Overview of Ultrasound Biometry
Biometry is a method used to measure certain parts of your eye, like the power of the cornea, and the size (called the axial length) of the eye among others. Doing these measurements helps doctors figure out the perfect power of the lens they need to put inside your eye during a surgery (intraocular lens power).
How big your eye is changes from person to person, but most people have an average axial length of 23.5 mm. People who are farsighted have shorter axial lengths, while people who are nearsighted have longer axial lengths.
If the axial length is measured wrong by just 1 mm, it can lead to a change in lens power by 2.5 (2-3) Diopters, the unit used to measure the power of the lens (the ‘D’ in your eyeglass prescription). Interestingly, shorter eyes experience a change of around 3.75 Diopters in lens power for the same error. Also, if the axial length changes by 1 mm, the error in vision (called the refractive error) changes by around 2.88 Diopters.
All these tiny details and corrections become really important when you have a cataract surgery. That’s a surgery where they need to replace the lens of your eye, so these measurements help make sure they get the new lens just right. This surgery is usually performed by eye doctors called ophthalmologists.
Anatomy and Physiology of Ultrasound Biometry
Ultrasound biometry is a method that uses sound waves to measure parts of the eye. This procedure involves a device with a special crystal that vibrates to create high-frequency sound waves. These waves enter the eye and bounce back when they hit different parts of the eye. The time taken and speed at which these sound waves return help calculate the length of your eye.
This ultrasound can be done in two main ways. The first method involves the device touching the surface of your eye. A second method, called the immersion technique, uses a shell filled with saline, a kind of salt solution, placed over the eye. The device then doesn’t touch your eye directly but measures through this salt solution. Results from the immersion method are generally more reliable, especially in children who have cataracts.
While a good ultrasound involves minimally varying measurements, physicians also look for specific echoes or “spikes” on the ultrasound result from different parts of the eye. These spikes help assure the accuracy of the measurement.
Interestingly, speed of the sound waves changes depending on where they are in the eye. This happens because different parts of the eye have different densities.
The ultrasound biometry competes with another method called optical biometry, which involves the use of light instead of sound. Studies show both methods are equally effective, although optical biometry is often better for patients who have a certain type of eye condition, like oil filled eyes or a bit of an outpouching at the back of the eye.
Optical biometry uses similar principles as ultrasound biometry but uses light waves instead. Two well-known techniques—Laser partial coherence interferometry (PCI) and Optical low-coherence reflectometry (OLCR) are used. PCI, for example, involves a laser that creates light beams. When the light travels through the eye and reflects back, it interferes with a mirrored part of the device. Using this interference, the machine measures the length of the eye from the front surface of the cornea to the back part of the eye called the retina. However, certain conditions, like dense cataracts, could interfere with the quality of optical biometry because they block the transmission of light.
In conclusion, both ultrasound and optical biometry are effective ways to measure the eye, with each one having its unique benefits and limitations. Your physician will decide which method is more suitable for your particular situation.
Why do People Need Ultrasound Biometry
Understanding the size and shape of your eye is very important when planning for surgeries like cataract removal, lens replacement, or to correct your vision. This is often done through a process called biometry. Biometry uses measurements of your eye to figure out the right strength of lens to put in during surgery. And don’t worry, these techniques have proven to be accurate and reliable.
For normal eye surgeries, two types of biometry are often used: ultrasound and partial coherence interferometry. However, when the eye has a severe cataract (a clouding of the lens), corneal scarring (scars on the front of your eye), or swelling, it might be tough to measure the length of the eye using partial coherence interferometry. In this case, ultrasound biometry is helpful. Increasing the ultrasound sensitivity can help measure the eye length through these conditions.
Knowing your eye’s length is especially important in children undergoing lens replacement, as a slight shift can change the lens strength needed. For small eyes (length less than 20mm), a change in eye length by just 1mm can cause a larger shift in lens strength.
It’s also interesting to note that in a study looking at eye measurements before and after surgery, more than half of the errors in predicted vision correction came from inaccuracies in measuring the length of the eye. Other aspects, like the shape of your cornea and the depth of the front of your eye, also played a role, but were less significant.
During operations, devices for measuring the eye can also ensure that everything is going smoothly. The goal after cataract surgery is to achieve perfect vision, also known as emmetropia, but we generally consider surgery successful if your vision is corrected within one diopter (a measure of vision) of perfect.
Patients with eye cancers also make use of biometry, as it can provide the doctor additional information for diagnosis.
When a Person Should Avoid Ultrasound Biometry
There are certain situations where ultrasound biometry (a technique using sound waves to measure your eyes) may not be appropriate. One of these situations is if there’s an active eye infection. Eye infections could include conditions like a corneal ulcer, which is a sore on the clear front surface of the eye, or conjunctivitis, commonly known as “pink eye”.
Equipment used for Ultrasound Biometry
Ultrasound biometry is a technique that uses a special tool called an A-scan probe. This probe is used to create a sort of ‘sound picture’ of your eye, which helps your doctor understand the specific characteristics of your eye more clearly.
In one type of this technique, known as ‘contact ultrasonic biometry’, the tip of the probe touches the central part of your eye, which is called the cornea. In another type, called ‘immersion ultrasonic biometry’, a sort of ‘eye cup’ is placed over your eye while you’re lying down, filled with a harmless saline solution. Once it’s filled, the A-scan machine automatically collects the data it needs, creating what is known as an amplitude scan or A-scan.
Each peak or spike on this A-scan corresponds to an area of your eye where the speed at which sound travels changes. Depending on the specific method used, the first peak usually shows the probe interacting with the cornea, followed by two peaks corresponding to the front and back surface of the lens, then the retina and finally the sclera, which is the white outer layer of your eye. Areas like the pupil show no echo, while the retina and sclera create high spikes. The difference between the front of the cornea and the retina gives the length of the eye.
The reading will be best if the machine is set effectively, which includes setting the correct velocity for your particular circumstances, including if you have a cataract, have had cataract surgery, or have an artificial lens implanted (pseudophakia). The peaks from the cornea, lens surface, and retina should be tall and sharp. The retina peak should rise sharply without any breaks. If the alignment is off, the final peak will be missing, a sign that the scan is off target. A strong final peak should be just a bit behind the retinal peak. The orbital peak creates a series of spikes that slowly decrease. The gain should be set at the lowest level that still gives a good reading. On average, a score of 5 to 10 is most reliable and gives the smallest amount of variation (less than 0.06 mm). Probing the eye with too much force can compress the cornea, which should be avoided.
The scan should be repeated if there is a big difference in eye length between the two eyes (more than 0.30 mm) or if repeated measurements vary by more than 0.2 mm. The most accurate scan, with the maximum depth of the front chamber of the eye (the part between the cornea and the lens) and the eye length, should be used. This helps avoid any compression caused by directly touching the cornea with the probe during the measurement.
Who is needed to perform Ultrasound Biometry?
Cataracts can negatively impact your sight and overall health, which can cause difficulties in day-to-day life. Therefore, it’s important for anyone with cataracts to understand what they are, what complications can occur, what treatments are available, and why it’s important to get treatment quickly. In addition, it’s necessary to have regular check-ups even after treatment because cataracts tend to be a long-term condition.
One way to diagnose and monitor cataracts is with ultrasound biometry. This is a procedure that uses sound waves to create a picture of your eye. The process requires specialized knowledge and skills since it’s greatly affected by how it’s done. In contact ultrasound biometry, the ultrasound probe is positioned in the center of the cornea (the clear front part of the eye). The patient is asked to look at a red light at the end of the probe. One downside of this type is that it requires actual contact with the cornea, which could increase the infection risk or lead to minor scratches.
Furthermore, during this type of scan, applying force to the cornea could affect the results. After the scan, the doctor will analyze the peaks for the cornea, lens, and back of the eye (retina). An ideal measurement would be if three scans have less than 0.1 mm difference among them and they show sharp, high-rising spikes of the lens and retina against clear spaces.
If there are any issues with the scan, such as too much force applied to the cornea, corrections are made using high-quality scans and a personalized adjustment factor while calculating the lens power for cataract surgery. The measurements can vary a bit depending on how the ultrasound is done, which can affect the lens’s position after cataract surgery. This can lead to some remaining vision problems. In comparison, immersion scans, where the scan is done without touching the eye, can be more reliable as they don’t come with the risk of accidentally applying too much pressure and resulting in abnormally low measurements of the eye length. In contact ultrasound biometry, usually, the scan that shows the largest-depth front chamber of the eye is used provided other ideal scan features are present, because this suggests the least amount of pressure on the eye.
Preparing for Ultrasound Biometry
Before the procedure starts, the doctor will explain everything to the patient and apply a special kind of numbing medicine to the area where the procedure will take place. It’s important for the doctor and the patient to work together to make sure the measurements taken during the procedure are as accurate as possible.
How is Ultrasound Biometry performed
If you have a cataract, you might experience a painless but progressive loss of vision. When you see an eye specialist, they will assess how bad your vision is and how it impacts your daily life. If your vision loss is severe enough to require treatment, they may recommend that you have surgery.
To prepare for the operation, certain steps need to be taken. The medical team will take some blood tests and use some technique called ‘biometry’ to determine the right strength for an artificial lens which is placed during the surgery. Optic and ultrasound biometry are utilized to measure the thickness of the cataract.
For one kind of biometry, you’ll sit down and look at the target. The medical staff will apply a pain-relieving eye drop; then, a special ultrasound probe will gently touch your eye. The doctor will tell you to avoid pressing your eye too hard as that may lead to an inaccurate measurement of the eye’s size. For the other type of biometry, you’re required to lie down; then, a small dish is placed over the eye, and saline solution is filled. You’ll be asked to look at the light at the end of the ultrasound probe, which will record some readings with a sound. The ideal readings will be chosen, and the less-than-perfect ones will be disregarded.
To further aid in the evaluation of your eye, an additional scan known as an optical coherence tomography may be done. This helps to evaluate the condition of the retina. On the surgery day, the eye specialist will examine your eye before surgery.
Once your surgery is over, they’ll prescribe you some eye drops which the nursing staff will administer, and they’ll make sure you understand how to take care of your eye after the procedure. The aim is to restore your vision and improve your ability to carry out your daily activities.
Possible Complications of Ultrasound Biometry
Ultrasonic biometry is a test your doctor may use to take measurements of your eye. However, there may be some complications. When the test is performed by making direct contact with your eye (applanation), possible issues might include damage to the clear surface of your eye (corneal epithelial defect), feeling uncomfortable, or even catching an infection.
An alternative technique, known as immersion biometry, avoids direct contact with your eye. While it does lower the risk of some complications, it still comes with its own problems. These could include bleeding under the thin outer layer of your eye (subconjunctival hemorrhage), infection, and discomfort.
What Else Should I Know About Ultrasound Biometry?
This text talks about a procedure that measures the size of your eye and the curve of your cornea, which is the transparent layer at the front of your eye. This information is used to choose the right strength (power) for the artificial lens (IOL) that replaces your natural lens during cataract surgery. This procedure is especially useful in certain situations, such as in eyes with thick cataracts, pediatric cataracts (in children), eyes filled with a specific type of oil, and patients who cannot cooperate during other types of measurements.
Using ultrasound to measure these values in your eye is a traditional method that has been used in cataract surgery for many years. However, it does have some challenges. For example, it doesn’t provide very high-resolution images, and it measures the physical length of the eye, not the length along the line of sight. Differences between the physical and sight-line lengths of the eye can cause the power of the IOL to be calculated incorrectly, particularly in larger eyes or those with a specific type of bulged retina.
Another challenge with using ultrasound is that it measures the front part of the retina (the back of the eye), while the light-sensing cells are closer to the back side of the retina. This difference is usually compensated for when calculating the IOL strength, but the compensation may not be accurate for everyone.
The use of ultrasound also requires the probe to touch the cornea, which may lead to infections or abrasions if not done correctly. The measurements obtained from an ultrasound are only ideal if the lens and the retina have steep, sharp spikes in their signals, and there’s a clear gap between them.
On the other hand, a method called optical biometry, which uses a laser, has better resolution and can therefore provide more precise measurements. However, it also has certain disadvantages compared to ultrasound. For example, optical biometry cannot work as well in eyes with thick cataracts or silicone oil.
In conclusion, selecting the right artificial lens for cataract surgery can be complex and requires careful measurements. The method of measurement can influence how well the artificial lens will work for you, and both ultrasound and optical biometry have their advantages and disadvantages. Your eye doctor will consider these factors when deciding the best approach for your eye surgery.
