statistical calculations, we used commercially available software
(Statview 5.01; SAS Institute, Cary, NC) for descriptive statistics and bivariate
scatter charts.
RESULTS
The results of our measurements are listed in Table 1. Correlations between axial length and magnification of
each device are presented in Figure 2
and Figure 3. The fundus camera
is a nontelecentric device and uses compensation lenses for high ametropia,
which results in a nonlinear relation of axial length and magnification and
additional changes of magnification when the compensation lenses are used.
The scanning laser ophthalmoscope and the Mainster lenses are also not pure
telecentric imaging systems, but they show an almost linear relation between
axial length and magnification for an axial length of 20 to 29 mm and can
be considered telecentric within this range. Figure 4 provides the correlation between the measured spot sizes
of both imaging devices and the required spot size for covering the 4-mm spot
at each axial length with both laser lenses. Figure 5 and Figure 6
show the differences between required and calculated spot sizes in relation
to axial length. When using the Mainster Standard lens, there is a maximum
deviation of -440 µm at a refraction of +10 D with the fundus
camera, whereas there is a maximum deviation of -300 µm at -10
D with the scanning laser ophthalmoscope. These deviations result in a calculated
spot size smaller than that required. When using the same spot size calculations
and the Mainster Wide Field lens, the differences between calculated and required
spot size will only produce larger spot sizes than those intended. The variation
of laser irradiance when using a Mainster lens for treating a 4-mm spot at
different axial lengths of the model eye is also shown in Figure 7 and listed in Table 1. The laser power required for PDT to treat a spot of 4 mm with
a laser irradiance of 600 mW/cm2 is 75 mW. According to our measurements,
the laser power used to treat a 4-mm spot at different axial lengths varies
from 47 to 116 mW for the Mainster Standard lens and 39 to 92 mW for the Mainster
Wide Field lens. This translates into a variation of laser irradiance between
a minimum of 312 and a maximum of 928 mW/cm2 or a laser fluence
of approximately 25 to 75 mJ/cm2 for axial lengths ranging from
20 to 31 mm.
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Variation of Magnification, Spot Size, and Laser Irradiance for Different
Values of Axial Length*
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Figure 2. Actual magnification of the fundus
image at different axial length values using the 40° field of the fundus
camera (CF-60 UV; Canon, Tokyo, Japan) and of the scanning laser ophthalmoscope
(SLO 101; Rodenstock Instruments, Düsseldorf, Germany). The magnification
of the scanning laser ophthalmoscope is divided by 10 to be in a comparable
range.
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Figure 3. Actual laser spot magnification
of the Mainster Standard and the Mainster Wide Field laser lenses (Ocular
Instruments, Bellevue, Wash) at different axial length values.
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Figure 4. Variation of spot sizes calculated
with the fixed-magnification factor for fundus images obtained at different
axial length values. Measurements were obtained using the fundus camera (CF-60
UV; Canon, Tokyo, Japan) and the scanning laser ophthalmoscope (SLO 101; Rodenstock
Instruments, Düsseldorf, Germany).
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Figure 5. Differences between the spot sizes
calculated from the images obtained with the fundus imaging systems and the
spot size setting of the photodynamic therapy laser when using the Mainster
Standard lens (Ocular Instruments, Bellevue, Wash).
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Figure 6. Differences between the spot sizes
calculated from the images obtained with the fundus imaging systems and the
spot size setting of the photodynamic therapy laser when using the Mainster
Wide Field lens (Ocular Instruments, Bellevue, Wash).
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Figure 7. Variation of laser power when
treating a spot size of 4 mm at different axial length values. Measurements
were obtained using the Mainster Standard and Wide Field lenses (Ocular Instruments,
Bellevue, Wash).
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COMMENT
It is well known that the calculation of the true size of an object
in the ocular fundus depends on the knowledge of the refraction, corneal curvature,
and axial length of the eye and the magnification of the fundus imaging system.5-7,9-10
Nevertheless, exact calculations of the size of fundus lesions have until
now mainly been performed to calculate the true area of the neuroretinal rim
of the optic disc11-13
or the size and volume of intraocular tumors.14
Although most laser surgeons have noticed, when using indirect condensing
laser lenses, that laser spots are significantly larger in myopic eyes than
in hypermetropic eyes, these findings were of little importance, since the
actual size of a laser spot is also affected by the accommodative status of
the laser surgeon,15 the optical quality of
the refractive media of the treated eye,16
and the optical quality of the laser delivery system.17
Most experienced laser surgeons are aware of these factors and include them
when changing the optimal setting for transpupillary laser photocoagulation
of the retina.
In the phase 1/2 trial investigating the correct dose of verteporfin
and laser fluence for the treatment of subfoveal CNV,18
and in the TAP2 and Verteporfin in Photodynamic
Therapy (VIP)19 trials, the transparent overlay
sheet generated by the Wilmer Reading Center for the Macular Photocoagulation
Study trials was used to measure the size of the lesion on the film negative.
This sheet uses circles of different diameters to represent areas of different
disc size and a small ruler with a total length of 2 cm and subunits of 0.1
mm. To calculate the largest linear dimension of the lesion, the measurement
performed with this ruler was divided by 2.5, when the Zeiss 30° (FF 4
fundus camera; Carl Zeiss, Oberkochen, Germany), the Topcon 35° (TRC-50
series fundus cameras; Topcon, Tokyo), or the Canon 40° image fields had
been used for obtaining the images, since the respective manufacturers indicated
that their cameras all had a magnification factor of 2.5 at these field angles.
The PDT protocol adds 1000 µm to the largest linear dimension to provide
a treatment area that would include the whole lesion and a safety margin of
500 µm. On the other hand, the aiming beam of the treatment laser used
a green or red laser and produced a bright spot exactly the size of the treatment
area, which allowed easy identification of lesion details. Thus, it was quite
easy for the investigator to verify that the treatment spot calculated according
to the study protocol indeed covered the whole lesion. In the TAP trial,2 the inclusion criteria required the study eye to have
an ametropia of less than 6 D, which prevented any significant deviation of
the measured lesion size due to an abnormal axial length of the study eye.
In the VIP trial,19 eyes that were seen with
a subfoveal classic CNV due to pathologic myopia were also included. Many
investigators in the VIP pathologic myopia trial noted that when examining
angiograms obtained from myopic eyes, the magnification of the image was much
different from those of eyes treated for subfoveal CNV in AMD. However, when
using the Mainster Standard and Mainster Wide Field lenses, which were the
lenses preferably used, the treatment-spot size always fitted the lesion in
a similar way as in all nonmyopic eyes. This experience prompted us to investigate
the exact relation of the lesion size measured using different fundus imaging
systems with the actual treatment spot size obtained usingh the Mainster lenses
and a laser commercially available for PDT.
The results of our study indicate that the current method of using a
fixed magnification factor does not produce clinically significant errors.
The observed maximum deviation of -440 µm with the fundus camera
and -300 µm with the scanning laser ophthalmoscope, when using
the Mainster Standard lens, is still much less than the 1000-µm margin
added to the largest linear dimension. Therefore, even in the extreme situation
of a deviation of -440 µm, there is still a safety margin of 560
µm, which should be sufficient for a complete treatment of the lesion.
The Gullstrand-type model eye used for our study has a fixed corneal curvature
and power of the intraocular lens, providing a model for ametropia only by
varying the axial length of the model eye. Since the axial length is the most
important factor for the change of the magnification due to ametropia,6 the deviations recorded in our setup are therefore
most likely the maximum deviations to be expected in vivo, where a wide variation
of corneal curvature, refractive power of the crystalline lens, and total
axial length provide the refractive status of the patient's eye. Our results
stress the importance of a correct definition of all lesion components (eg,
classic and occult CNV, pigment epithelial detachment, blocked elevated hypofluorescence,
and blood) to achieve a treatment-spot size that covers the whole lesion.
On the other hand, the use of a fixed magnification factor with a Goldmann-type
fundus lens, which has a magnification independent of axial length,20 would produce a treatment spot too small to cover
the whole lesion in myopic eyes and a treatment spot much larger than necessary
in hyperopic eyes. Thus, indirect condensing laser lenses (Mainster Standard
and Wide Field, Volk Transequator, and PDT lenses) should be preferred for
PDT treatment to achieve a correct treatment spot size. Still, it might be
useful to examine other fundus imaging systems and other laser lenses for
their deviations of calculated and required laser spot size.
The laser irradiance observed in our experimental setup suggests that
in myopic eyes with an axial length of 31 mm, the irradiance is more likely
close to 300 mW/cm2, which is only half the irradiance intended
by the TAP protocol (600 mW/cm2).2
On the other hand, the phase 1/2 trial demonstrated that there is indeed a
strong treatment effect within a range of 300 to 1200 mW/cm2, and
the 600-mW/cm2 dose was chosen because there was a maximum of vision
improvement at 4 and 12 weeks after treatment. The VIP study group has also
reported the first results of the randomized treatment of subfoveal CNV in
pathologic myopia. The percentage of eyes with less than 3 lines of visual
loss was significantly higher in the treatment than in the placebo group (86%
vs 66%), a difference of the same magnitude as in the 12-month results of
the TAP study.19 This indicates not only that
a slightly reduced laser irradiance in myopic eyes is sufficient for a treatment
effect similar to the results of the TAP trial, but also suggests that there
might be a different situation when activating the photosensitzer in myopic
eyes, where the choroid is significantly thinner and the CNV is much smaller
than in eyes with classic CNV due to AMD. Thus, the current protocol for verteporfin
therapy seems to provide an optimized laser irradiance also for eyes with
high axial myopia, although the influence of a reduced laser irradiance on
the outcome of verteporfin therapy should be investigated further in future
studies. On the other hand, we cannot comment on the effect of the variation
of laser irradiance caused by variation of the axial length when using other
photosensitizers for PDT for choroidal neovascular lesions.
The effect of the axial length on laser irradiance when using indirect
condensing laser lenses might be much more important when considering TTT.
At present, no standardized treatment protocol for TTT exists, and although
the first reported study used a Goldmann lens for TTT,3
others might prefer to use wide-field indirect condensing laser lenses to
cover the whole lesion with the maximum spot size of 3 mm currently available
with the laser adapter for TTT. Since most eyes currently recruited for TTT
are eyes with AMD, and since AMD is associated with hypermetropia, it is likely
that in some hyperopic eyes with a short axial length the laser irradiance
during TTT using an indirect condensing laser lens will be significantly higher
than when using the same spot and the same laser power in an emmetropic eye.
Therefore, we recommend the use of the Goldmann-type lens for every TTT application,
but also would remind users that any lesion size calculated with a fixed magnification
factor cannot be used for treatment with a Goldmann-type lens. This setup
would require the knowledge of the axial length of the eyes considered for
treatment and the calculation of the true size of the lesion using the formulas
provided already by several authors.6-7
AUTHOR INFORMATION
Accepted for publication February 1, 2001.
Corresponding author and reprints: Michael Stur, MD, Department of
Ophthalmology, University of Vienna Medical School, Allgemeines Krankenhaus,
Währinger Gürtel 18-20/8i, A-1090 Vienna, Austria (e-mail:
michael.stur{at}akh-wien.ac.at).
From the Department of Ophthalmology, University of Vienna Medical
School, Vienna, Austria. Both authors have no financial interest in any of
the products discussed in this article.
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