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Imaging the Central Nervous System of the Fetus and Neonate

2006 Edition, April 13, 2006

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ISBN: 978-0-8247-2856-4
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Product Details:

  • Revision: 2006 Edition, April 13, 2006
  • Published Date: April 13, 2006
  • Status: Active, Most Current
  • Document Language: English
  • Published By: CRC Press (CRC)
  • Page Count: 275
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:

Preface

One of the most significant areas of advance in clinical medicine over the last 20 years has been in the imaging technologies. It is difficult to point to the single method or application that has benefited most from those advances because nearly all specialities in medicine have been involved. Perhaps the most significant trend, however, has been the development and introduction of imaging methods that do not use ionizing radiation, such as ultrasonography (US) and magnetic resonance imaging (MRI). Imaging methods using x-rays or nuclear medicine products are the largest single man-made source of radiation burden to the population. Research and development into other methods that can replace techniques that use ionizing radiation is a justifiable goal in itself, for the benefit of both patients and hospital staff involved in imaging. It is difficult/impossible to provide accurate estimates of risk of imaging with ionizing radiation on an individual basis, but suffice it to say, the risks are small but present and must be taken into account when devising imaging policies.

The effects of ionizing radiation used in medical imaging in adults are divided into two types depending on the mechanism of risk accumulation with increasing dose. Non-stochastic effects occur in body areas such as the cornea where radiation will definitely have a deleterious effect that is dose-dependent. This is not usually a concern for patients but it is for staff involved in high-exposure procedures such as interventional angiography. Most of the concern for patient risk is the induction of malignant processes, which are described as being stochastic events, i.e., they are chance occurrences with the risk increasing with exposure. The risk of tumor development is not equal all over the body, some parts being quite resistant to radiation and others being very sensitive (ova, bone marrow, breast, thyroid, etc.). Irradiation of the ovaries and testicles presents concerns other than tumor generation in the individual, as there is risk of DNA damage that will be passed to a future offspring and could manifest as malformation or tumor.

When experts in radiation biology produce dose-related risks for radiation exposure in a medical environment they take account of the relative exposures of different parts of the body with different imaging methods because of the variation of radiation sensitivity, as described above. They take account of the patient's age as well, as it is highly likely that cells that are dividing rapidly are more susceptible to the damaging effects of radiation. Because an induced malignancy may take many years to develop, a lower age is an increased risk because there is more time for a problem to develop. Hence clinical imagers are very keen to limit radiation exposure in children wherever possible. It is imperative that the developing fetus should not be exposed to unnecessary radiation because of the known risks of teratogenicity and tumor formation. Although x-ray based methods of accessing the fetus have been used in the past, this is not the case now and even irradiating the mother during pregnancy should be performed only for well-considered reasons. Fetal assessment by imaging, therefore, is the realm of methods that do not use ionizing radiation

The introduction of obstetric US into clinical practice in the late 1970s and early 1980s and its subsequent expansion has had a major effect on clinical practice. The majority of women are offered fetal screening in the second trimester, which may be supplemented by detailed anomaly scanning if problems are found. The widespread use of ultrasonography in neonatal imaging has cemented that method as the primary method of assessing the fetus and neonate. It is easy to predict that MRI should have a role in fetal and neonatal evaluation, if it is reasonable to extrapolate from the pediatric and adult population where US and MRI are used in a complementary fashion. The problems with using MRI have been many-fold including price, limited access, and practical issues that frequently require anesthesia/sedation in neonates and, until recently, have made imaging of the fetus a non-option. Many things have changed and the introduction of ultrafast MR imaging in the late 1990s made MRI of the fetus a realistic option. MRI generally is exceptionally good for neuroimaging and it is not surprising that most of the early work in fetal MRI has concentrated on the brain and spine.

This book describes the status of ultrasonography and MRI of the central nervous system of the fetus and neonate in the early part of the new millennium and attempts to explore the relative roles of each method.We have tried to be as current as possible and have included a section where we describe our view of future developments, but this is a rapidly expanding field and still driven by technological refinements in a major way.

We do have one request for workers who are active in this field or who are contemplating becoming involved, and that is the continuing need for vigilance about safety issues. All imaging methods work by perturbation of human tissues by the input of energy—be it x-rays in CT, sound waves in US, or radiofrequency (RF) pulses in MRI—and none of these can be considered totally safe. US, is thought to be totally safe for the majority of pregnant mothers and very few doctors using US are concerned about damaging effects of any description and they are probably right. Most of the new developments in US technology use greater power deposition when compared to the studies confirming the safety of US in earlier studies and continuing safety studies are required. MRI presents a range of potential safety issues, the two most important being temperature increases due to RF deposition and acoustic noise damage from the very noisy sequences used. Both of these are unresolved at present and are surprisingly difficult to study in the current environment. The acoustic effects on subsequent hearing acuity should be very easy to study; however, it must be appreciated that, as far as we know, "normal" pregnancies are not referred for fetal MRI. Any study, therefore, must rely on the small number of cases where a brain problem has been described on US but no abnormality was shown on in utero MRI or post-natal imaging and the child is shown to be otherwise developmentally and neurologically normal. Unless there are obvious and gross effects on hearing (which seems unlikely), that study will require a long-term multicenter study.