In g ro u p A the resu lts o f two children had to be excluded because they w ere unable to c a n y o u t the tests correctly. O ne gro u p B child refused to take th e tests, whilst a second child’s resu lts had to be excluded d u e to a m echanical fault. T h irte en sets of lung function tests in each group w ere thus available fo r analysis. F o r each child, th e resu lts w ere c o rrelated with his o r h e r age, weight a nd height and c a lc u la te d a s a p e r c e n ta g e o f t h e p re d ic te d v a lu e s a c c o rd in g to Schoenburg. 8 T able 2 com pares th e average values fo r the two groups. G raphic repre se n tatio n o f th e average perce n ta g e values reveals no significant differences in lung function betw een th e two groups (F igure 1). In both groups, however, th e F E V i, F E V i% a nd F E F 50 a re low er th an th e norm al average o f 100% p redicted by Schoenburg. O nly th e FV C in b oth g roups and the P E F R in gro u p B reach ed norm al values. 120 100 FVC FEV1 FEV1% FEF PEFR H S eries A W Series B Figure 1: Lung function values of 8 to12 year old children DISCUSSION AND CONCLUSIONS T h e higher re p o rte d incidence o f respiratory disease in g roup B children, w ho lived clo ser to th e petrochem ical complex, cannot be regarded as statistically significant d u e to th e small sam ple size. The higher incidence o f sm okers in g roup B parents may also have played a role in the higher incidence o f disease in this group. A lthough no co rrelation was found betw een the n u m b er o f sm okers in the house and the n u m b er o f children w ho suffered from asthm a, a previous study has shown th at ch ild ren ’s lung functions a re adversely affected w hen th e ir parents, and in p a rticu la r th e ir m others, sm oke . How­ ever, a study carried o u t in O h io 10 also show ed a hig h er re p o rte d incidence o f a cu te a nd c h ro n ic re sp irato ry disease in children a tte n d ­ ing school in an area o f raised S O 2 a nd N O 2 levels. N o significant difference could be found betw een th e lung func­ tions o f th e tw o groups o f children, but b oth g roups d e m o n stra ted low er values th an th e pre d ic te d norm s.8 Since th e possibility o f a d e g ree o f a ir pollution in th e a re a o f th e c o n tro l g ro u p could n o t be excluded, a fu rth e r study o f a larg e r sam ple o f children from suburbs b o rdering on th e petrochem ical complex is recom m ended, with a c o n tro l g ro u p from fu rth e r afield. A lthough th e 1986 C S IR study o f th e a rea show ed pollution a t th a t tim e to be within acceptable lim its,1 M ostardi 10 has suggested th at th e a cc eptable limits fo r atm ospheric SO 2 and N O 2 be redefined. Acknowledgements T hanks a re re co rd e d to P ro fe sso r M A d e Kock, fo rm e r H e ad o f th e D e p artm en t o f Internal M edicine, U niversity o f Stellenbosch, for the provision o f th e E L F a n d fo r training in its use, and also to D r S W alsh o f th e sam e D e p a rtm e n t fo r h elp in analysing th e results. REFERENCES 1. Klopper JLM, Harrison JA, Rip MR. Epidemiological studies of health effects associated with air pollution in the G reater Cape Town area, Pretoria, CSIR, 1986. 2 LeederSR , Corkhill RT, Irwin LM e ta l. Influence o f family factors on the incidence o f lower respiratoiy illness d u rin g the first year o f life. B r J Prev Soc M ed 1976;30:203-212 3. Sharratt MT, Cherny F. Pulmonary function and health status:a pilot study 1971- 1974. Arch Environ Health 1979;34:114-119. 4. Kagawa JU N .Toshio MD, Toyama MD. Photochemical air pollution - its effects on re s p ira to ry fu n c tio n o f ele m e n ta ry school ch ild ren . A rch E nviron H ealth 1975;30:117-122 5. Alarie Y. Classification of airborne chemicals that stimulate respiratoiy tract nerve endings.O i/ R ev Toxicol 1973;2:229. 6. Saruc M, Fugas M, Hrustic O. Effects o f urban air pollution on school-age children. Arch Environ Health 1981;36:101-108. 7. Ferris BG. Epidemiology standardisation project. A m R ev Resp Disease 1978; 118(6) Part 2 8. Bouhuys A, Schoenburg JB, Beck, GJ. Growth and decay of pulmonaiy function in healthy blacks and whites. Resp Physiol 1978;33:367-393. 9. Ware JH , Dockeiy DW, Spiro A et al. Passive smoking, gas cooking, respiratoiy health of children living in six cities. A m R ev Resp Disease 1984;129:366-374. 10. Mostardi RA, Ely DL, Woebkenberg NR et al. The University of Akron study on air pollution and human health effects (1 & 2). Arch Environ Health 1981;36:243-255. CHARACTERIZATION OF THE ACOUSTIC OUTPUT OF THERAPEUTIC ULTRASOUND EQUIPMENT M G van der Meiwe (MSc Physics Stellenbosch) N Bhagwandin (MSc Physics Natal) J E van der Spuy B Eng Hons Stellenbosch) PRI e Roux (PhD Physics Cape Town). Directorate of Radiation Control SUMMARY The safety and efficacy of ultrasound therapy may be compromised if the output from therapy transducers differs considerably from the indicated value. Although the total power output of a transducer can be easily measured using a pressure balance, it is also important to know how this energy is distributed through space. B y'using a hydrophone scanning technique, beam profiles of the energy dis­ tribution can be obtained. From the beam profiles various parameters such a the effective radiating area (ERA) and the beam non-uniformity ratio (BNR) can be determined. Since the spatial-average intensity selected for treatment is a ratio of the emitted ultrasound power and the effective radiating area, it is essential to be able to measure parameters like the effective radiating area. In this study ERA and BNR measurements for commercially available devices were performed with a hydrophone scanning technique. OPSOMMING Die effektiwiteit en veiligheid van ultraklankterapie kan bevraagteken word indien die lowering vanaf terapie-omsetters betekenisvol afwyk vanaf die aangeduide waarde. Alhoewel die totale drywingslewering vanaf 'n omsetter maklik gem eet kan word met behulp van 'n drukba- lans, is dit 00k belangrik om te weet hoe die energie ruimtelik versprei is. Bundelprofiele van die energieverspreiding kan verkry word deur gebruik te maak van 'n hidrofoon-aftastings-tegniek. Vanaf die bun­ delprofiele kan verskeie parameters soos die effektiewe stralingsarea (ESA) en die nie-uniformiteitsverhouding van die bundel (BNV) verkry word. Aangesien die ruimtelik-gemiddelde intensiteit, w atgew oonlik as 'n behandelingsparam etergekies word, die verhouding tussen die uitgestraalde drywing en die effektiewe stralingsarea is, is dit van belang om parameters soos die effektiewe stralingsarea te kan bepaal. In hierdie studie is van 'n hidrofoon-aftastings tegniek g e ­ bruik gem aak om ESAen BNV metings van kommersieel beskikbare terapie toestelle te verkry.________________________________________ DEFINITIONS E ffective radiating a rea (E R A ) m eans the a rea o f th e effective radiating surface that consists o f all points at which the ultrasonic Bladsy 4 Fisioterapie, Februarie 1992, deel 48 no 1 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) intensity is equal to o r g re a te r th an 5% o f the maximum spatial ultrasonic intensity a t the effective radiating surface, expressed in c m 2. Beam non-uniform ity ratio (B N R )1 m eans the ra tio o f the highest intensity (spatial peak) in the ultrasound field to the average intensity. INTRODUCTION O f all the techniques available to m easure and characterize the acoustic o u tp u t o f ultrasound therapy equipm ent, the d eterm ination o f to tal pow er with a radiation pressure balance to g eth e r with the spatial a nd tem poral field characterization using a calibrated h ydro­ p h one 2,3 have found m ost w idespread use and acceptance. T hese two techniques permit the m easurem ent o f m ost param eters a c ­ cepted to be o f im portance in ultrasound therapy applications. T he principle o f the radiation pressure balance is the m easure­ m ent o f the force produced on a target intercepting the whole ultrasound beam. T h e force may be related to th e total pow er in the ultrasound beam. V arious designs o f ultrasonic pressure balances a re widely used to assess the accuracy o f the total o u tp u t pow er of ultrasound therapy devices. How ever, m ajor shortcom ing in the use o f pressure balances is the fact that no inform ation is gained on the distribution o f u ltra ­ sound th roughout the acoustic field. T he determ ination of th e d is­ trib u tio n o f acoustic energy in b oth space and tim e is im portant in the assessm ent o f param eters like th e effective radiating a rea and the beam non-uniform ity ratio. F o r the m easurem ent o f these p a ram e ­ te rs a beam plot system is used. In essence, the system consists o f a w a ter tank into which the ultrasound beam radiates and a calibrated m e a s u r in g h y d r o p h o n e ( u n d e r w a t e r m ic r o p h o n e ) s c a n n in g th e beam by m echanical means. A hydrophone is a device that produces an electrical signal in response to an applied acoustic field. T he sensitive elem ent o f the hydrophone is usually a small piezoelectric elem ent and the electrical voltage developed is related to the acoustic pressure at the elem ent. By scanning a hydrophone across an ultrasound field, an indication o f the distribution o f the acoustic pressure across the field can be obtained (a beam profile). In this study these beam profiles a re used to determ ine: • The Effective R adiating A rea (E R A ); and • T he Beam N on-uniform ity R a tio (B N R ). MEASUREMENT APPARATUS Figure 1: Schematic and block diagram of the measurement apparatus T h e m easurem ent a p p ara tu s a t the D ire cto rate R adiation C o n ­ trol is shown schematically in Fig. 1. T he ultrasound scans a re done in a tank, 60 x 26 x 26 cm to which fixtures a re a ttached fo r m ounting hydrophones a nd transducers. T he hydrophone m ount is a ttached to a g ear system which provides x, y and z translation. The hydrophone can also be ro tated a bout two axes. Scans used for m easurem ents consist o f a two-dimensional array o f d ata accum ulated in a ra ster fashion in the x,y plane, at a fixed z-distance from the tran sd u ce r face. T he scan size and step size a re variable. Typical step sizes a re between 1 mm and 2 mm and typical scans consist of approxim ately 2000 data points. All m easurem ents a re m ade in tap w a te r with a nom inal 0 . 5 mm d iam e te r M edisonics (M edisonics (U K ) L td., H aslem ere, Surrey, U K ) hydrophone, which has a frequency range betw een 200 kHz and 15 MHz. RESULTS Effective Radiating Area (ERA) T he m easurem ent o f the effective radiating a rea (E R A ) o f u ltra ­ sonic physiotherapy devices is a crucial facet o f th e ir calibration. T he spatial averaged intensity, form ulated as the ra tio o f th e ultrasonic pow er to the effective radiating a rea (E R A ), is one o f th e fu n d a m e n ­ tal trea tm e n t p a ram ete rs chosen in ultrasound therapy. T e m p e ra ­ tu re rises in tissue, which play a considerable role in ultrasound therapy, a re p ro portional to this q u antity4. T he intensity levels for therapy a re also in th e range w here adverse biological effects have been observed. Problem s can th ere fo re arise with both the safety and efficacy o f tre a tm e n ts if th e spatial average intensities deviate c o n ­ siderably from th e ir indicated values. Som e o f th e difficulties e n co u n tere d with E R A m easurem ents have been discussed elsew here5. A sam ple plot o f a two-dim ensional ra ste r scan, o b tained with the hydrophone scanning technique to d eterm ine the E R A , is shown in Fig. 2. Beam Non-uniformity Ratio (BNR) T h e ultrasonic beam distribution produced by a th era p eu tic tra n s ­ d u c e r is nonuniform in nature. T he intensity w ithin the ultrasonic beam varies; th at is, som e po in ts a re higher o r lower than others. Thus, w hen an ultrasonic therapy unit is set to produce a p a rticular intensity, say 2 W/cm , th ere will be places in the beam w here the intensity is actually higher than the indicated value. A num erical in d ic a to r-o f this non-uniform ity is provided by the beam non-unifor- mity ratio, abbreviated BN R. T he B N R is simply th e qu o tie n t of the highest intensity in the field to the average intensity indicated on the m eter. F o r example, if the Physiotherapy, February 1992 Vol 48 no 1 Page 5 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) Boehringer Ingelheim nebulising solutions R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) New—Atrovent Inhalant Solution Unit Pose Vials • Added always to your standard B2 solution • Precise dose every time • Simple and convenient • Preservative-free Bisolvon® solution • Reduces bronchial and nasal secretion viscosity • Facilitates mucocilliary transport and expectoration Boehringer /J £ \ Inhalation Ingelheim Therapy Creating a better clim ate for your patients S2 S2 S2 A tro ve nt 0,025% Inhalant Solution. Each ml con tains 0,250 m g ipratro p iu m b rom ide Reg. No. Q /1 0.2.1/117 A tro ve nt U.D.V. 0,5 m g/2 m l Inhalant Solution. Each 2 m l c o n tains 0,5 m g ipratro p iu m bro m id e (preservative free) Reg. No. X /1 0.2.1/322 Bisolvon S olution Each 5 m l con tains b rom hexine HC110 m g Ref. No. G642 (Act 101/1965) For fu rth e r info rm a tio n a b o u t these and our other pro d u c ts , please co n tact; Boehringer Ingelheim (Pty) Ltd Reg. No. (69/08619/07) Private Bag X3032, R andburg, 2125 S PE C T R U M 113122 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) B N R is 4.0 a nd the unit is se t fo r an indicated intensity o f 2.0 W /cm 2, th en a t som e p oint in the beam the intensity is actually 8.0 W /cm 2. By looking a t the intensity d istribution o f an ultrasound beam , som e qualitative inform ation a bout w here the highest intensity is located c an be acquired. T he B N R is a useful indicator o f the degree of non-uniform ity. It is th ere fo re vital in th e th erapeutic application of ultrasound th at the applicator (soundhead) be moved continuously over the a rea being treated as a result o f non-uniform ity o f the beam. T his causes the energy distribution to be m ore uniform and thus prevents high tem p e ra tu re buildup in tissues. A sam ple p lot o f a two-dimensional ra ste r scan, o b tained with the hydrophone scanning technique to d eterm ine the B N R , is shown in Fig. 3. DISCUSSION M easurem ents w ere m ade on 6 ultrasonic beam s, o ne from each com m ercially available ultrasonic physiotherapy device inspected. It is recom m ended th at the m easured effective radiating a rea (E R A ) s h o u ld b e w ith in 2 0 % o f th e la b e lle d value**. T h e d e v ic e c o d e , o p e r a t in g fre q u e n c y , m e a s u r e d E R A a n d m a n u f a c t u r e r 's r a te d values a re given in T able 1. F o r o n e of the six devices, th e E R A was n ot w ithin 20% of th e ra te d value. F o r devices C and D th e rated value of the m anufacturer was n ot known. DEVICE CODE FREQUENCY (MHz) MEASURED ERA (CM2) RATED^RA PERCENTAGE DEVIATION BNR A 1.1 3.9 4.4 13 6.16 B 3.3 3.3 3.9 18 6.16 C 1.1 3.25 unknown - 7.72 D 1.1 3.25 unknown - 7.84 E 1.1 3.05 5 64 8.96 F 0.87 3.5 4 14 8.06 Table I: Results of measurements m ade on 6 ultrasonic beams T he quantity term ed % deviation is defined as % D eviation = [(E R A m - E R A i)/E R A m] x 100 w here ERAm is the value o f the E R A as calculated from the m easured ra ste r scan and E R A i is the E R A value indicated by the m anufacturer. V arious international safety guidelines recom m end an u pper limit to radiated ultrasound energy to protect the p atient against adverse biological effects. T he W orld H e alth O rganisation7 (W H O ) limits the spatial average intensity to a maximum o f 3 W /cm , while the International E lectrotechnical Com m ission states the sam e limit 2 o f 3 W /cm fo r b o th the continuous wave m ode and th e pulse wave m ode. However, a safety aspect that is not considered in these limits, is the o ccurrence o f high spatial peak intensities within th e beam. H igh spatial p eak intensities (also known as “hot sp o ts”) may cause dam age to the p a tie n t’s tissues and should th ere fo re b e avoided. As discussed earlier, these hot spots a re usually quantified by th e beam non-uniform ity ratio (B N R ). A lthough m ost safety stan d ard s do not specify a limit on the B N R , th e T N O M edical T echnology U nit of th e N e th erlan d s9 used a B N R ra tio o f 8 as a limit in a survey done on ultrasound therapy devices. This value is used as a guideline in the c u rre n t study. T he B N R values fo r the six ultrasound beam s u n d e r study a re listed in T able 1. O f these, one had a B N R ra tio above 8. Sum m arizing the m easurem ent o f the E R A and B N R , it may be concluded th at only two o f the units comply w ith the requirem ents se t fo r safety and accuracy, th a t is, a m easured E R A deviating less th an 20% from the labelled E R A , and a m o d era te B N R . N o c o n clu ­ sion could be m ade on tw o fu rth e r u n its d ue to a lack o f m anufac­ tu re r’s data, while two units did n o t com ply with the re q u ire m e n ts set. T his result is in a ccordance w ith resu lts from sim ilar investiga­ tio n s a b ro a d 9’*®'**. Q uality control a nd a ccep tan ce testing o f equipm ent, dosim etry and fundam ental studies o f ultrasonic techniques all re q u ire the m easurem ent o f acoustic o u tp u t. H ence, the m easurem ent and specification of the acoustic o u tp u t o f medical ultrasonic e q uipm ent is an a re a o f growing in te rest and concern. REFERENCES I. Food and Drug Administration, Departm ent o f Health and Human Services, United States o f America. Performance Standards for Sonic, Infrasonic and Ultrasonic R adiation Emitting Products Part 10S0. Code o f Federal Regulations 21 Parts 8 0 0 to 1 2 9 9 ,1985:365-369. 2 Preston RC. Measurement and characterization o f the acoustic output o f medical ultrasonic equipment Part 2. M ed & B id E ng& C om put 1986;24:225-234. 3. H arris GR. A discussion o f procedures for ultrasonic intensity and power calcula­ tions from miniature hydrophone measurements. Ultrasound in M edicine and Biology 1985;11(6):803-817. 4. Nyborg W L Interaction mechanisms:Heating. In Repacholi MH, G randolfo M, Rindi A , eds. Ultrasound:M edical applications, biological effects and hazard poten­ tial. New York:Plenum Press;1987:73-84. 5. Bly SHP, Hussey RG, Kingsley JP et al. Sensitivity o f effective radiating area measurement for therapeutic ultrasound tra n sd u c e r to variations in hydrophone scanning technique. Health Physics 1989;57(4):637-643. 6. Canadian Government Publishing Centre. Standard for Ultrasound Therapy D e­ vices. (MICanada G azette Part I I 1984. 7. World Health Organisation. Ultrasound. Environm ental Health Criteria22. Geneva, 1982 8. International Electrotechnical Commission. Medical Electrical Equipment Part 2: Particular Requirements for the Safety o f U ltrasonic Therapy Equipm ent fitb tica - tion 601-2-5 1984. 9. Hekkenburg RT, Oosterbaan WA, Van Beekum WT. Evaluation o f Ultrasound Therapy Devices. Physiotherapy 1986;72(8):390-394. 10. Steward HF, Harris GR, Herman BA- Survey of use and performance o f ultrasonic therapy equipment in Pinelles County. Physical 77i«rapyl974;S4(7):707-715. I I . Repacholi MH, Benwell D A Using surveys o f ultrasound therapy devices to draft performance standards. Health Physics 1979;36:679-686. PHYSIO FORUM IN 1992 FORUM DEADLINES In 1992 Physio Forum will continue to be published 8 times a year, in January, M arch, A pril, June, July, S eptem ­ ber, O ctober and D ecem ber. However, due to pressure of work, and to allow a margin for last-minute submissions, the schedule for deadlines has been slightly altered. Please note that the deadline is 12h30 on the days listed below, and late submissions should be cleared by tele­ phone. We cannot guarantee the publication of any late contribution. ISSUE DEADLINE March 5 February April 4 March June 29 April July 3 June September 5 August October 2 September December 4 November January 2 December It would be advisable to keep a copy of these dates in the back of your diary for future reference. Physiotherapy, February 1992 Vol 48 no 1 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )