Neurotherm NT-1100 Service manual

NEUROTHERM

RADIO FREQUENCY

LESION GENERATOR

MODEL NT 1100

jService Manual

Neurotherm Radio Frequency Lesion Generator Service Manual

Model NT1100

1.0CONTENTS

1.1	List of Figures..................................................................................................................			3
2.0	WARNINGS AND CAUTIONS .........................................................................................			2-1
3.0	INTRODUCTION AND APPLICABILITY OF THIS MANUAL...........................................			3-1
3.1	Introduction and applicability of this manual.................................................................			3-1
3.2	Summary of Equipment Revision Changes..................................................................			3-2
3.3	Summary of Board Revision Changes .........................................................................			3-3
3.3.1		Revision A – All Boards	NT1100 ..............................................................	3-3
3.3.2		Revision B – Boards in rack .................................................................................		3-3
3.4	Manual Updates ...........................................................................................................			3-4
3.4.1		Neurotherm Radio Frequency Lesion Generator Unit Manual Changes..............		3-4
3.4.2		Record of Manual Updates carried out.................................................................		3-5
4.0	GENERAL DESCRIPTION AND PRINCIPLE OF LESIONING .......................................			4-1
4.1	Specifications ...............................................................................................................			4-1
4.2	Principles of Lesioning ...............................................................................................			4-11
4.2.1 The basic physical principles of radiofrequency ablation ..........................................				4-11
4.2.2		Pulsed radiofrequency........................................................................................		4-14
4.2.3		The Pulse Dose Concept....................................................................................		4-17
4.3	General Description....................................................................................................			4-22
4.4	General Signal Information.........................................................................................			4-23
4.5	Connector Panel Layout.............................................................................................			4-29
4.6	Back Panel Layout .....................................................................................................			4-30
5.0	DETAILED DESCRIPTION OF MODULES .....................................................................			5-1
5.1	Power Entry Module, Isolation Transformer and Power Supply Board ........................			5-1
5.2	Fuse Board...................................................................................................................			5-3
6.0	CIRCUIT DIAGRAMS AND COMPONENT LISTS...........................................................			6-1
6.1	Power Supply Board - RF100C ....................................................................................			6-2
6.2	Fuse Board – RF101D .................................................................................................			6-5
6.3	Impedance Board –RF 102D........................................................................................			6-8
6.4	Stimulate Board RF103D + Sub Board ......................................................................			6-13
6.5	RF Amplifier Board RF 104 C.....................................................................................			6-19
6.6	RF Voltage and Current Metering Board RF 105C.....................................................			6-24
6.7	Temperature Board RF106 D.....................................................................................			6-29
6.8	Interlock Board RF107D.............................................................................................			6-35
6.9	Connection Board RF109B.........................................................................................			6-41
6.10	Interface Board RF110D ............................................................................................			6-44
6.11	Computer Mother Board RF111B...............................................................................			6-47

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual

Model NT1100

1.1List of Figures

4.1	Output Power/Load Resistance Curve
4.2	Typical available Stimulate Output Voltage/Load Resistance
4.3	Fundamental Radiofrequency Circuit
4.4	Mechanism of Radio Frequency Heating
4.5	Schematic of Tissue Temperature v Distance from Electrode Tip
4.6	Increase in Lesion Size v Time
4.7	Typical Pulsed RF Signal
4.8	Frequency Spectrum of a 2Hz Rectangular Pulse
4.9	Frequency Spectrum of a 500 KHz Signal
4.10	Pulsed RF – Amplitude Control
4.11	Pulsed RF – Pulse Width Control
4.12	Pulse Dose
4.13	General Schematic of the Neurotherm NT1100

5.1	Power Supply Unit - Circuit Diagram
5.2	Fuse Board – Circuit Diagram

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual

Model NT1100

6.1	Power Supply Board -	-RF100C -	Circuit Diagram
6.2	Power Supply Board	-RF100C -	Component Layout
6.3	Fuse Board	-RF101D -	Circuit Diagram
6.4	Fuse Board	-RF101D -	Component Layout
6.5	Impedance Board	-RF102D -	Circuit Diagram
6.6	Impedance Board	-RF102D -	Component Layout
6.7	Stimulate Board	-RF103D -	Circuit Diagram
6.8	Stimulate Board	-RF103D -	Component Layout
6.9	RF Amplifier Board	-RF104C -	Circuit Diagram
6.10	RF Amplifier Board	-RF104C -	Component Layout
6.11	V/I Board	-RF105C -	Circuit Diagram
6.12	V/I Board	-RF105C -	Component Layout
6.13	Temperature Board	-RF106D -	Circuit Diagram
6.14	Temperature Board	-RF106D -	Component Layout
6.15	Interlock Board	-RF107D -	Circuit Diagram
6.16	Interlock Board	-RF107D -	Component Layout
6.17	Connection Board	-RF109B -	Circuit Diagram
6.18	Connection Board	-RF109B-	Component Layout
6.19	Interface Board	-RF110D-	Circuit Diagram
6.20	Interface Board	-RF110D-	Component Layout
6.21	Computer Motherboard	-RF111B-	Circuit Diagram – Bus
			Interface
6.22	Computer Motherboard	-RF111B-	Circuit Diagram - Digital
			Outputs
6.23	Computer Motherboard	-RF111B-	Circuit Diagram-Ribbon
			Connectors and PSU
6.24	Computer Motherboard	-RF111B-	Circuit Diagram – Digital
			Inputs
6.25	Computer Motherboard	-RF111B-	Circuit Diagram– Analog
			Inputs
6.26	Computer Motherboard	-RF111B-	Circuit Diagram– Analog
			Outputs
6.27	Computer Motherboard	-RF111B-	Component Layout

6.28Front Panel Wiring

6.29General System Schematic

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	2-1
Model NT1100

2.0WARNINGS AND CAUTIONS

Warning indicates a potentially harmful situation to yourself or others.

HAZARDOUS ELECTRICAL OUTPUT: The equipment is for use ONLY by qualified medical personnel.

Do NOT under any circumstance perform any testing or maintenance on the equipment while it is being used on a patient.

Do NOT use extension cords or adapters of any type. The power cord and plug must be intact and undamaged.

Should the power cord or plug become cracked, frayed, broken of otherwise damaged, it must be replaced immediately.

If the equipment has in any way suffered mechanical damage it should be returned to the Supplier for Inspection and Test before further use.

Unplug the power cord before cleaning or service.

The operator should not perform any servicing of the equipment. Any servicing should only be carried out by qualified personnel.

EXPLOSION HAZARD: Equipment not suitable for use in the presence of a flammable anaesthetic mixture with air or with oxygen or nitrous oxide.

ELECTRIC SHOCK HAZARD: Always turn the equipment off before cleaning and DO NOT allow ANY fluid to enter the ventilation holes or sockets.

ELECTRIC SHOCK HAZARD: Do not touch any exposed wiring or conductive surface, while cover is off and the equipment is energised. The voltage present when the electric power is connected to the equipment can cause injury or death. Never wear a grounding wrist strap when working on energised equipment.

FUSE REPLACEMENT: For continued protection against fire hazard, replace only with same type and rating of fuse as displayed on the rear Serial Number Plate.

IMPROPER LINE VOLTAGE: The voltage selector on the mains input socket is factory set and should not be changed by the user. The serial number plate shows the correct mains input voltage for the machine and the rating of the fuses to be used in the mains input fuse holder. An incorrect voltage setting may result in Neurotherm malfunction and potential damage.

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	2-2
Model NT1100

2.0WARNINGS AND CAUTIONS (continued)

A CAUTION indicates a condition that may lead to equipment damage or malfunction.

Servicing of the equipment in accordance with this service manual should never be undertaken in the absence of proper tools, test equipment and the most recent revision of this service manual which is clearly and thoroughly understood.

When repairing circuit boards, great care should be taken in handling boards as all boards contain static sensitive devices. Before servicing a board, ground yourself and the relevant tool to discharge any accumulated static charge by wearing a wrist strap and placing the board on a static mat. If a board has to be returned, use anti-static bags or containers.

The tests and repairs outlined in this manual should only be attempted by trained personnel. Unauthorised service may void the warranty of the unit.

Check the voltage rating on the rear Serial Number Plate before connecting the equipment to AC Mains Power. The equipment must never be operated at the wrong mains voltage.

Use insulated tools when adjusting the internal controls on the equipment.

When cleaning the outer casing or display panel of the equipment do not use abrasive agents or solvents.

To reduce risk of electrical shock do not remove back panel of generator. Refer servicing to qualified personnel.

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	3-1
Model NT1100

3.0INTRODUCTION AND APPLICABILITY OF THIS MANUAL

3.1Introduction and applicability of this manual

This service Manual (Document No. R1000) gives the information required to maintain and repair the Neurotherm Radio Frequency Lesion Generator Unit, Model NT1100. The main body of this manual deals with the present production revision of the equipment. Differences between equipment revision are summarised in Section 3.2. Section 3.3 lists the technical changes made to the equipment.

The revision of the whole equipment is changed if such technical changes are made which make some spare parts incompatible with earlier units. The initial equipment numbering, as shown on the rear Serial Number Plate contains no revision letter (e.g. Serial No. NT1100- 7020-05) however later revisions will contain a revision letter (e.g.Serial No. NT1100-7020-05 Rev A). If the whole machine is upgraded such that early machines cannot be easily amended, then the upgraded machines will start from a nominated serial number as indicated in Section 3.2.

Within the equipment, function units such as Printed Circuit Boards will be changed or updated from time to time, these may or may not introduce a revision of the whole equipment. Each printed circuit board contains an identity number and a serial number of the board together with the issue number - designated by a letter. The initial issue letter was A for all Boards. In all cases the spare parts order code is also the Board Identity Number and Issue Number (e.g. RF 102E).

In order to understand this manual it is necessary to have a complete understanding of the function and operations of the Lesion Generator Machine. This information can be obtained from the Operators Manual which contains full operating instructions.

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	3-2
Model NT1100

3.2Summary of Equipment Revision Changes

Initial production revision of this equipment – Model NT1100

Initial production DEC 2005

Start from

NT 1100 – 7007-05

From

NT1100 – 7150 – 06 (May 2006)

Metal Rack used and all cards in rack were changed to move components away from guides

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	3-3
Model NT1100

3.3Summary of Board Revision Changes

3.3.1 Revision A – All Boards	NT1100
Initial production revision of this equipment
Power Supply Board		RF100C
Fuse Board		RF101C
Impedance Board		RF102C
Stimulate Board		RF103C
RF Power Amplifier Board		RF104C
RF Voltage and Current Board		RF105B
Temperature Board		RF106C
Interlock Board		RF107C
Front Panel and Connection Board		RF109B
Interface Board		RF110C
Computer Motherboard		RF111B
Card Rack Backplanes		RF115A and RF116A
3.3.2 Revision B – Boards in rack
Fuse Board		RF101D
Impedance Board		RF102D
Stimulate Board		RF103D
RF Voltage and Current Board		RF105C
Temperature Board		RF106D
Interlock Board		RF107D
Interface Board		RF110D
Card Rack Backplanes		RF115B and RF116B

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	3-4
Model NT1100

3.4Manual Updates

3.4.1 Neurotherm Radio Frequency Lesion Generator Unit Manual Changes

This is the NT 1100 Manual applicable to the machines from serial numbers shown below:-

Issue No.	From	Page	Change	Date
	Serial No
	NT1100 – 7007-05		As issued	DEC 2005
2	NT1100 – 7150-06			JUNE 2006

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	3-5
Model NT1100

3.4.2Record of Manual Updates carried out

Update Number	Carried out by Name	Date
2	Howard Clarke	June 2006

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-1
Model NT1100

4.0GENERAL DESCRIPTION AND PRINCIPLE OF LESIONING

4.1Specifications

SIZE:
Width	400 mm (15 ¾”)
Height	300 mm (11 ¾”)
Depth	415 mm (16 ½”)
WEIGHT:
12.5 kg (28 lbs)
ELECTRICAL:
EUROPE	230 Volts 50Hz Fused 1 Amp on live and neutral
USA/CANADA	110 Volts 60Hz Fused 2 Amp on live and neutral
Voltage change via rear connector
Power Consumption	150 watts

The power supply is built to Class 2 standard. The mains transformer and all mains related parts are doubly insulated from the Main Enclosure. The mains transformer has separate isolated bobbins for mains and low voltage windings. Thermal fuses (rated to fail at 125°C) are fitted into all primary and secondary windings.

The machine is not connected to mains earth (class2).

STANDARDS:

This instrument complies with

EN60601-1:1997

IEC60601-1-2:1993

IEC60601-2-2:1998

IEC60601-2-10: With Canadian deviations

With respect to electrical shock, fire and mechanical hazards only in accordance with UL60601-1, IEC60601-1, CAN/CSA C22.2 No.601.1 and IEC 60601-2-2

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual			4-2
	Model NT1100
IMPEDANCE
Measuring frequency		53KHz (± 3KHz)
Measuring source voltage		Less than 500 mV AC
Measuring Display		50-2000 ohms (one ohm resolution)
Accuracy		±5%
Features	(a)	Internal 500 ohm Test Resistor
	(b) Impedance in all Lesion Modes and in Stimulation
		Mode when stimulation is off
	(c) Audible Tone available where frequency		varies
		with impedance over full impedance range (50-
		2000 ohms). Audible tone is adjustable and
		mutable.
	(d) Warning on screen if impedance is less than 50
		ohms or greater than 2000 ohms.
STIMULATION MODE
Signal Shape		Biphasic square wave with negative edge leading.
		This wave is available in a variety of frequencies and
		widths.
Output Range	Voltage	0-5v ± 3% for motor frequencies (2Hz and 5 Hz)
		0-3v ± 5% (Default) for all other frequencies
		0-0.5v ± 10% for all other frequencies
	Current	0-10mA±5% 50-2000 ohms
		0-6mA± 5% 50-2000 ohms
		0-1mA± 5% 50-2000 ohms
Pulses Rates	Motor	2 or 5 Hz (Default 2Hz)
	Sensory	10,20, 50, 75, 100, 150, 180, 200 Hz (Default 50 Hz)
Pulse Rate Accuracy		± 3%
Pulse Widths		0.1, 0.2, 0.5 and 1.0 mS (Default 1.0 mS)
Pulse width Accuracy		± 5% for 0.2, 0.5 and 1.0 mS
		± 15% for 0.1mS

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual			4-3
	Model NT1100
Features	(a) Hardware and Software lockout if
	voltage / current control not initially set
	to zero.
	(b) Warning on screen if stimulation control
	is not	initially at zero.
	(c) Flashing LED on front panel indicates
	machine is delivering stimulation pulses.
	(d) Stimulation Test Socket is provided on
	front of machine to interface with the
	standard stimulation test kit.
	(e) Various screen displays for displaying
	amplitude of		each
	stimulation procedure.
LESION MODE
RF Waveform	480 KHz ± 5% Sinusoidal
Power Output	Continuously variable. Maximum power output 30
	watts ± 5% into 200 ohms. Power is displayed in
	certain Lesion Modes.
Voltage Display on screen	0-99RF volts (RMS)
Current Display on screen	0-999RF milliamps (RMS)
Self Test	150 ohm dummy load resistor built into machine
Lamp Indicator	LED flashes when Lesion Power is being delivered.
Temperature Range	Selectable 50-90°C for Thermal Lesion (Default 80°)
	Selectable in 5° C steps in initial screen set ups
	Selectable in 1°C steps when in Lesion Mode using
	“Temp up and Temp down” buttons.
Time	Selectable 0:30 to 10:00 mins (Default 1:00 minute)
	Selectable in 30 seconds steps in initial screen set
	ups
	Selectable in 1 second steps when in Lesion Mode
	using “Time up and Time Down” buttons
Special Temperature Profiles	A series of fixed temperature/time profiles are
	programmed into the generator: P1, P2, P3. The
	user can also program a custom profile with the
	following characteristics:

Start Temperature 50-60°C (Default 50°C) Step Time 00:10 to 3:00 mins (Default 2 mins) Step Rise 1°C or 5°C (Default 5°C)

Final Temperature 65° - 90° C (Default 65°C) Final Dwell Time 1:00–10:00 Minutes (Default 4.00 mins

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-4
Model NT1100

Lesion Start	Lesion starts as soon as temperature is within 5°C of
	desired temperature.
Auto Mode	With Lesion Power Control off, the procedure can be
	carried out under Automatic control by pressing the
	“Auto start” button. The temperature will ramp up at
	8°C per second and time will start when the
	measured temperature is within 5°C of desired
	temperature.
	The lesioning can be stopped at any time by
	pressing the “Auto Stop” Button.
Display	Temperature is displayed against time as a curve on
	the screen together with a display of “Measured
	temperature” and “Time to completion of lesion”. RF
	Lesion power (or voltage and current) together with
	impedance are also displayed.
Audible Indicator	An alarm tone (with a volume adjustment) will
	indicate the end of the procedure.

PULSE RF MODE

In pulsed mode the waveform is pulsed rather than continuous.

Pulse Widths	5ms, 10ms, 20ms, 50ms (default 20 ms)
Pulse Frequencies	1Hz, 2Hz, 5Hz, (default 2 Hz)
Temperature Range	Selectable in 42-65°C range, (default 42°C)
Time	Selectable 00:30 to 20:00 minutes
	(default 2:00 mins)
Set Volts/ Current	Pulsed RF can be carried out in Auto Mode at fixed
	voltage or current.
	Voltage range 30-70 Volts (default 45 Volts)
	Current range 50-350 mA

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-5
Model NT1100

PULSE DOSE MODE

In Pulse Dose Mode the numbers of Pulses of Pulsed RF are counted. Pulse Dose Procedures are carried out in Auto Mode.

Set Temperature	42°C
Pulse Counts	120-1200 count (Default 240 counts)
Rate	2Hz
Width	20 mS
Set Voltage Range	30-70V (Default 45V)
Set Current Range	50-350 mA

MULTIPLE PROBES

The Neurotherm can be operated with 1,2 or 3 probes. When in Stimulation Mode each probe is selected by the operator for Stimulation. In RF Lesion, Pulse RF or Pulse Dose Mode the generator energises all connected probes in a time interlacing method. In multiple probe operation not all pulse rates are available.

Features	(a)	Hardware and Software lockout if RF Power
		Control not initially set to zero.
	(b)	Warning on screen if RF Control is not
		initially set to zero or if Auto is selected and
		RF control is not off.
	(c)	LED Flashes on front panel to indicate
		machine is delivering power.
	(d)	Three output sockets to accept a variety of
		probes, (including cordotomy (optional

extra)) and enable multiple probe peration.

(e) Hardware lockout if temperature exceeds 95°C.

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-6
Model NT1100

MAJOR FEATURES

Touch Screen Operation – User interface set up and software control via TP 400 processor.

Windows CE4.2. NET Operating System.

12.1” LCD Sceen with Back lighting and wide antiglare visibility.

Printer Support	Via Bluetooth adaptor internally fitted.
Remote Mimic	Optically isolated running over CAT5 Cable to
Screen	External TFT screen up to 300 metres.
Storage Device	USB Memory Stick for downloading log files.
Service Ports	Only accessible by service engineers for keyboard +
	mouse.

Any equipment connected to rear sockets must comply with IEC60950 and IEC60601-1

Use only parts supplied by Neurotherm Ltd. Any other parts will void the warranty and may cause danger to the patient.

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-7
Model NT1100

Earth Leakage

1	Enclosure leakage current
	Normal	40 microamsps	100 microamps
	Reverse	40 microamsps	100 microamps
	Single fault condition
	Normal	40 microamsps	500 microamps
	Reverse	40 microamps	500 microamps
2	Patient leakage current
	Normal (AC)	5 microamps	100 microamps
	Reverse (AC)	4 microamps	100 microamps
	Single fault condition
	Normal (AC)	7 microamps	500 microamps
	Reverse AC)	7 microamps	500 microamps
3	Patient Leakage current
	Normal (DC)	4 microamps	10 microamps
	Reverse (DC)	4 microamps	10 microamps
	Single fault condition
	Normal (DC)	4 microamps	50 microamps
	Reverse (DC)	4 microamps	50 microamps
4	Patient Auxiliary Leakage Current
	Normal (AC)	4 microamps	100 microamps
	Reverse (AC)	4 microamps	100 microamps
	Single Fault Condition
	Normal (AC)	6 microamps	500 microamps
	Reverse (AC)	6 microamps	500 microamps
5	Patient Auxiliary Leakage Current
	Normal (DC)	4 microamps	10 microamps
	Reverse (DC)	4 microamps	10 microamps
	Single Fault Condition
	Normal (DC)	4 microamps	50 microamps
	Reverse (DC)	4 microamps	50 microamps
6	Patient Leakage Floating Type
	Normal	27 microamps	5000 microamps
	Reverse	27 microamps	5000 microamps
	Single Fault Condition
	Normal	36 microamps	5000 microamps
	Reverse	35 microamps	5000 microamps

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-8
Model NT1100

Environmental Conditions

7	Transport	Temperature	-10°C to 70°C
		Humidity	0-95%RH	Non-Condensing
		Pressure	140-760mmHg	(0-12,200 metres)
				(0-40, 000ft)
8	Storage	Temperature	10°C to 60°C
		Humidity	10 to 80% RH
		Pressure	520-760mmHg	(0-3000 metres)
				(0-10,000ft)
9	Operating	Temperature	10°C to 40°C
		Humidity	10 to 80% RH

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual		4-9
	Model NT1100
Figure 4.1	Output Power/Load Resistance Curve

	60
	50
Wattage	40
	30
	20
	10
	0
	50	100	150	200	300	500	1000	1200
				Impedance

Morgan Automation Ltd

June 2006

	Neurotherm Radio Frequency Lesion Generator Service Manual			4-10
		Model NT1100
Figure 4.2		Typical available Stimulation Output Voltage / Load Resistance and
		Stimulate Output Current/Load Resistance
Load Resistance		Stimulate Output Voltage	Stimulate Output Current
		(5v Constant Voltage)	(10mA Constant Current)
0
100		4.88	10.2
200		4.98	10.2
500		5.04	10.1
1000		5.05	10
2000		5.07	9.84

16
14														Stimulate Output
12														Current
10
8
6														Stimulate Output
4														Voltage
2
0
100			200		500		1000		2000

Load Resistance - Ohms

Morgan Automation Ltd

June 2006

Figure 4.3 Fundamental Radio Frequency circuit.

Neurotherm Radio Frequency Lesion Generator Service Manual	4-11
Model NT1100

4.2Principles of Lesioning

4.2.1 The basic physical principles of radiofrequency ablation

Figure 4.3 shows the fundamental radiofrequency circuit. The RF lesion generator or power source provides a source of RF. It is connected by wires to 2 electrodes: one inserted into the body, referred to as the active electrode; another in contact with the surface of the body, referred to as the dispersive electrode. This is the so-called monopolar configuration. The RF voltage

causes current to flow through the wires, through the electrodes, and to the patient’s body. The patient's body is a conductive electrolytic media, and thus the patient’s body becomes part of the RF circuit. This current spreads out from the electrodes and flows through the electrolytic tissue medium of the body. The active and dispersive electrodes have a similar physical role in delivering and receiving the current, but functionally, because of their differing areas, they have very different effects with regard to the RF heating process.

The active electrode with its smaller surface area has much higher field densities in the tissue adjacent to it. This higher field density causes significant heating near the active electrode surface. The dispersive electrode has a much larger area, and, as a consequence, the field density is much lower in the tissue adjacent to it. This results in a lower radiofrequency heating effect, and thus if the dispersive electrode is large enough no appreciable heating will occur near it. In fact, a large area surface plate to join to the skin with a conductive gel for good conductivity will not heat appreciably even though this same radiofrequency current will cause intense heating near the much smaller active electrode. It is recommended for most radiofrequency procedures that the dispersive electrode, therefore, should have an area of greater than 150 square centimeters to be safe from any significant heat elevation when RF lesions delivering 50 watts or less are used.

The mechanism for radiofrequency heating is shown in figure 4.4. The electric field lines emanate from the active electrode tip and are created by the voltage impressed upon it by the radiofrequency generator. This electric field creates an electric force on the charged ions within the electrolytic medium of the tissue. According to the physics laws this force produces a motion, and the motion is oscillatory at the frequency of the RF current.

Figure 4.4 mechanisms for

Radiofrequency heating

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-12
Model NT1100

It is this radiofrequency motion which causes the ions to rub against the surrounding fluid medium, causing friction which results in the tissue heating. The temperature at any point is controlled by the frictionally induced power dissipation at that point, mediated by thermal diffusion and thermal convection. The thermal diffusion and convection are typically caused by blood circulation.

The temperature distribution around the electrode tip can be calculated by making certain simplifying assumptions. One of the assumptions is that the medium is homogeneous and that the factors of thermal diffusion and circulation are also uniform. Under these circumstances one can calculate the isotherms (surfaces of constant temperature) surrounding the electrode tip for a given impressed radiofrequency voltage. These isothermal surfaces are critical to determining the lesion size. As it is known that living tissue will be permanently destroyed for sustained temperatures of approximately 45 C, the 45 degree isotherm can be considered to be the outer border of the lesion since tissue within this volume will be thermally destroyed and tissue outside of this volume will experience lower temperatures than are necessary for cellular destruction. Isothermal surfaces are indicated by the -- lines in figure 4.4

It is important to understand that the radiofrequency field, and thus power dissipation in the tissue actually heats the tissue as opposed to the electrode itself. The heated tissue in turn raises the temperature of the electrode tip and thus heats the tip. Therefore, it is not the electrode tip which heats the tissue, but rather the tissue which heats the electrode tip. If the electrode is properly designed so as not to sink away too much of the thermal energy, the electrode will give an accurate representation of the tissue temperature at its surface. It is for this reason that thermal monitoring of the radiofrequency tip is a good indication of the hottest portion of the lesion volumes, namely the isotherm that lies closest to the surface of the electrode. With the simplifying assumptions of a homogeneous medium, the lesion size represented by the 45 degree isotherm increases with increasing tip temperature and also increases with increasing tip dimension particularly the radius of the electrode tip.

Figure 4.5 Schematic of Tissue Temperature v Distance for Electrode Tip.

Morgan Automation Ltd

June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-13
Model NT1100

Figure 4.5 shows a schematic diagram of the temperature of the tissue as a function of distance from the electrode tip. It should be noted that the temperature is not isotropic for a non-spherical electrode, and thus these curves may differ at different orientations on a non-spherical electrode tip. The temperature at the surface of the electrode, such as T2, measures very nearly the hottest tissue nearby, and the tissue at greater distances falls off until it asymptotically approaches body temperature at large distances from the electrode tip. By raising the radiofrequency voltage, one will increase the temperature to T1 near the electrode tip, and thus the distances to the 45 C isotherm will increase accordingly. With knowledge of these characteristics and the temperature vs.distance curve, one can therefore judge the size of the lesion volumes by choosing the appropriate tip temperature for a given tip geometry. This is the reason temperature measurement has been historically essential to produce consistent and quantified lesion volumes.

Measurement of the tip temperature has another very important benefit. By avoiding tip temperatures near 100 C (the temperature at which water boils), one avoids the undesired effects of charring, sticking or the formation of a hemorrhage or explosive gas which may be also be accompanied by sparking. In the early days of neurosurgical lesions, before reliable tip temperature measurement was possible, neurosurgeons would establish the end point of their lesion making by listening for the “popping” lesion. The popping was caused by the tip temperature exceeding 100 C and the subsequent gas formation at the tip to the electrode. This obviously was not a controlled lesion technique and led to unpredictable and dangerous destructive conditions.

In pain management there are now well-established prescriptions for appropriate electrode size and tip temperature to achieve desired lesion volumes. It has been historically clear that prescriptions which involve power and current did not have lasting value, but rather prescriptions that involve temperature, electrode size and accounting for the heat washout caused by blood flow. The importance of temperature control was not always recognized. For instance, in the early days of percutaneous cervical cordotomy, elaborate prescriptions of current, power and time for making RF lesions were established. The subsequent clinical results were not consistent in the early days of cordotomies, and it was only when temperature was measured at the tip of the cordotomy electrode that consistency and reproducibility was finally achieved.

Figure 4.6 Increase in Lesion Size v Time

Another important aspect of controlled radiofrequency lesion making is illustrated in Figure 4.6. This shows experimental data of the increase in lesion size for a fixed electrode geometry and a fixed tip temperature. The lesion size in this situation is defined as the width of the prolate ellipsoidal width of the prolate ellipsoidal lesion volume. The graph clearly shows that for constant tip temperature the lesion size grows and asymptotically reaches a maximum value in a time between 30 and 60 seconds. The 45C isothermal surface can then be referred to as the equilibrium lesion size. Leaving the radiofrequency power turned

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June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-14
Model NT1100

on indefinitely beyond 60 seconds will not increase the equilibrium lesion size. In the past, so-called time-dependent lesions were made in which a certain power was held by the radiofrequency generator for 10-20 seconds. This too led to inconsistent results, and resulted in the acknowledgment of the equilibrium lesion size as being the optimum methodology.

It is noteworthy that impedance monitoring has a great value in assessing the progress of a heat lesion. The impedance seen by the electrode tip depends on tissue interfaces and this property has been used very effectively to distinguish between the interface of electrolytic fluids and tissue. For example this has been used with percutaneous cordotomy electrodes to clearly tell when the electrode has traversed from the cerebral spinal fluid to a position of contact with the spinal cord. Impedance monitoring has also been used to identify when an advancing electrode has progressed from the annulus of the disk into the nucleus pulposus. The change of impedance during the heating process is dramatic. It has been shown that as the tissue or medium heats up, the impedance will drop. This is very much related to the phenomena that the engine oil in an automobile will become less viscous as the temperature of the engine increases. There is a point, however, as the temperature at the lesion tip approaches 100 C, when the impedance will cease to decrease and, in fact, will rise precipitously as the temperature approaches the boiling point. The reason for this is that the protein coagulation has a rapid onset in this temperature range causing a decrease in Ionic mobility. Near the boiling point, gas suddenly forms around the electrode tip, acting as an electrically insulating barrier thus sending the impedance to very high levels. At the onset of boiling, the impedance rises very rapidly. In summary, it is clear that the monitoring of temperature and impedance are both of great significance.

4.2.2Pulsed radiofrequency

Historically, radiofrequency was neuroablation. This was true for percutaneous cordotomy, the treatment of trigeminal neuralgia, and the destruction of the medial branch nerve for facet pain. Mysteries remained however. It was not understood why RF lesions were so often followed by long periods of discomfort before any beneficial clinical effect appeared. In the 1990s, additional unanswered questions were added. The mode of action of RF lesions of the lumbar sympathetic change (other than for vascular disease) was not understood since there were acceptable success rates, though the results did not correlate with the degree of sympathetic block.

This led to the hypothesis by Sluijter that heat might not be the element causing the clinical effect of an RF lesion. The next obvious steps were to define a method to apply radiofrequency at high intensity without allowing the tip temperature to rise to neurodestructive levels. The method that was chosen by Sluijter was placing the output setting of the RF generator in the same range as was customary for making heat lesions but interrupting the output, thus allowing for sufficient time for the generated heat to be washed away by thermoconductivity and circulation.

This method has been commonly referred to as pulsed radiofrequency (PRF). Pulsed radiofrequency is a relatively new technique that applies short pulses of radiofrequency (20 ms) at a high voltage of 45 to 60 volts to neural tissue. Figure 4.7 shows the currently accepted paradigm of 20 milliseconds of RF followed by 480 milliseconds of off time. In this way high intensity radiofrequency is delivered but with a short enough on time so as not to cause heating above 42 C.

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June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-15
Model NT1100

Figure 4.7 Typical Pulsed RF Signal

It is a natural reaction to think of pulsed radiofrequency as being analogous to the neural modulation effects achieved using spinal cord stimulators or TENS units. However, these two modalities are very different. In neural modulation the therapeutic effect is achieved by applying low frequency (< 1000Hz) rectangular pulses.

Strength

2

4

6

8

10

12

Frequency Hz

Figure 4.8 Frequency Spectrum of a 2Hz Rectangular Pulse.

Figure 4.8 shows the frequency spectrum of a two Hz rectangular pulse. As can be seen from the figure, the major frequency component is at two Hz and falls off as the frequency increases. At frequencies above 1000 Hz, the amplitude of the frequency component is getting very small.

There is a very different situation with pulsed radiofrequency. In this case, the rectangular pulses have radiofrequency inside of them. This changes the frequency spectrum entirely. As can be seen from figure 4.9, the major frequency component is now 500 kHz and decreases at higher and lower frequencies. At frequencies below 1000 Hz, the contribution can be shown to be negligible. In conclusion, the frequency spectrums of pulsed radiofrequency are entirely different than the frequency spectrums of low frequency stimulators.

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June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-16
Model NT1100

Strength

500 kHz

frequency Hz

Figure 4.9 Frequency Spectrum of a 500KHZ Signal.

There is very little conclusive research to date on the mode of action of PRF. A few preliminary studies have shown that a modification of CFOS and DNA expression has been observed in cells of rat DRG after exposure to PRF. However, additional basic research needs to be done to come to any good scientific conclusion as to the mechanism of action of PRF.

In this era of evidence based medicine, pulsed radiofrequency has not yet been validated. Several retrospective studies and audits have been conducted and the initial results are positive. Because of a lack of uniform treatment guidelines, the anecdotal results for pulsed have ranged from magnificent to abysmal. It is this author's belief that standardization of many of the PRF parameters will at worst result in uniform treatment for all patients and at best significantly improve the clinical outcomes. In any case, controlled clinical studies are long overdue for this potentially promising modality.

When the study of PRF began, the parameters were arbitrary. For the voltage, a value is taken that was within the range of the voltage during the initial heating phase of an RF heat lesion. The values of 20 milliseconds on time and 480 milliseconds off time were chosen because they were thought to provide good conditions for preventing heating above 42 degrees Centigrade. The initial choice of 120 second duration of the procedure was completely arbitrary, and it was just taken as a starting point. With no scientific basis, these parameters have been arbitrarily modified by clinicians and thus there is no consensus as to what the optimum parameters are.

One of the major variables in pulsed radiofrequency treatment is the voltage level when the pulses are “on”. This arises from the desire to keep the temperature below 42 degrees C. If 42 degrees C. is reached, it is necessary to either manually or automatically reduce the pulsed amplitude or the pulsed duration in order to ensure the temperature does not exceed 42 degrees. Using the pulsed dose method, every pulsed is ensured to be of the same amplitude and duration. This method is explained in detail in the following paragraph.

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June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-17
Model NT1100

4.2.3The Pulse Dose Concept.

Whenever Pulsed RF is used, if the selected temperature limit is reached, the pulse must be modified in some way to prevent the selected temperature limit from being exceeded; this can be done by either.

1.Modifying the pulse amplitude of the pulses- i.e. if a 45 volt amplitude was set, and if the temperature limit was set to 42 degrees and was reached, the next pulses will be reduced in voltage to prevent the temperature from increasing above the 42 C temperature limit.

2.Modifying the pulse width whenever the temperature limit was reached, thus insuring that each pulse delivered was the full set voltage amplitude.

In Pulse dose the two conditions shown above are avoided. A FULL pulse is always given, i.e. if the setting is 45 volts amplitude for 20 milliseconds, you will always deliver this pulse amplitude and duration. If the set temperature limit is reached, the generator will wait until the temperature drops below the set temperature limit, and then again will give a FULL amplitude and duration pulse.

Because the generator is only delivering full pulses, in this mode initially one sets the “number of pulses” that is desired for the procedure as opposed to procedure time, since procedure time can vary depending on whether the set temperature limit was reached.

The following diagrams depict the different modes-

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June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-18
Model NT1100

Figure 4.10 Shows Historical Pulsed RF amplitude control

Figure 4.10 Pulse RF Amplitude Control

The beginning pulses are the desired pulses of 45 volts amplitude and 20 ms duration. Note that the moment the set temperature limit is reached the voltage is changed (reduced) in order to keep the temperature below this limit. (Note that this implies that every patient gets unpredictable and variable pulse amplitude which is undesirable).

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June 2006

Neurotherm Radio Frequency Lesion Generator Service Manual	4-19
Model NT1100

Figure 4.11 Shows Historical Pulsed RF – pulse width control

When the temperature limit is reached the pulse width is changed as opposed to the pulse amplitude. Note that the pulse width is changed the moment the temperature limit is reached, the width is varied to keep the temperature limit. Though this is better than amplitude control, it still implies that treatments will not be consistent and uniform between patients.

Morgan Automation Ltd

June 2006