Ultrasonic investigation with eye dummy
TEAS
9.5.04
-00
Related Topics
Cornea and retina distance, lens thickness, biometry, eye sonography, mean ultrasound velocity, time of
flight, A-mode, ultrasonic echography, reflection coefficient.
Principle
This experiment shows a typical application of A-scan ultrasound biometry in medical diagnostics for
ophthalmology. On the eye dummy the different parts of an eye can be identified and the time of flight is
measured in each part so that the different dimensions in the eye structure can be determined.
Equipment
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1
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Basic Set Ultrasonic echoscope
consisting of:
1x Ultrasonic echoscope
1x Ultrasonic probe 1 MHz
1x Ultrasonic probe 2 MHz
1x Ultrasonic test block
1x Ultrasonic cylinder set
1x Ultrasonic test plates
1x Ultrasonic gel
Extension set: medical ultrasonic diagnostics
consisting of:
1x simplified heart dummy for echocardiography
1x breast dummy
1x eye dummy
Additionally needed
PC with USB port, Windows XP or higher
Fig. 1:
13921-99
13921-04
Equipment for ultrasonic investigation of eye dummy, experimental set-up
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P5950400
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TEAS
9.5.04
-00
Ultrasonic investigation with eye dummy
Caution!
This is not medical equipment, don’t apply to human body. Pay attention to the special operation and
safety instructions included in the user manual of the ultrasonic sonograph.
Tasks
1. Measure the different parts of the eye, determine the time of flight and depth of the structures (lens,
iris, retina).
2. Compare the dimensions measured on each separate part with those obtained using average velocity for the whole eye.
Set-up and Procedure
The ultrasonic eye dummy was designed, in terms of size and used materials, to be measured with
the ultrasonic echoscope and a 2 MHz probe. It is also possible to use the 1 MHz or the 4 MHz
probe, but it should be more difficult to find out the appropriate parameter set to inspect the device.
Connect the 2 MHz probe to the “Probe (Reflection)” plug.
Switch the selection knob to “Reflection”.
Select medium gain and output power.
Switch on the echoscope
Start up the measure Ultra Echo software in A-scan mode, measure in time mode
Couple the probe in the middle of the cornea to the test dummy using sufficient coupling gel, to get a
good ultrasonic contact between the probe and the dummy.
Slowly move the probe over the cornea, without any pressure, observing the A scan image. Change
the position until the two large echoes of the front and the back of the lens and the smaller echo of
the retina can be seen (Fig.: 2).
Adjust the transmitter and receiver amplifier settings until the peak position, corresponding to the
lens and the retina can be clearly determined. The respective peak amplitudes can also be
optimised adjusting the time-TGC (Time Gain Control) parameters.
Often, the small echo of the iris appears in front of the first lens signal peak. The two echoes of the
lens and the retina echo shall be seen as clear peaks, without any supplementary structure. Only in
this condition the incident sound angle is perpendicular to the lens. If the direction of the ultrasonic
wave is not perpendicular to the lens surface, the peaks will appear larger and small additional
peaks can appear. Do not interpret these artefacts as additional boundary surfaces in the dummy.
Measure the time of flight in the different parts of the dummy and calculate the dimension of these
parts assuming the following velocity values: aqueous/vitreous humour 1410 m/s, lens 2500m/s
Determine the average velocity
Set the ultrasound velocity to the average value, toggle from Time to Depth scale and measure
again.
Compare the results calculated with real values to them measured with average value.
Software
The measure Ultra Echo software records, displays and evaluates the data transferred from the echoscope. After starting the program the measure mode is active and the main screen “A-Scan mode” is
open. All actions and evaluations can be selected and started in this window.
The main screen shows in the upper part the A-scan signal, the frequency of the connected transducer,
the mode (reflection/ transmission). Actual positions of cursors (red and green line) are displayed at the
bottom of the window. The cursors can be positioned by mouse click. The time of flight is displayed under the cursor buttons.
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Ultrasonic investigation with eye dummy
TEAS
9.5.04
-00
Note:
The cornea of the eye dummy is made of a sensitive ultrasonic phantom material. The surface can be
deformed easily when pushing the probe to the surface. Some pressure to the probe can increase the
coupling between the probe and the bent cornea but too strong pressure can lead to cracks in the material and damage the dummy. Don’t touch the dummy with sharp objects. Clean the cornea with a wet,
soft cloth. Don’t use any cleaning agents or hot water, it could destroy the dummy.
Theory and Evaluation
Diagnostic ultrasound is now used routinely in the investigation of patients with opacification of the ocular
media or with orbital problems. Ultrasonic frequencies within the range 5-20 MHz are generally used for
ophthalmic diagnosis. In order to couple the high-frequency sound to the eye, a gel can be applied to the
anaesthetized eye or to the closed eyelids or the coupling can be realised by means of a saline bath.
Ultrasound is mostly used in ophthalmology in the area of biometry, to measure distances in the eye.
The distance between cornea and retina has to be known to calculate the characteristics of an artificial
lens, which can be implanted to patients with cataract. Sonography is necessary in this case since the
Fig. 2: Schematic representation of the eye dummy and the corresponding reflexes in a Ascan image
cornea or the lens is too cloudy to be inspected with optical methods.
The measured time of flight, determined in the A-scan, can not be transformed directly, using “time to
depth” button, into distances, because the ultrasound velocity is different in cornea, lens and vitreous
humour. Therefore some corrections are necessary.
Two velocities are given for the dummy:
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Ultrasonic investigation with eye dummy
-lens: 2500 m/s,
-humours: 1410 m/s.
Using the time of flight measured in A-scan and the sound velocity for each part of the dummy, the thickness of each individual part can be measured using the equation:
(1) s  v
t
2
In medical diagnostics „average“ values are often used to get quick results. For the eye dummy an average velocity can be calculated using the following equation:
(2)
v
v1 (t1  (t 3  t 2 ))  v2 (t 2  t1 )
t3
Where tx and vx are defined in (Fig.: 2) Results
The time of flight was measured for each peak and the averaged velocity was calculated using equation
Fig. 3:
Screenshot of the measured reflexes
Fig. 4:Multiple echoes: probe not in perpendicular position
(2). The result was introduces in the ultra echo software and the X axis switched to the depth scale.
The depth was measures for each peak.
It can be seen that read out values obtained with the average velocity are similar to the values obtained
from the regions 1410 m/s sound velocity. However, it should be noted that for lens thickness
Fig. 5:
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“Iris”echo
Fig. 6:
Retina echo
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TEAS
9.5.04
-00
Ultrasonic investigation with eye dummy
measurements, a correction must be applied to the measured values. This is especially important in
practice when a good accuracy of lens thickness is needed. Over the years, many approaching formulas
for "eyes surveying” measurements have been developed. Often, a fixed value (0.32 mm) is added to the
axial length measured with a average speed value (for a real eye, not for these dummy).
This average value should be determined experimentally, using the formula (2), for the eye dummy; this
value is 1518 m / s.
Table 1: Results :
Velocities [m/s]
(Aqueous/vitreous humour)
1410 m/s
(lens)
2500 m/s
Calculations
Lens front
Lens back
Retina
Time [µs]
13,7
21,1
74,8
Average velocity [m/s] formula (2)
1518
Measured depth using average
velocity [mm] formula (1)
10,4
16,0
56,8
Thickness measurement for each
part [mm] formula (1) diff. velocities
9,66
9,25
37,9
Measured depth adding the
thicknesses [mm]
9,66
18,9
56,8
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Ultrasonic investigation with eye dummy
PHYWE Systeme GmbH & Co. KG © All rights reserved
P5950400