Nexus - 0906 - New Times Magazine-pages

Page Content (OCR)

ULTRASOUND TECHNOLOGY AND TECHNIQUES pragmatic reasons. It offers a reasonably accurate due date— The term "ultrasound" refers to the ultra high frequency sound- although dating is most accurate at the early stages of pregnancy waves used for diagnostic scanning. These waves travel at 10-20 when babies vary the least in size—and the baby is big enough for million cycles per second, compared to 10,000-20,000 cycles per = most abnormalities to be seen that are detectable with ultrasound. second for audible sound (de Crespigny, 1996). Ultrasound = However, at this stage, the EDD (expected date of delivery) is waves are emitted by a transducer (the part of the machine that is only accurate to a week either side, and some studies have put onto the body), and a picture of the underlying tissues is built suggested that an early examination or calculations based on a up from the pattern of "echo" waves that return. Hard surfaces woman's menstrual cycle can be as accurate as RPU (Olsen, 1997; such as bone return a stronger echo than soft tissue or fluids, giv- Kieler, 1993). ing the bony skeleton a white appearance on the screen. And while many women are reassured by a normal scan, RPU Ordinary scans use pulses of ultrasound which last only a frac- actually detects only between 17% and 85% of the | in 50 babies tion of a second, with the interval between waves being used by who have major abnormalities at birth (Ewigman, 1993; Luck, the machine to interpret the echo that returns. In contrast, 1992). A recent study from Brisbane showed that ultrasound at a Doppler techniques, which are used in specialised scans, foetal major women's hospital missed around 40% of abnormalities, monitors and hand-held foetal stethoscopes ("sonicaids"), feature with most of these being difficult or impossible to detect (Chan, continuous waves, giving much higher levels of exposure than 1997). Major causes of intellectual disability such as cerebral "pulsed" ultrasound. Many women do not realise that the small palsy and Down's syndrome are unlikely to be picked up on a rou- machines used to listen to their baby's heartbeat are actually using tine scan, as are heart and kidney abnormalities. Doppler ultrasound, albeit with low dose parameters. When an abnormality is detected, there is a small chance that More recently, ultrasonographers have been using vaginal ultra- the finding is a "false positive", where the ultrasound diagnosis is sound, where the transducer is placed high in the vagina, much — wrong. A UK survey showed that, for 1 in 200 babies aborted for closer to the developing baby. This is used mostly in early preg- major abnormalities, the diagnosis on post-mortem was less nancy, when abdominal scans can give poor pictures. However, severe than predicted by ultrasound and the termination was prob- with vaginal ultrasound, there is little intervening tissue to shield —_ ably unjustified. In this survey, 2.4% of the babies diagnosed the baby, who is at a vulnerable with major malformations, but not stage of development, and exposure aborted, had conditions that were sig- levels will be high. Having a vagi- nificantly over- or under-diagnosed nal ultrasound is not a pleasant pro- we (Brand, 1994). cedure for the woman; the term When an abnormality Is There are also many cases of error describe how some women exper. | detected, there is a small can eatne anxiety and lea to repeated ence vaginal scans. chance that the finding Is scans, and there are some conditions Another recent application for u ttl which have been seen to resolve spon- ultrasound is the nuchal translucency a false positive , where the taneously (see, for example, Saari- test, where the thickness of the skin ultrasound diagnosis is wrong. Kemppainen, 1990). fold at the back of the baby's head is As well as false positives, there are measured at around three months. A also uncertain cases where the ultra- thick nuchal (neck) fold makes the sound findings cannot be easily inter- baby more likely, statistically, to preted and the outcome for the baby is have Down's syndrome. When the baby's risk is estimated to be —_ not known. In one study involving women at high risk, almost over 1 in 250, a definitive test is recommended. This involves 10% of scans were uncertain (Sparling, 1988). This can create taking some of the baby's tissue by amniocentesis or chorionic immense anxiety for the woman and her family, and the worry villus sampling. may not be allayed by the birth of a normal baby. In the same Around 19 out of 20 babies diagnosed as "high risk" by nuchal study, mothers with "questionable" diagnoses still had this anxiety translucency will not turn out to be affected by Down's syndrome, three months after the birth of their baby. and their mothers will have experienced several weeks of In some cases of uncertainty, the doubt can be resolved by fur- unnecessary anxiety. A nuchal translucency scan does not detect __ ther tests such as amniocentesis. In this situation, there may be up all babies affected by Down's syndrome. to two weeks' wait for results, during which time a mother has to decide if she will terminate the pregnancy if an abnormality is INFORMATION GAINED FROM ULTRASOUND found. Even mothers who receive reassuring news have felt that Ultrasound is mainly used for two purposes in pregnancy: this process has interfered with their relationship with their baby either to investigate a possible problem at any stage of pregnancy, (Brookes, 1994-95). or as a routine scan at around 18 weeks. As well as estimating the EDD and checking for major abnor- If there is bleeding in early pregnancy, for example, ultrasound —_— malities, RPU can also identify a low-lying placenta (placenta may predict whether miscarriage is inevitable. Later in praevia) and detect the presence of more than one baby at an early pregnancy, ultrasound can be used when a baby is not growing or stage of pregnancy. However, 19 out of 20 women who have pla- when a breech baby or twins are suspected. In these cases, the centa praevia detected on an early scan will be needlessly wor- information gained from ultrasound can be very useful in _ ried: the placenta will effectively move up and not cause prob- decision-making for the woman and her carers. However, the use _ lems at the birth. Furthermore, detection of placenta praevia by of routine prenatal ultrasound is more controversial, as this RPU has not been found to be safer than detection in labour involves scanning (or "screening") all pregnant women in the (Saari-Kemppainen, 1990). hope of improving the outcome for some mothers and babies. No improvement in outcome has been shown for multiple preg- The timing of routine scans (at 18 to 20 weeks) is chosen for _ nancies, either; the vast majority of these will be detected before detected, there is a small chance that the finding is a "false positive", where the ultrasound diagnosis is wrong. INFORMATION GAINED FROM ULTRASOUND Ultrasound is mainly used for two purposes in pregnancy: either to investigate a possible problem at any stage of pregnancy, or as a routine scan at around 18 weeks. If there is bleeding in early pregnancy, for example, ultrasound may predict whether miscarriage is inevitable. Later in pregnancy, ultrasound can be used when a baby is not growing or when a breech baby or twins are suspected. In these cases, the information gained from ultrasound can be very useful in decision-making for the woman and her carers. However, the use of routine prenatal ultrasound is more controversial, as this involves scanning (or "screening") all pregnant women in the hope of improving the outcome for some mothers and babies. The timing of routine scans (at 18 to 20 weeks) is chosen for 18 ¢ NEXUS ULTRASOUND TECHNOLOGY AND TECHNIQUES When an abnormality is www.nexusmagazine.com OCTOBER — NOVEMBER 2002