Biomedical. Measurement and Design ELEC4623/ELEC9734. Electrical Safety and Performance Standards

Transcription

1 Biomedical Instrumentation, Measurement and Design ELEC4623/ELEC9734 Electrical Safety and Performance Standards

2 Contents Physiological Effects of Electrical Currents Safety Standards for Medical Instrumentation ECG Performance standards

3 Physiological effects of electricity E is proportional to m Ionic Concentration (outside) Ionic Concentration (inside) Effects of electrical currents on the body manifest themselves in three different ways: Injury to tissue Uncontrollable muscle contraction Ventricular fibrillation All living mammalian cells exhibit a potential difference across the cell membrane (Nernst)

4 Nerve excitation Excitation of a nerve occurs at a threshold level of ~ - 60 mv, and the action potential is propagated along the whole nerve membrane. At the end of the nerve, the ACTION POTENTIAL causes a release of the chemical ACETYCHOLINE A WAVE of DEPOLARISATION propagates down the entire muscle membrane. This wave causes movement of specific ions (CALCIUM) into the muscle cell to activate the contractile machine

5 Muscle contraction The muscle can be stimulated by contract by: activating the nerve connected to the muscle direct stimulation of the muscle with ELECTRIC current. The temporal dependence of the various phases of action potential (i.e., it takes a finite time for ions to pass across the membrane) the properties which define the excitation of nerve and muscle cells are described by the STRENGTH- DURATION CURVE, i. e. the relationship between the strength (magnitude) of current and the duration of times required to produce excitation and therefore contraction.

6 Strength/Duration curves Current 2I 0 I 0 τ Excites T (secs) There is a minimum value of current (I O = constant rheobase current) to produce excitation τ = Chronaxie time constant It τ = I0 1+ t Hence the frequency of stimulation is important in terms of the effectiveness of a current in producing excitation The optimal frequency range is Hz (!)

7 Stimulation frequency Tetanic or subtetanic contractions can cause respiration to stop or uncontrolled movement of the limbs. High frequency and short duration stimulation does not produce muscle contraction but is used to produce local heating as in the process of DIATHERMY ( Electric Scalpel ) Tetanic contraction = contraction of maximum amplitude

8 Ventricular Ectopic Beats Ectopic = occurring in an abnormal position or in an unusual manner or form Ventricular ectopic beats (premature ventricular contractions) are common in healthy people Under normal conditions the heart is refractory for about 300 msec. A VEB will normally annihilate itself and will not be self-sustaining.

9 Ventricular Fibrillation If the heart tissue becomes damaged (e.g. from ischaemia or infarction) it can affect the normal conduction pathways By the time the ectopic activity travels around, the transient refractory area is sensitive again, causing a CIRCUS MOTION leading to FIBRILLATION VF is almost invariably fatal unless defibrillated: the uncoordinated ventricular contractions result in little or no blood flow to the body. In the ECG VF appears as an erratic rhythm where the waves cannot be identified.

10 Sensitivity to electric current

11 Perception current The minimum value of current perceived by a subject. Mucous membranes are the most sensitive, but current is usually measured in the grasped hand 45 μa for the tongue 1 ma for the hand The frequency dependence of perception current is a direct result of the strength-duration curve for membrane excitation. Slide 6 The perception current, in itself, is not hazardous, but accidents may occur indirectly. It is important in determining levels of leakage current for household and industrial appliances

12 Let-go current The current at which a subject is unable to release an energised conductor grasped in the hand. The current stimulates all the forearm muscles, causing tetanic contractions. Since the flexor muscles are stronger than the extensor muscles, the subject is unable to release the grip. The following have little effect on let-go current The location of the return electrode (hand or foot) The size of the stimulating conductor Moisture conditions of the site of the electrode contact t

13 Let-go current Let-go current is also frequency dependent Relative minimum in the range Hz mean ~ 16 ma for men 2/3*16 for women. The let-go current increases at both LOW and HIGH frequencies e.g. 24 ma at 5 Hz and 1000 Hz

14 Physiological sensations at DC Physiological sensations Percentage of test subjects 5% 50% 95% Slight prickle in palms and Finger-tips at ma Feeling of warmth and increased prickle in at ma palms, slight pressure in wrists Pressure increasing to shooting pain, developing in wrists and palms Prickle in forearm, pressure in wrists, shooting pain in hands, increased feeling of warmth at ma at ma Increased pain caused by pressure in at ma wrists, prickle reaching up to elbow Acute pain caused by pressure in wrists, shooting pain in hands Current path: hand-body-hand; current in ma at ma

15 Fibrillatory Current K IRMS =, ma (0.083 < t < 5 secs) t K (ma.s -0.5 ) 50 Kg 18 Kg Adult child Experiments may not be conducted directly in humans Non-Fibrillating 116 ma* 52 ma Extrapolation ti from animals or analyses of accidental Fibrillating 185 ma 69 ma electrocutions For a 95% probability of fibrillation or a 95% probability of non-fibrillation with shock Hz ma duration of 1 sec Fibrillatory current is also frequency dependent < ~

16 Factors affecting fibrillatory current 1. Electrode Position Experiments are usually performed in dogs with saline filled catheters (see next slide) Most sensitive areas are: apex of right ventricle (for internal placement) midpoint of right ventricle (for external placement on the pericardium). Five fold variation of current with position and orientation of contact with electrode. A current of 20 microamps caused ventricular fibrillation in dogs. (Hence LEAKAGE current set to 10 microamps by many electrical standards).

17 Canine ventricular arrhythmia, pump failure and fibrillation threshold currents of 60 Hz and 3 seconds duration Catheter position Random Most Sensitive: (n = 54) (n = 50) Ventricular arrhythmia 65 ± ± 2.5 Pump failure 125 ± ± 4.4 Fibrillation 335 ± ± 36 Source: Ledsome et al. Cardiovascular Research 8: , * Values in microamperes, mean ± SD.

18 Factors affecting fibrillatory current 2. Electrode size and current density. Threshold for fibrillation is inversely proportional to current density Fibrillation threshold for application of current to exposed heart of >3 sec: ~ 80 microamps / mm 2 for area of 2 mm 2 ~ 5 microamps / mm 2 for area > 200 mm 2 3. Time factors Current must flow through the heart during the RELATIVE REFRACTORY PERIOD (T wave) for fibrillation to occur. Threshold decreases with increasing duration of current over range 83msec sec (AC) and msec - 21 sec (DC)

19 Fibrillatory window There is a variable upper limit of current above which fibrillation will not occur i.e. fibrillation occurs only in a current window. The longer the duration of applied current, the wider the window. For short pulses, the window is 3-4 times the threshold limit. Upper limit is ~2-3A through the intact chest, above this limit the heart will remain contracted during time of current application. Current greater than 2 Amps through the intact chest may not result in fibrillation but can cause temporary cardiac arrest and paralysis. Current greater than 6 Amps can cause cardiac arrest, paralysis and deep burns On removal of the large super-fibrillation threshold applied current, heart usually resumes its normal rhythm DEFIBRILLATION apply a large high-current pulse simultaneously to ALL heart muscle cells All depolarise at once and relax together Hopefully sinus rhythm returns

20 Impedance of the human body TOTAL BODY IMPEDANCE, essentially consists of the sum of THREE impedances; SKIN impedance at the TWO contact points: extremely variable with moisture, contact area, pressure, temperature, current duration, touch voltage, time. p,, g, INTERNAL impedance: essentially resistive (approx. 500 Ohm) and not too variable.

21 Total body impedance at 50Hz V1= R V(Volts) Z t (Ohms) I (ma) Estimated total body impedances at 50Hz

22 Internal body resistances Friedberger (1934): simplified representation for a reference body (Height 165 cm; Weight 56 kg)

23 Protection against electric shock with floating circuits A floating or isolated circuit itis a one that thas no connection to any other circuit or to ground. The secondary circuit it is isolated from all other circuits by the transformer functional insulation, and the other functional insulations. However, there is a very small leakage current in the stray impedances of the functional insulations. A floating or isolated circuit

24 Protection Circuits Pole-to-opposite pole. Figure 2A depicts the circuit path when a man simultaneously touches both poles of the floating or isolated circuit. In this situation, there is no insulation. The current is limited it only by the impedance of the body.

25 Protection Circuits If we interpose an insulating barrier between one pole of the floating circuit and the man, then we can define that barrier as Basic Insulation. In the event of failure of that t Basic Insulation, there is an electric shock current in the man. If we extend that same insulation such that it is interposed between the OPPOSITE pole and the man, then we can define the OPPOSITE pole portion of the insulation as Supplementary Insulation (because it provides insulation against the SECOND body connection). Failure of ONLY one insulation leads to no shock current.

26 Biomedical Instrumentation, Measurement and Design ELEC4483 Lectures 21 and 22 Diagnostic Ultrasound

27 Physical properties of sound Ultrasound = 20KHz 1 GHz Diagnostic Ultrasound is usually in the range of 1 10 MHz Remember sound is a longitudinal pressure wave v = f*λ, f = frequency, λ =wavelength Velocity of ultrasound in soft tissue is relatively constant ~1540m/sec Variations in v are functions of the density and elasticity

28 Sound waves

29 Acoustic (characteristic) impedance Acoustic (characteristic) impedance Z = ρ *v (kg/m 2 /sec, v = velocity, ρ = density) Think of this as the substance s resistance to penetration of sound waves When a sound wave reaches the boundary between two media of different characteristic impedance, it may be transmitted, reflected, refracted or scattered. What actually happens depends on: The difference between the characteristic ti impedance of the two media. The orientation of the sound beam relative to the boundary plane. The smoothness or roughness of the boundary. The distance between sound source and boundary The frequency of the ultrasound wave.

30 Reflection and Transmission When a sound wave strikes a boundary at right angles, part of the energy is transmitted and the rest is reflected back The ratio of the amplitude of the reflected wave (echo) to that of the incident wave is called the amplitude reflection coefficient R a, and is given by Z Z R a = Z + Z Z 1 = Z 2 means no echo Liver to kidney R a ~ 6%, Soft tissue to lung R a ~ 50% Soft tissue to bone R a ~ 40%. Soft tissue to air R a ~ 99% the use of a gel as a coupling medium is essential.

31 Schematic illustration of the reflection of an ultrasound pulse

32 Refraction When the sound sits the interface at an angle, reflected and transmitted beams travel in different directions to the incident beam incident angle = reflection angle transmitted wave is refracted angle of refraction depends d on the difference of the characteristic impedances of the two media, and hence to the difference in wave velocities. The angle of refraction is smaller than the incident angle when the wave velocity in the second medium is slower than in the first medium When the refraction angle becomes 90, no energy is transmitted. Critical angle cannot see any deeper structures

33 Types of echoes (reflections) Specular - originate from smooth, large, regularly shaped objects Such echoes are relatively strong and depend on incident angle Scattered - originate from relatively small, weakly reflective, irregularly shaped objects are less angle dependant and less intense. (ie. blood cells)

34 Attenuation All media attenuate ultrasound Attenuation of ultrasound increases with frequency (thus penetration drops) Ultrasound amplitude attenuation coefficient α Ultrasound intensity attenuation coefficient μ NOTE: Intensity is proportional to (amplitude) 2 So μ = 2 α Ultrasound rule of thumb for soft tissues α = A db cm -1 MHz -1 For many tissues A ~ 1 I( x) = I e, Q( x) = Q e μ x 0 0 α x

35 Absorption and power Absorption results in the conversion of the wave energy to heat This causes a temperature rise Ultimately the best images come from highest S/N ratio So just use more power! Regulations? IEC and FDA Power restricted to about 200mW max for pulsed, 50% that for CW Can cause up to a C temperature rise

36 Transducers Piezoelectric effect transducers always used Quartz and some ceramic materials are piezoelectric Apply voltage get a deformation or vice versa AC voltage applied will produce vibrations at the same f This produces sound waves

37 Continuous wave (CW) vs. pulsed wave (PW) ultrasound Previously we looked at CW ultrasound for measuring blood flow. CW Doppler separate transmitter and receiver, continuous signal transmission PW Doppler same transducer for transmit and receive Pulses sent in bursts with time delay in between

38 CW vs PW Continuous Wave Advantages Accurately measures high velocity flows Disadvantages Lacks range resolution o Pulsed wave Ability to measure velocities at a specific location (range resolution) Aliasing of velocities above the Nyquist limit (inability to measure high velocities accurately)

39 Pulsed wave signal Typically, the pulse duration is 1 micro second and the pulse repetition period is 1 msec. Since the duty factor is of the order of to 0.005, the same element can transmit the ultrasound and also receive the echo. This technique is known as the Pulse-Echo mode.

40 Time gain control Voltage controlled It accounts for the attenuation of sound in the tissue Time it takes for pulse to return is proportional to distance travelled Simplest form is a logarithmic voltage ramp

41 Near and far field zones Far field zone at distance 2 D > 4 4λ λ

42 Mechanical focusing Internal Focusing - the piezoelectric element is formed in a curved element that focus the beam. External Focusing - a lens is placed on the transducer face

43 Ultrasound scan heads (a) Rotating mechanical device 5 to 10 rotations per second (b) linear sequenced array (c) curved linear array (d) phased array (beam steering)

44 Linear sequenced arrays Pulse is applied simultaneously to all elements in a small group (aperture) E.g. 1 to 4 then 2 5 etc. Each element is about a wavelength wide Rectangular image formed, with width equal to scanning head width

45 Curved linear arrays Curved linear arrays send pulses out in different directions Cover wider area than linear arrays for size of head Operation identical to linear sequenced array

46 Linear phased arrays Pulses are applied to ALL elements (not a small pp ( group) Each element has a programmable phase delay End result is to steer or focus beam in different directions

47 Phased array focussing and steering Focussing and steering an acoustic beam using a phased array. (a) transmit mode (b) Receive mode. Dynamic focussing allows the focus to change the focal point.

48 Lateral resolution The ability to resolve objects side by side. Minimum reflector (object) separation perpendicular to beam direction that produces two separate echoes Equals the beam width in the scan plane

49 Axial resolution The ability to resolve objects that lie behind one another in the direction of the beam. Equals half the spatial pulse length (Length of a pulse from front to back = length of each cycle times the number of cycles in the pulse) Can improve by Reducing number of cycles per pulse (can t reduce below 1 cycle!) increasing frequency (but decreases depth of penetration)

50 Axial resolution