Current Status of Paediatric Imaging

Projection Radiography

The development of digital imaging in plain film radiography is advantageous within paediatric imaging. First introduced in computed radiography (CR) and later in direct readout radiography (DR) systems (utilising flat-panel detector [FD] technology), this technology helped provide greater efficiency in converting incident x-ray energy into image signal. This, together with its inherent wide dynamic range, greatly improved image quality, when compared with conventional screen-film-based systems, if equivalent exposure parameters were used. This has the potential for lowering radiation dosage for the patient and reducing the risk of failed (i.e. non-diagnostic) exposure. Using post-processing capabilities, both bone and soft-tissue anatomy can now be optimally displayed on the same image, thus eliminating the need for repeated radiation exposure.

However, care must be taken when setting exposure factors because, unlike film-based techniques, overexposure can easily occur with digital imaging. This happens without adverse effect on image quality, and may not be recognised by the operator as the image brightness can be freely adjusted, independent of exposure level.

In general, FD technology is an efficient method for obtaining high-quality image data and enabling immediate image preview, storage and distribution over local area networks for viewing by clinicians, thus enhancing efficiency and productivity within high workflow departments.

Other applications of FD technology include digital tomosynthesis (or digital tomography), providing quasi three-dimensional images, adapted for use in chest imaging. As a chest radiograph is a two-dimensional image, sensitivity may be reduced because of overlapping anatomy. This can be overcome by computed tomography (CT) applications but with an inherent increased radiation dose. Tomosynthesis evolved from conventional geometric tomography and was introduced as a low-dose alternative for chest radiographic examination in monitoring children with cystic fibrosis, and in the detection of pulmonary nodules. This technique, involving the acquisition of a number of projection images at different angles during a single vertical motion of the x-ray tube (between a given angular range of −17.5 and +17.5 degrees) directed at a stationary digital FD, results in up to 60 coronal sectional images at an arbitrary depth. Anatomical structures within each image section are sharply depicted, whilst structures located anteriorly and posteriorly are blurred. Spatial resolution is higher in tomosynthesis than in CT in the acquired imaging plane, but depth resolution is inferior, due to the limited angles used.

Further limitations to this imaging technique include the necessity of a 10-second acquisition time, increasing the likelihood of respiratory motion artefacts in non-compliant patients, which will exclude younger children who are unable to hold their breath. Although the radiation dose for tomosynthesis is much reduced compared with CT, it is three times higher than that for a frontal chest radiograph. Whilst this can be offset by a higher nodular detection rate than that seen with the plain radiograph, the higher sensitivity of CT to small nodules and other fine detail, and the increasing use of low- and very-low-dose CT techniques has limited the uptake of chest tomosynthesis, with few departments installing the technology for chest imaging.

Fluoroscopy

The introduction of digital fluoroscopy with its high-speed digitisation of the analogue video signal has revolutionised real-time fluoroscopy that relied on the use of image intensifier/TV systems to display the diagnostic image. Development of fluoroscopy FD technology with its fast digital readout and dynamic acquisitions at high frame rates (up to 60 frames per second) has become a well-established application in paediatric cardiac angiography. The other important application is within minimally invasive interventional procedures, due to their less invasive nature, when compared with conventional surgery. Advantages of FD compared with image intensifier systems that help minimise radiation dose include pulsed fluoroscopy and last-image hold and screen capture, which negate further diagnostic image exposures. Other features that improve image quality include homogeneous image uniformity with lack of geometric distortion across the entire image, reduced veiling glare, and a rectangular or square field of view (FOV) utilising the full width of the image monitor. The small compact size of FD mounted on a dedicated C-arm system increases operational flexibility and ease of patient access, both features which are particularly pertinent within paediatric imaging.

Modern C-arm angiography systems utilising FD technology are equipped with rotational angiography applications, providing three-dimensional CT image capture (FD-CT) that is used mainly in interventional procedures. The ability to combine two-dimensional fluoroscopic and three-dimensional CT imaging within a single unit is advantageous for providing planning, guidance and monitoring of interventional procedures and intraoperative imaging. The image quality is lower in FD-CT than in clinical CT, but in situations where a quick CT control diagnosis is required, an alternative lower spatial resolution image is acceptable. In addition, due to the slow rotation of FD-CT, patient movement and respiratory artefacts in body imaging further reduce spatial resolution. The radiation dose of FD-CT is higher than that of modern conventional diagnostic CT systems due to lower detector efficiency, although the milliamperes per second (mAs) per single image acquisition is much lower. The cumulative dose of exposures throughout a procedure is crucial in this instance, with variation seen in each individual investigation/treatment.

Ultrasound

The real-time, high spatial and temporal resolution capabilities of modern ultrasound equipment, with dynamic and quantitative information from colour-phase, spectral Doppler and M-mode imaging, have consolidated the place of ultrasound as the primary imaging investigation for most paediatric studies at diagnosis and follow-up. In many countries several subspecialties have taken on the role of ultrasound and the technique is no longer wholly within the remit of radiology (namely, echocardiography and fetal ultrasonography), while many subspecialty applications remain firmly within the repertoire of the paediatric radiologist, including cranial, spinal and infant hip ultrasound. Newer techniques, such as ultrasound elastography and contrast-enhanced ultrasonography, continue to provide new ways of quantifying and clarifying diagnoses not previously possible, without the need for biopsy.

As ultrasound becomes increasingly available, it is more important than ever that radiologists remain central to the practice and teaching of ultrasonography. In the hands of highly trained and experienced non-radiologists, several applications of ultrasound have made significant improvements to modern medicine (e.g. focused assessment with sonography for trauma (FAST) and ultrasound-guided nerve blocks in anaesthesia). However, increasing use by non-experts has led to several more questionable uses. For some years there has been a rapid increase in the publication of studies suggesting a role for ultrasonography in the diagnosis and follow-up of pneumonia and other parenchymal lung diseases. In resource-poor areas there is a clear role for a cheap, portable imaging investigation with no need for consumables beyond coupling gel and battery power, while the role of ultrasound for pneumonia becomes less clear in areas with ready access to conventional radiography. Although some studies report sensitivities and specificities of ultrasound in the diagnosis of pneumonia in excess of those for plain chest radiography, they tend to ignore the inability of ultrasound to demonstrate central problems, not in direct contact with the pleural surface, such as lymphadenopathy and cavitating pneumonias. Many of these studies have the admirable aim of reducing exposure of young children to ionising radiation, while the imbalance of the small risk of exposure from a simple chest radiograph with the risk of missing a pulmonary abscess or necrotic mycobacteria-infected central lymph node are potential hazards.

Computed Tomography

CT is a proven essential diagnostic imaging technique and is considered the most sensitive method for evaluating airway and parenchymal lung diseases in children and there is an increasing role for contrast-enhanced CT in the evaluation of complex congenital heart disease. Outside the thorax, the most common uses for CT are in the imaging of acute neurological problems (particularly following trauma or in the post-operative period following neurosurgery) and imaging children and infants on high levels of cardiorespiratory support.

Two CT configurations exist in clinical use: namely, single-source multidetector CT (MDCT) (including newer large detector scanners with up to 320 detector rows) and dual-source MDCT (DSCT) technology. Increasing temporal and spatial resolution has extended the role of CT applications in young children to include electrocardiogram (ECG)-gated cardiac imaging. Advantages of these systems include sub-second tube rotation times (down to 0.2 second). This improved acquisition speed has helped reduce artefacts from patient movement, cardiac pulsation and respiratory motion and improved image quality. The overall reduction in examination acquisition time allowed by the newest machines generally obviates the need for sedation or general anaesthetic for most diagnostic studies. The availability of small detector elements (0.5 mm) combined with thin-slice collimation provides isotropic resolution that allows image data to be manipulated/reformatted in any orthogonal plane and displayed as either two- or three-dimensional images that have the same spatial resolution as the base axial data set and with reduced partial volume artefact.