|
FIRMWARE FOR LASER SURGICAL OPHTALMIC
MICROSCOPES
(Ophthalmology, Surgical, Operating)
SORIN LAURENŢIU STĂNESCU1, VASILE SAVA2, PAUL DAN CRISTEA1
,MIRCEA VIRGIL UDREA3
Key words: Laser surgical microscope, Ophthalmic
microscope
(ophthalmological
surgical microscope,Surgical, operating,
ophthalmology, ophthalmologic)
This work presents the technical aspects for the development of
a firmware for a
Nd:YAG laser based ophthalmic (Ophthalmology,
Surgical, Operating)
microscope.
Software design was developed for a Microchip® PIC18F4550
microcontroller (operating,ophthalmology).
It interfaces the electronic blocks of the device and allows the
calibration of laser
energy attenuator.The
calibration data are stored into EEPROM’s for each Nd:YAG laser
unit. The fast
developing time of the software is achieved by using of Flowcode®
4 – a graphical
programming tool for microcontrollers (ophthalmology,Surgical, operating,ophthalmological
surgical microscope,
ophthalmologic)
with customizable code
software blocks. The
software undertakes the specific requirements of a medical
device.
1. INTRODUCTION
The last years’ studies targeted the development of new
equipments for
healing a larger range of ophthalmic(ophthalmology,Surgical,operating)diseases.This represents a necessity because
of the current aging population trend in developed countries
(Western Europe,
USA, Japan) which is leading to an increase of the incidence of
ocular diseases
(e.g., cataracts and glaucoma). Following this situation, many
researches have been
made on the instruments which use laser radiation as an active
tool in ophthalmic (ophthalmology,
Surgical, operating,ophthalmological
surgical microscope,
ophthalmologic).
2. EXPERIMENTAL
The microcontroller (ophthalmology,
Surgical, operating,ophthalmological
surgical microscope) chosen for this firmware implementation is
PIC18F4550
from Microchip® due to its large number of digital I/O ports(15ports)and analog
input channels(13 channels), high memory capacity (32 Kbytes
program memory
and 255 bytes EEPROM) and 10 bits resolution of the CCP
(Capture/Compare/PWM) modules[2]. Further are described the main
functions
implemented in the software
(operating,ophthalmology), together with their specific
hardware.
2.1. INITIALIZATION
After the successful initialization of the microcontroller’s
(ophthalmology,
Surgical, operating, Ophthalmologic) software,a
continuous 5 Vcc output voltage is generated at its E0 port. External hardware is
signaled this way that the microcontroller’s (operating,ophthalmology) software has
started well and continues
the initialization of other hardware blocks.
2.2. SETTING THE ENERGY LEVEL
The system
(Ophthalmologic,operating,ophthalmology,
Surgical,
Ophthalmological
surgical microscope) uses an innovative way of setting the laser energy.
In order to
continuously control the laser energy to a required value,
without using moving
parts, or to setting a different voltage for the laser’s flash
lamp, we used a Pockels cell. Its functioning is based on the
rotation of the polarization(operating,ophthalmology,Surgical)
plane of the laser beam which passes
through it. The incident laser beam is already polarized at the
laser output so the rotation of polarization plane will produce
an attenuation of the beam’s power.
3Firmware for laser surgical ophtalmic microscopes
(ophthalmological
surgical microscope, ophthalmologic,ophthalmology,
operating,
Surgical) 447 preferential direction at angle β.
When passing the laser beam through a second linear polarizer
(A) acting as the analyzer, with the preferential axis at the
angle Ψ, the P-W-P (Polarizer-Wave plate-Polarizer) combination
stands as an adjustable attenuator
(operating,ophthalmology)
where:
2 2
0 (cos cos cos sin sin ) (cos cos sin ) ;
tan cos cos sin ;
cos cos cos sin sin
, e o
D =A β Ψ Δφ+ β Ψ + β Ψ Δφ
β Ψ Δφ
χ =
β Ψ Δφ+ β Ψ
Δφ = φ − φ
(5)
(6)
(7)
where:A isthe amplitude intensity of the beam;k0 is the
coefficient constant of the
oridinary
(operating,ophthalmology,
Surgical) beam; ke is the coefficient constant of the
extraordinary beam; r is the
vector position; φ is the phase shift;0 φ is the ordinary phase
shift;e φ is the
extraordinary
(operating,ophthalmology,
Surgical)pahse shift.For use, the energy of the laser should be set between
0.5 mJ and
9 mJ,whereas the pulse width is fixed at 5 ns because of to Q-switch working regime. This function is achieved by modifying the duty cycle of
a 15 kHz square
wave signal which is generated by CCP1module of the
microcontroller (Surgical,
Ophthalmologic, operating, ophthalmology,
Ophthalmological
surgical microscope). The duty
cycle is modified with a 10 bits resolution in function of
desired energy and of the
correction offsets.
Fig. 2 shows the theoretical energy variation versus the duty
cycle, without
taking into account the specific properties of the attenuator.
The correspondence (Surgical,operating,ophthalmology,
Ophthalmological
surgical microscope)of
the output energy to the duty cycle fill factor has been chosen
0.1 mJ for 1 %, in
the range 0.5–9 mJ.The peripheral hardware block which uses
this signal
integrates the square wave in function of its duty cycle and
gives a high voltage
output in the range 0 to 6kV.This high voltage is applied to a Pockels cell which, together with the two polarizers, determines the desired
(operating,ophthalmology,
Surgical)energy
attenuation. The
energy can be incremented from the menu with selectable step of
0.1 mJ or 1 mJ.448 Sorin Laurenţiu Stănescu et al. 4
Both the step and the energy values are recorded in the EEPROM.
When the
microscope
(Surgical,
Ophthalmologic, operating,ophthalmology,
Ophthalmological
surgical microscope)
is on, it will recall the last energy used and
the last selected energy step
from the EEPROM [5].Fig. 2 – Theoretical energy variation versus the duty cycle.2.3. ENERGY CORRECTION
A square wave of 5 V amplitude, with variable duty cycle,
controls the high
voltage which drives the Pockels cell. The laser energy provided
at the output of
the attenuator in function of the voltage differs for each
attenuator. Fig.3 shows the
energy of the output of the system-laser,Pockels
(ophthalmology, Surgical, ophthalmologic,operating)
cell and
polarizers, versus the
voltage applied on the Pockels cell. The laser used for testing
was a Solar TII
LF117 Q-Switch with Nd:YAG as active medium and 1064 nm
wavelength. The
Pockels cell was a CIQS 8IM99 made by Linos. The energy was
measured with a
Coherent Field Max II energy meter. It can be observed that the
energy dependence
of high voltage
(ophthalmological
surgical microscope,Operating, Ophthalmology, Surgical)
is almost linear. However,for eye surgery,
linearity and accuracy
are very important. Because the high voltage is controlled by a
square wave with
variable duty cycle, we use a software correction based on
adding or subtracting
offsets from the duty cycle value, at each value of the energy
in the range of 0.5 mJ
to 9 mJ.
CAPACITORS MONITORING CIRCUIT
The capacitors monitoring
(Ophthalmology, Surgical,
Ophthalmologic,
Operating)
circuit gives a signal between 4.5V
and 5 V when
the capacitors are charged enough to supply the flash lamp. This
signal is read on
port C3. If the signal received is not in the desired range, an
error message is
displayed in the LCD and the laser triggering is stopped.
LASER ENERGY CHECK
A signal between zero and 5V is read from transducer of an IR
photodiode. The checking routine is performed three times at the
initialization of
the microscope
(surgical,operating,
ophthalmological
surgical microscope,ophthalmology)
and each time after a new energy value is set
from the keyboard.
OTHER FUNCTIONS
In addition,following hardware blocks,not related to microscope
(ophthalmologic,
operating,
surgical,
Oophthalmology)
functions, must be implemented:
– an HS oscillator (a 16 MHz quartz crystal [2]) chosen in order
to have the
maximum gain and frequency response, even if the power
consumption is higher,
– a power supply based on a 5 Vcc voltage regulator of 1 A.
The block scheme of all software functions and additional
hardware blocks
that we obtained is shown in Fig. 4.
The firmware
(ophthalmological
surgical microscope,surgical,oophthalmology,
ophthalmologic,
Operating)
was designed based on logic diagrams in Flowcode®
4. The
components (logic blocks from the diagrams)are C code blocks
which code and
variables had been customized which respect to the needs.Flowcode®4 permits
452 Sorin Laurenţiu Stănescu et al. 8
3. CONCLUSIONS
The compiled hexadecimal file has 17 KB out of 32 KB available
from the
program memory of microcontroller
(ophthalmological
surgical microscope,surgical,oophthalmology,Operating,
ophthalmologic)
. It was burned in the
PIC18f4550 using a
Matrix Multimedia® PicMicro USB MultiProgrammer with PPP version
3
software.
Very good shielding of signal and supply wires was provided in
order to
avoid any parasitic (Operating,Ophthalmology) interference from the laser’s and Pockel’s
cell supply sources. The entire system is shielded with a
metallic case such that it complies with the 9 Firmware for
laser ophtalmic
microscopes
(Operating,
Ophthalmological
surgical microscope , Surgical , Ophthalmology,
Ophthalmologic ) 453
communitary reglementation stipulated in second edition of EMC
Directive 2004/108/ EC which refers to the electromagnetic pollution
problem approaches
with implications mainly in biological effects [7].The firmware developed was simulated and tested with all the
necessary
hardware.
The software modelling was done by using visual programming,
which made
the effort similar
(
Ophthalmological
surgical microscope, Surgical,
Oophthalmology,
Ophthalmologic,
Operating,) to the one necessary for code-write
programming. Significant
time was saved due to this approach, which – on our best
knowledge, was applied
here for the first time in designing a medical device.
We also used an optical attenuator based on a Pockels cell, for first time
in a laser surgical microscope
(Ophthalmological
surgical microscope,
surgical,
oophthalmology,operating,ophthalmologic).This allows a simple and
efficient software control
and a good calibration procedure
(operating,ophthalmology) and makes object of a
pending patent
(A/00839/21 . 10.2009).
ACKNOWLEDGEMENTS
The paper was partially supported from the “Oftalas” Innovation
Contract
No. 180/26.08.2008 with AMCSIT – “Politehnica” University of
Bucharest [5].
Received on 11 July 2010
REFERENCES
1. J.F. Bille, C.F.H Harner, F. Loesel, New Frontiers in Vision
and Aberration-Free Refractive, Springer Press, Heidelberg, Germany, 2002.
2. * * *, PIC18F2455/2550/4455/4550 Data Sheet, Microchip®
Technologies, 2004.
3. D. C. Dumitraş, Ingineria fasciculelor laser, Editura All,
2004.
4. F.H. Loesel, M.H. Niemz, J.F. Bille and T. Juhasz,
Laser-Induced Optical Breakdown on Hard and
Soft Tissues and Its Dependence on the Pulse Duration:
Experiment and Model, IEEE J.
Quantum Electron., 32, pp. 1717-1722(1996).etc…… |
