A Scan Biometry
A Scan Biometry
A Scan Biometry
Author: Rhonda G Waldron, MMSc, COMT, CRA, ROUB, CDOS; Chief Editor:
Timothy Jang, MD more...
http://emedicine.medscape.com/article/1228447-overview
Ultrasound Principles
Sound is defined as a vibratory disturbance within a solid or liquid that travels in a wave
pattern. When the sound frequency is between 20 hertz (Hz) and 20,000 Hz, the sound is
audible to the human ear. To be considered ultrasound, sound waves must have a frequency of
greater than 20,000 Hz (20 KHz), rendering them too high in frequency to be audible to the
human ear.[1] In ophthalmology, most A-scan and B-scan ultrasound probes use a frequency of
approximately 10 million Hz (10 MHz) that is predesigned by the manufacturer. This
extremely high frequency allows for not only restricted depth of penetration of the sound into
the body but also excellent resolution of small structures. This meets unique needs, because,
at times, the probe is placed directly on the organ to be examined, and its structures are quite
small, requiring excellent resolution.
The velocity of sound is determined completely by the density of the medium through which
it passes. Sound travels faster through solids than through liquids, an important principle to
understand because the eye is composed of both. In A-scan biometry, the sound travels
through the solid cornea, the liquid aqueous, the solid lens, the liquid vitreous, the solid
retina, choroid, sclera, and then orbital tissue; therefore, it continually changes velocity.
The known sound velocity through the cornea and the lens (average lens velocity for the
cataract age group, ie, approximately 50-65 y) is 1641 meters/second (m/s), and the velocity
through the aqueous and vitreous is 1532 m/s. The average sound velocity through the phakic
eye is 1550 m/s. The sound velocity through the aphakic eye is 1532 m/s, and the velocity
through the pseudophakic eye is 1532 m/s plus the correction factor for the intraocular lens
(IOL) material.[2] The cornea is not routinely factored in because of its thinness. If one were to
consider 1641 m/s at about 0.5 mm, only 0.04 mm would need to be added to the total eye
length, which in no way alters the IOL calculation.
In A-scan biometry, one thin, parallel sound beam is emitted from the probe tip at its given
frequency of approximately 10 MHz, with an echo bouncing back into the probe tip as the
sound beam strikes each interface. An interface is the junction between any two media of
different densities and velocities, which, in the eye, include the anterior corneal surface, the
aqueous/anterior lens surface, the posterior lens capsule/anterior vitreous, the posterior
vitreous/retinal surface, and the choroid/anterior scleral surface.
The echoes received back into the probe from each of these interfaces are converted by the
biometer to spikes arising from baseline. The greater the difference in the two media at each
interface, the stronger the echo and the higher the spike.[2] If the difference at an interface is
not great, the echo is weak and the displayed spike is short (eg, vitreous floaters, posterior
vitreous detachments). No echoes are produced if the sound travels through media of
identical densities and velocities, eg, young, normal vitreous or the nucleus of a
noncataractous lens, in which the A-scan display goes down to baseline. See the image below.
surfaces are seen as separate spikes to the right of the display. The density of the cataract
determines the need for changing the gain setting due to absorption of the sound. The more
dense the cataract, the higher the necessary gain. Patients who are aphakic require less gain to
prevent merging of the retinal and scleral spikes. Therefore, the gain setting may vary not
only from patient to patient but from one eye to the next in the same patient, depending on
cataract density.
See the image below.
average length eye. In eyes that are shorter or longer than average, this method of measuring
produces an innate error.
When setting the measurement mode to aphakic, 2 gates will be present (on the respective
corneal and retinal surfaces), and the biometer will calculate the distance at a velocity of 1532
m/s, the correct velocity for the aqueous and vitreous.
When setting the measurement mode to pseudophakic, depending on how many
pseudophakic options the equipment possesses, the eye length is calculated using 1532 m/s
for the aqueous and vitreous, then the correction factor for the given implant material is
added. If only one pseudophakic mode option is available, it only will be accurate for
polymethyl methacrylate (PMMA) IOLs.
Routinely using automatic mode on most equipment increases the risk of error because every
biometer will capture poor quality scans. Biometers are programmed to capture any scans
with spikes that are of high amplitude within their given area. However, they often cannot
determine if the spike arose steeply from baseline or if a slope or step is present in the spike
origin. Manual mode is sometimes preferable, in which the examiner presses a foot switch to
capture the scan when it is seen to be of high quality. Equipment varies greatly with some
manufacturers only using a 4-gate system on automatic mode, which means that anterior
chamber depth can be monitored only in automatic mode. If this is the case, automatic mode
is preferable, but the examiner must carefully edit the scans stored by the machine.
Once the eye length is measured, compare it to the precataract refractive error of the patient
to ensure that the readings appear accurate. The precataract refractive error is important
because the cataractous lens changes can induce a more myopic prescription. The reference
range between the right eye and the left eye of the same patient is within 0.3 mm, unless
evidence suggests the contrary (eg, previous scleral buckling, anisometropia, corneal
transplantation, keratoconus, refractive surgery, hypotony).
The average anterior chamber depth is 3.24 mm but varies greatly.[2] If the biometrist is
documenting a shallow anterior chamber depth, examine the medical chart for clinical
correlation of this finding. The average lens thickness is 4.63 mm but this also varies, and,
with cataractous changes, the lens will increase in thickness to as much as 7.0 mm in
extremely dense cases.
The average keratometry (K) reading is 43.0-44.0 D, with one eye typically within a diopter
of each other. Check these readings against the refractive error of the patient for accuracy. If
one eye is found to differ from the other by more than 1 D, immediately begin researching the
cause and alert the physician. For instance, if the patient had refractive surgery, corneal
transplantation, an injury with a resultant corneal scar, or has keratoconus, the K readings
may vary between the eyes. It is rare for the patient to have disparate K readings biologically.
If any of the above eye measurements is found to be unusual, another technician should
recheck the measurements and immediately alert the physician.
Just as precise keratometry and biometry are critical for good surgical outcomes, correct IOL
placement by the surgeon is essential. A 0.19 D postoperative refractive error occurs for every
0.1 mm posterior chamber intraocular lens (PCIOL) displacement. A 0.12 D postoperative
refractive error occurs for every 0.1 mm anterior chamber intraocular lens (ACIOL)
displacement. Lens displacement can also be caused by the patient's ciliary body pushing the
lens out of position rather than by the surgeon's placement of the lens.
Using the correct IOL calculation formula is important for good surgical outcomes. Current
2-variable formulas that are considered the most accurate include the Hoffer Q, SRK/T, and
Holladay I. Two-variable formulas are those that only take into consideration the axial length
and the corneal curvature. Multivariable formulas have proven to be the most accurate due to
more of the eye anatomy being considered.
The Haigis formula is a 3-variable equation, using not only axial length and corneal curvature
but also the anterior chamber depth of the eye. The Holladay II formula is a 7-variable
equation widely thought to be the most accurate formula; it takes into account axial length,
corneal curvature, horizontal white-to-white, anterior chamber depth, lens thickness,
precataract refractive error, and age of the patient.
Predicting lens position is one of the most common causes of a postoperative surprise; by
taking more of the eye anatomy into account, this can be more accurately predicted. For
average-length eyes with average Ks, these formulas give almost identical calculations.[3]
However, when the eye is small, formula selection is more critical. In eyes that are less than
22 mm in length, the Hoffer Q and the Holladay II IOL Consultant formulas are the most
accurate. For long eyes, the SRK/T and the Holladay II IOL Consultant formulas are the most
accurate.
The Holladay II IOL Consultant formula is also the only formula that calculates for
piggyback IOL procedures (ie, when 2 IOLs are implanted, which may be necessary when the
eye is so small that 1 implant does not contain enough converging power, or when a
piggyback lens is inserted to correct a postoperative surprise that results from the first
implant) and is highly recommended for patients that have had previous refractive surgery.
certain the axial length is correct is by verifying with both immersion A-scan and B-scan
biometry.
the correct axial length is 23.72 mm. If the IOL is acrylic, the correct axial length is 23.52
mm. If it is low-velocity silicone, the correct length is 22.52 mm.
When any new implant material is produced, the correction factor can be calculated using the
CT of the IOL and the sound velocity of the material at body temperature (35C), which must
be supplied by the manufacturer. The formula for this calculation is the CT multiplied by 1
minus 1532 divided by the velocity of that material, or CT X (1-1532/vel).[2] For example, if
the IOL has a CT of 0.8 mm, and the sound velocity of the material is found to be 1040 m/s,
then 0.8 X (1-1532/1040) = 0.8 X -0.473 = -0.378. Therefore, the correction factor for this
eye is -0.378 from the length obtained on aphakic setting.
Another problem arises when the implant material is unknown. If the patient has a wallet card
showing the implant used, the manufacturer may need to be called to determine implant
material if the model is unfamiliar to the examiner. If the patient does not have a wallet card,
contact the surgeon's office to determine the implant used. If the patient cannot recall the
surgeon's name, it may be necessary to contact a family member in the case of an IOL
exchange. However, the implant reverberation pattern may prove helpful because PMMA has
a longer chain of reverberation echoes, followed by acrylic and then silicone.
Corneal compression is
demonstrated in the A-scan on the right. Note the more shallow anterior chamber depth of
2.63 mm as compared to the scan of the same eye on the left, with an anterior chamber depth
of 3.20 mm, indicating 0.57 mm of corneal compression. Note also that the total eye length is
shortened from 24.60 mm in the scan on the left to 24.18 mm in the scan on the right. This
error would result in an unwanted postoperative refractive error of about -1.25 D.
The second most common error is misalignment, either by not obtaining perpendicularity to
the macular surface or by not directing the sound beam through the visual axis.
Perpendicularity to the macular surface is achieved when the retinal spike and scleral spike
are of high amplitude, and the retinal spike arises steeply from baseline. No sloping of the
retinal spike should be present and no jags, humps, or steps should be present on the
ascending edge of that spike.
See the image below.
Misalignment demonstrated by
the decreased amplitude of the posterior lens spike (arrow). When either of the lens spikes is
too short, the sound beam is aligned at an angle through the lens rather than through its
center, and thus not aligned along the visual axis.
Misalignment along the optic nerve is an error that is easily recognized, since the scleral
spike will be absent. The retinal spike will be present and of high amplitude and can even
appear steeply rising, but, if the scleral spike is not as high in amplitude as the retina, the
sound beam is misaligned along the nerve. No sclera is present at the optic nerve; the sound
beam is passing through the nerve cord with only short amplitude echoes present, because the
sound beam is striking blood vessels within the nerve cord. In the normal eye, there will
generally not be a great difference in axial length when aligned along the optic nerve, but, in
cases of a full optic disc, papilledema, or optic disc drusen, this will result in an erroneously
short axial length measurement. In cases of optic nerve cupping, as seen in glaucomatous
eyes, this error results in an erroneously long axial length measurement.
See the image below.
outward into the sclera, most commonly in the posterior pole. This causes the macula to be
sloped in configuration, which in turn causes reflection of the sound beam away from the
probe tip and a poor retinal spike. It is impossible to obtain perpendicularity to a macular
surface that is sloped; thus, it is impossible to obtain a proper retinal spike. Also because of
the sloped surface, the measurements will be not only long but extremely variable. Patients
must be alerted that because their eye is misshapen, they have a higher risk of the
postoperative result not being as accurate as a patient with a normally shaped, round globe.
In these cases, a B-scan examination is necessary, with a horizontal macular scan performed
and the axial length measured from the B-scan. The proper B-scan probe position for this
measurement is to have the patient in primary gaze with the B-scan probe face (using a
generous amount of gel-type tear solution) centered on the corneal vertex and the probe
marker aimed nasally. (The probe marker is either a line or a dot on one side of the probe,
near the probe face.)
When this probe position is achieved, the B-scan display will demonstrate the epithelial and
endothelial corneal echoes centered to the left, the posterior lens surface just to the right, and
the optic nerve void just above the center to the far right. The macula will lie centered on the
right, about 4.5 mm below the center of the optic disc. Simply place calipers on the vertex of
the epithelial corneal echo and on the macula to measure the axial length at average sound
velocity of 1550 m/s. Compare this axial length measurement to the various biometry
measurements, and use the measurement that has the most comparable vitreous length in the
IOL calculation, preferably within 0.1 mm.
See the image below.
posterior spike, since the retina should lie back into this position once repaired. The examiner
should inquire if the surgeon plans to place a scleral buckle around the globe to repair the
detachment, and if so manually add another 0.5-1.0 mm to the total eye length to account for
lengthening of the globe by the buckle postoperatively.
Another method, referred to as the topography method, as published by Wang et al, involves
measuring the eye with topography, multiplying that number by 1.114, and then subtracting
the correction factor of 6.1.
Another is the method from Wake Forest University, sometimes known as the corneal bypass
method. In this method, the original K's prior to the refractive surgery are entered into the
calculation formula along with today's axial length measurement. In order to achieve
emmetropia, however, the refractive target entered into the calculation is the amount of
correction from the refractive surgery. In their study, all patients had better outcomes than
using the standard clinical history method and the implants were calculated to be closer to the
perfect IOL power.[7]
Another method faring well in recent studies is the Masket Formula.[8] This formula involves
an adjustment in the final IOL power rather than an adjustment of the keratometry readings in
the refractive surgery patient. In Dr. Masket's study, keratometry and biometry were
performed with the IOL Master as with standard cataract patients, and then the following
formula was used to adjust the calculated power from those measurements: IOL adjustment =
LSE X (-.326) + 0.101, where LSE = the spherical equivalent of change after laser vision
correction.
If the patient had myopic correction, the LSE should be a negative number, and if hyperopic
correction, a positive number. In other words, if a myopic refractive patient has standard
calculations of +16.0 D, and LSE was 6 D of myopia, -6 X (-0.326) + 0.101 = +2.057, which
should be added to the calculated +16 D for a final IOL power of +18 D. If a patient had
hyperopic refractive surgery and the calculation yielded a power of +22 D, and LSE was 3 D
of hyperopia, +3 X (-0.326) + 0.101 = -0.877, which is added to the calculated implant power
of +22 D, for a final IOL power of +21 D.
An online calculator is recommended for these patients because mathematical errors can
occur. The most popular calculator is from the American Society of Cataract and Refractive
Surgeons. This calculator is constantly maintained and contains the most highly
recommended methods. Multiple methods are embedded with instant results for comparison,
with the user merely inputting the required measurements. The methods used and their results
are divided into 3 columns: Those that require history for both the change in corneal
curvature and refractive change, those that require history for the refractive change only, and
those that require no history at all. The general consensus is that the nonhistorical methods
tend to outperform those that require history. Because records are difficult to obtain in many
cases, this is a tremendous relief. These patients should be forewarned that they have a higher
risk of an imperfect resultbecause of their previous refractive surgery and that another
procedure may be required afterward to get them back to their desired visual acuity. These
procedures could include an IOL exchange, a piggyback lens, or more refractive surgery.
Velocity Conversion
The velocity conversion equation is helpful in many biometric circumstances, including cases
of silicone oil in the vitreous, using an incorrect velocity setting on the biometer or measuring
an eye filled with silicone oil.[2] The equation is as follows:
Velocity (correct)/Velocity (measured) X Apparent Length = True Length
In the event of an incorrect eye type setting, this equation is quite simple to use and will
preclude the need for the patient to return for repeat measurements. For example, an aphakic
eye was measured incorrectly with the phakic average setting. The correct velocity for this
eye is 1532 m/s. The velocity used was 1550 m/s. If the eye length obtained was 24.1 mm on
the phakic average setting, then 1532/1550 X 24.1 = 23.82 mm = true eye length.
For eyes that have silicone oil in the vitreous cavity, this formula is used to determine the true
vitreous length. Silicone oil is used surgically to replace the vitreous in some cases of
recurrent retinal detachment and macular hole. The oil is removed several months later, but
while in the eye, it causes a cataract, often requiring the removal of the cataract at the time of
oil removal. The velocity conversion equation is necessary because the velocity through
silicone oil is only 980 m/s, much slower than the 1532 m/s the biometer uses in determining
vitreous length. Therefore, the biometer measures the vitreous erroneously long, and,
consequently, the total length also is erroneously long. In a 4-gate system with silicone oil,
the ACD and lens thickness are accurate, so they should be subtracted from the total length to
isolate the erroneous apparent vitreous length. Then, the formula used is as follows:
980/1532 X Apparent Vitreous Length = True Vitreous Length
The corrected vitreous length is now added back to the anterior chamber depth and lens
thickness for an accurate total eye length. Biometry is best performed with these patients
sitting upright so the bubble will not separate from the retinal surface, causing a spike to arise
at the back of the bubble, which can be confused easily with the retinal spike. If an aphakic
patient has silicone oil in the eye, it must be determined whether or not the oil is in the
anterior chamber or only the posterior chamber. If the oil is only in the posterior chamber, the
ACD should be subtracted from the total length, the vitreous length corrected using the
velocity conversion equation, then added back to the ACD.
See the image below.