prota_med* Medical LaserScanner Project Developed in Equity with my  partner in '94-'95 as proof-of-concept project

 
 
 

History	Results
Theory	Scanner technology
Advantages	Technical data
Software advantages	Gallery

 

History

Prosthesis allow disabled people to take part in everyday life. In  order to do so, a prosthesis must not only be light and follow the basic kinetic movement of the human body, but as it is worn throughout longer periods of time, it must be of perfect fit. This fit is guaranteed by highly skilled manual technicians that acquire their skill through training-on-the-job,  by an iteration of improvements to each masterpiece. Most of the time, however,  the skills demanded are of most basic nature and annoying:  plastering, demoulding, pouring the mould for a first positive, taking a negative. The real skill is demanded of the expert as he knows where to include recesses in the shape of the shell of the prosthesis in order to relieve sensitive portions. Also, in the vicinity of bones and especially in regions where force must be exerted to walk normally.

Our project was designed to reduce the expenditure of  making a prosthesis in applying 21st.  century techniques in this demanding field also. Through digitizing the contact surface to the prosthesis, in fact, all that the computer is good at  can be made use of, as all modern software tools are capable of volumetric  reduction, editing of shapes, defined recessing, splines etc. Speed is a factor also for direct milling of a negative which can virtually be fitted within minutes after the scan. But also the  advantages of following and archiving patient development, the reproductibility of the shapes  etc. are important advantages that made the approach seem very worthwhile.

back to top of page

Theory

The initial objective was to scan the contour of the disabled persons limb when walking with in  an idealized prosthesis with perfect fit. This is important because the human body and its  extremities resent being scanned and digitized as "glass"-hard surfaces. They have a strong  tendency to yielding to pressure. Therefore a scan of a free limb is a waste of time as the deformations occurring under stress must be manually added by the expert thus deminishing  the advantages -especially that of speed- to a minimum.
The basic idea that followed is to be able to scan a "soft" surface with the deformations  occurring in operation. In order to scan the surface of the amputation with uniform pressure  applied, it is inserted into a pressure vessel that incorporates the scanner.
 
 

During operation  the enclosed air must be sealed off in order to be able to build up pressure, making a latex  diaphragm to cover the extremity indispensable. When the scanner is pressurized it forces the membrane to apply pressure to the surface of  the amputation causing a deformation of the latter dependant of the compliance of the local tissue. This state resulting from the application of evenly distributed pressure is scanned and thereby digitized for later editing and fully automatic shaping of a basic plastic for the prosthesis.

 back to top of page

Scanner Technology

Mechanics

The main component of the scanners is a rotating cylinder carrying the laser sensor. The  distance sensor, based on the triangulation principle, is mounted onto a vertically driven sled  that can be positioned by a spindle drive activated by a stepper-motor.
 
 

In order to keep scans acceptably short the device spins at approx. 400 r.p.m. and therefore must be equipped with powerful brakes on the mechanical side.
 
 

Through the resulting superposition of vertical- and rotational movement of the laser the derived points are located on a screw-shaped line around the object circumference with 480  measurements taken each revolution.
 
 

Electronics

A sophisticated µcontroller mounted onto the rotating platform maintains coordination of the translatory and  vertical movement of the laser dependant on the gain in angle of rotation in order to rule out  possible variances in r.p.m. The µ-controller also performs storage of the derived data in an on-board RAM to eliminate interference and noise during operation.
 
 

Pneumatics

The scanning system is located inside of a pressure vessel which is sealed on the topside by a  latex-membrane that seals the vessel in respect to the amputation. In order to prevent  expulsion of the thin latex membrane when the vessel is put under pressure and for  stabilization and positioning of the patient, the patients limb is supported by an inflatable collar.

The pressure vessel was tested at 2 bar pressure. During operation normal applied pressure would  typically only be 0.1 - 0.2 bar. Astoundingly, a pressure of only 0.3 bar turned out to be the absolute sustainable maximum for an athletic patient.

Pneumatics include a free entry and exit support into and out of the latex membrane through application of a vacuum. This results in opening the latex membrane, allowing easy entry. The idea could also be applicable for easy entering into latex surgeons gloves etc.

The machine features a main pressure reduction valve, one pressure adjustor for the main  chamber and another one for the supporting and sealing ring on top. The pneumatic layout grew more complicated as failsafe behaviour was demanded above all, remembering that we are dealing with a human being having a limb inside.

As a result of the very high r.p.m. a pneumatically actuated braking system was designed. This proved to be too efficient resulting in the inertia turning the whole vessel around 45°. This made a two stage braking system indispensable that differentiates between normal braking and emergency braking.

back to top of page

Software

The software package consists of the following components:

Controller-Software
for initialization of the scanner, controlling of the scanning process and transmission of complete sets of data to the host-computer.

Visualizing- and Editing-Software
for visualizing and editing of the derived dataset. (f.i. pressure-sensitive regions are relieved through modeling recesses).

Conversion-Software
for conversion of the edited sets of data to control-programs for CNC milling machines.

CNC Milling Machine

for automated milling of the 3D external contour out of any machinable material such as  plastic material.  The positive or negative contour of the edited shape is the basis for further  completion of the prosthesis by the technician.

back to top of page

Results
Different scans were made during calibration without membrane or pressure.
Hereby different objects  were 3D-scanned with stunning accuracy theoretically enabling a 3D scan of a bouquet of  flowers and being a unique device to do so. Other applications could be turbine blades or  other objects with free-form shapes. Operation under pressure showed that the surface showed high details such as veins and  flexes, muscles etc. Deformation of soft tissue could be clearly displayed. A patient weighing  80kgs is supported only by the machine at approx. 2.1.bar, resulting in his whole weight to be  carried by the amputation, similarly to walking action which one leg at a time.

back to top of page

 

Advantages of the system:

• scanning of the amputation under pressure - perfect fit
• normal upright position during the scan
• no more modeling and casting with plaster (neg-pos-neg shape)
• fast first positive shape
• less time wasted for tasks demanding low skills from skilled personnel
• free choice of materials ("CNC-machinable materials")
• intentional and deliberate use of materials during manufacture
• high accuracy and resolution
• reproducibility
• scanning also during muscular contraction for sports-prosthesis (short scans)
• archivation of patient data and prosthesis geometry on diskettes
• clean in application (plaster vs. plastics)
• scanning of skeleton structure optional through an optionally integrated ultrasonic scanner

 back to top of page

Advantages of the Software

• simple percentile volume-reduction
• automatic close adherence to preset volume-contents
• display of the different zones of compliance through a series of scans at varying pressure levels
• editing can be performed anytime and repeatedly sophisticated editing tools can be developed enabling the use of macros (f.i. to shape the recesses at the lateral u. medial condules)
• collection of relevant data in knowledge-based data bases thereby influencing editing phase
• use of a medical record through archived data of former scans
• several scanners can be linked with one central CNC mill via data line / modem
• results are reproducible
• scanning and editing computers may be different machines in order to relieve personnel to allow later editing in a clean and undisturbed environment

back to top of page
 

Gallery

The following objects were scanned
(all scans uncorrected and not edited = raw data !)
 

• Test without diaphragm
• for resolution verification and
• optical and mechanical adjustment

• BAZAR
• different objects for reproduction proof

• Hands
• fortunately lacking an amputation something else had to do:

• Lower Extremity
• the first successful scan:

• Upper Extremity
• another fine example

• Selective Compliance
• a superposition of two scans of the same object at different pressure levels
• tissue compliance hilighted with color variation
• blue:      hard compliance
• green:   intermediate compliance
• red:       soft compliance

 
 

*prota_med
is a proof-of-concept- project developed by
K. Lauth and my  partner
1994-1995

back to top of page

Technical Data
(preliminary)

normal operating speed	480 revolutions per minute
emergency braking distance	< 1 revolution
operating voltage	12 V DC
Scan time	appr. 50 s
Actuation	pneumatic
measurements each revolution	480
depth range	40 mm
depth resolution at 16 bit	256 steps
depth resolution	> 2 µm
raw data file size	1524 kB
Data transfer	Serial
Data storage	On board RAM 4 MB

 unused internal links:

 

mechanics

electronics

pneumatics

software

cnc milling machine