When imaging children it is important to consider the age and development of the child under investigation. This is vital when considering the differential diagnosis of the any particular clinical symptom and assessing the appropriateness of any imaging investigation. Procedures and processes need to take account of the expected changes that occur within the growing child. For example, in younger children there is a greater amount of unossified cartilage within the skeleton and a different body fat composition, so exposure parameters for radiographs need to be altered: additionally the faster respiratory and heart rate should be taken into consideration when performing chest radiography. As a child is more sensitive to radiation than an adult, repeat X-rays should be avoided. Similarly magnetic resonance imaging (MRI) sequence selection in the neonatal brain (which contains more water and less myelinated tissue) is different from that in the older child.
Imaging departments should be as child-friendly as possible, though consideration must be given to the different requirements of the older child and adolescent compared to that of the young infant. Decorations and signing should ideally be at a lower level to help the orientation of the child. Explanation of any procedure should be aimed predominantly at the child wherever possible. Children should not be lied to and obtaining the child's cooperation, while a more time-consuming process, is ultimately more productive than forcing them into situations against their will.
Sedation and general anaesthesia
For some investigations, primarily MR imaging (but also angiography, nuclear medicine and computerized tomography [CT]) achieving a suitable level of cooperation in the younger or developmentally delayed child, so that they remain stationary for the procedure, may not be possible. In this instance sedation or general anesthesia may be required to ensure the child remains stationary.
Sedation is an artificially induced level of decreased consciousness. Increasing the amount of sedation and thus lowering the level of consciousness will lead first to a reduction of muscle tone of the oropharynx, and then to a loss of the glottic reflex. This has the risk of aspiration and respiratory compromise. The level of sedation should be aimed at keeping the child asleep but does not result in the loss of vital reflexes and the child is still rousable if necessary. Any sedated child should be closely monitored, particularly their respiration rate and blood oxygen saturation, and appropriate resuscitation facilities should be available. A variety of different sedation regimes have been described.
Anaesthesia is an unrousable state with loss of normal airway reflexes. The method of anaesthesia and monitoring of the child will vary but must be closely supervised by an appropriately trained paediatric anaesthetist. If used within a MR scanner then suitably compatible machinery is required.
Standard radiographs are the commonest imaging investigation undertaken. X-rays are part of the electromagnetic spectrum: they are a form of ionizing radiation and as such can damage any tissue they pass through. The use of X-rays should be carefully scrutinized and inappropriate and unnecessary examinations avoided. Children are particularly sensitive to ionizing radiation and have a greater risk of developing malignancy, following exposure to any ionizing radiation. ₺ Radiographs should only be performed if they will have an alteration on the child's clinical management.
Conventional X-ray films (radiographs) should be viewed on a light box, and digital images on high-quality monitor screens. The appearance of digital images can be adjusted by altering the windowing, contrast, brightness and magnification of the images.
The basic principle of fluoroscopy is similar to conventional radiographs: the image is produced following the passage of an X-ray beam through the body. However, instead of producing a static image, an instantaneous image is generated on a fluorescent screen, allowing visualization of a real time moving image. Fluoroscopy is used for dynamic investigations and is often used with contrast agents to improve the visualization of organs and viscera. In paediatric practice common fluoroscopic examinations are barium swallows and meals, micturating cystograms and air enemas.
• Readily available.
• Relatively cheap.
• Very good anatomical detail.
• Good detail of bone injuries.
• Side marker. By convention when viewing radiographs, the patient's right side is on the left side of the radiographic image (as if the patient were present).
• Radiographs are described in the manner in which the X-rays pass through the body. For example a posterior anterior (PA) chest radiograph indicates that the X-rays initially pass through the posterior surface of the patient and then expose the detector, which is against the anterior surface of the chest.
• Radiographic exposure. This is a measure of the amount of radiation that passes through the patient to produce the image. The two major variable factors are the kV and mAs which are settings on the X-ray tube.
• Radiographic (image) contrast. The difference in contrast between separate parts of the image is what forms the image: the greater the contrast, the more visible features become. Image contrast is dependent on subject contrast and film contrast. Subject contrast is the difference in physical/chemical properties of two structures which allows different intensities of radiation to be transmitted. Film contrast is related to the way the image is produced, processed and viewed.
• Radio-opaque. This refers to structures that absorb X-rays and appear white on the image. Typical radio-opaque structures are metallic objects, the bones, calcification, lung consolidation/contusion/fluid.
• Radiolucent. These are typically areas that allow X-rays to pass through them and appear either dark grey or black on X-ray, such as the lungs. Structures such as fat and muscle are predominately radiolucent but do absorb some X-rays and therefore appear relatively grey.
These can be used with radiographs, ultrasound, MR imaging and CT. They improve the visualization of structures and can assess the vascularity and permeability of tissues. Contrast agents can either be instilled into body cavities or injected into the bloodstream. The type and use of a contrast agent will depend on the imaging modality, the anatomical and the clinical indication.
On radiographs or CT will appear dark. It is typically introduced either per rectum or into the stomach to cause distension and improved visualization of the bowel. Water that has been aerated is easier to detect on ultrasound.
This compound is radiopaque on X-rays and CT. It is used for both upper (swallows, meals, small bowel studies) and lower (enemas) GI contrast studies. The concentration of barium is altered depending on the type of investigation (very low for CT). Air can be instilled at the same time to produce a double contrast effect. Contraindicated for suspected perforation or when there is a risk of aspiration as it can cause a fibrotic reaction in the peritoneum and lungs.
Organic iodide preparations
Iodine based compounds are radio-opaque. They can be used orally or per rectum (as a substitute for barium sulphate) or can be administered into the urinary tract (micturating cystourethogram MCUG). They are formulated such that they can be administered for intravascular studies (venography, angiography, cardiac studies).
In CT post-contrast imaging will demonstrate the vascularity and cell wall permeability of tissues. In general inflammation within a tissue will result in increased contrast uptake and higher attenuation on CT.
An unwanted side-effect of intravascular administered iodinated contrast is allergic reactions. These range from mild symptoms such as flushing, urticaria and headache to severe anaphylactic shock. Children who have suffered a previous contrast reaction should not have repeat doses.
Iodinated contrast can also exacerbate renal failure and should be avoided in children with known or suspected renal failure.
• The examination can be stressful and is not routinely indicated in children >1yr. The examination must be appropriate and justifiable.
• Detection of vescio-ureteric reflux (VUR).
• Suspected posterior urethral valves (PUV).
• Possible urethral stricture/narrowing.
• Bladder abnormalities.
Aseptic catherization of the bladder, in some institutions prophylactic antibiotics are given. Under fluoroscopic screening iodinated contrast is slowly instilled into the bladder. Oblique views of the bladder when it is partially filled to detect early VUR or small ureterocoeles are required. Images of any reflux are stored. The examination should be continued until the child micturates. In boys sagittal images of the whole urethra need to be obtained in micturition (to exclude the presence of PUV). A post-micturition cross-kidney image should be taken to demonstrate any reflux into the kidneys that may not have been noticed during any dynamic screening.
Contrast swallow and/or meal
• In children the aim is to depict the anatomical orientation of the oesophagus, stomach and duodenum.
• To detect any congenital abnormality of the oesophagus, trachea and vasculature of the adjacent vessels.
Either barium or iodinated contrast can be used. If there risk of aspiration, a tracheo-oesophageal fistula (TOF) or perforation then iodinated contrast is indicated.
Depending on the child's cooperation, contrast is given orally with a cup, straw or bottle. In the uncooperative child it may be necessary to inject slowly into the mouth with a syringe or use a nasogastric tube (NGT).
Regardless of the age of the child, after they have initially taken a small amount of contrast it is important this is allowed to reach the duodenal–jejunal flexure (DJF) and its position confirmed with a correctly centred AP image of the upper abdomen to exclude a malrotated gut. If a large amount of contrast is used this can opacify the stomach and it may obscure the DJF. The inability to confirm the presence of a normal DJF indicates an inadequate examination.
AP and lateral images of the oesophagus should then be obtained and the presence of reflux determined. If a TOF is suspected this may not be visualized unless there is sufficient distension of the oesophagus so an NGT should be introduced into the oesophagus and iodinated contrast injected, whilst withdrawing the tube and simultaneously imaging in the lateral plane. Some institutions image the child prone.
Ultrasound uses high frequency sound (region 1-15MHz) and their resultant echoes to create an image. Different structures and tissues have different acoustic properties, and therefore reflect varying amounts of sound energy which creates a structure and contrast between tissues and allows a image to be formed. For good contact and to allow sound to pass from the ultrasound probe to the skin surface a viscous gel is used.
• No ionizing radiation.
• Relatively quick.
• Good anatomical detail.
• Dynamic so can be used to assess function and useful for guided procedures such as biopsies and aspirations.
• When images are acquired, the transducer should be held such that the right or cranial portion of the body is viewed on the left side of the image.
• Depth indicates the distance from the transducer surface to the body part in question.
• Those structures that reflect a large amount of sound waves back are termed hyperechoic and appear bright on an image. Structures which reflect a small amount of sound waves are dark on the image and are hypoechoic.
• Hyperechoic structures:
• Vessel walls.
• Acute haemorrhage.
• Calcification or stones.
• Fatty lesions.
• Hypoechoic structures:
• Fluid will allow easy passage of sound waves through it so more sound is reflected back from behind a fluid-filled structure. Consequently, behind the fluid-filled structure will be relatively bright. This is termed posterior acoustic enhancement and is a typical feature of cystic fluid-filled structures.
• Some dense structures will not allow any sound to pass through them, so there is no reflected sound from behind them. This creates an acoustic shadow. This is a feature of stones and metallic fragments within organs.
• Resolution is the ability to discriminate between two separate points. The higher the resolution of an image, the nearer two points can be within the body before they can not be distinguished as separate identities on the image. There are a variety of different frequency probes available: the higher the frequency of the probe the better the resolution, but the less depth of tissue that can be imaged.
• Doppler effect this utilizes the change in frequency of sound that occurs when it interfaces with a moving object (typically blood). This enables an assessment of both direction and speed of flow of blood within tissues.
• Colour flow Doppler will determine the direction of flow of blood. Arbitrarily red is towards the transducer and blue is away.
• Pulsed Doppler can provide an estimation of velocity of flow.
Computerized tomography (CT) utilizes X-ray radiation to produce an image. As opposed to conventional radiographs where a single X-ray beam is passed through the body, with CT multiple beams are passed through the patient and are absorbed by a number of detectors. A CT examination involves a considerably higher radiation dose when compared with radiographs. The body is imaged in a number of slices but the use of computer software allows the data to be manipulated and images produced in any plane or three-dimensional (3D) reconstructions.
• Relatively quick and easily available.
• Good anatomical detail.
• Good bony detail.
• High-quality imaging.
• Multiple planar reconstruction.
• By convention, the images are viewed as if looking from the foot of the patient up towards the head, so the left side of the patient is on the right side of the image.
• Spatial resolution is the ability to discriminate between two separate points or a structure against its background. The higher the resolution of an image, the nearer two points can be within the body, before they are unable to be distinguished as separate identities on the image. Typically the resolution of CT is in the region of 0.5–1mm.
• Attenuation refers to how many X-rays are absorbed by a piece of tissue. Dense tissue such as bone will absorb a large amount of the X-ray beam: they are of a high attenuation and appear relatively bright on the CT image. Low attenuation tissues (e.g. air) will appear relatively darker. The attenuation value of a tissue is measured in Hounsfield units (HU). Water has a value of 0 HU.
• High attenuation:
• Intravenous contrast.
• Metallic objects.
• Intermediate attenuation:
• Solid abdominal organs.
• Low attenuation:
• CT images are produced in greyscale in which the more radio-opaque (higher attenuation tissue) appears relatively bright compared with lower attenuation tissue. The human eye is only able to resolve a limited number of levels of grey, far less than the computer images can produce. Windowing is the method used to optimize the level at which the images are viewed. The two adjustable parameters are the window level and the window width.
• The window level is the attenuation value of the the midpoint of the window width. Window width is the difference between the highest and lowest attenuation values that will appear grey on an image. Tissue with attenuation values outside the window width will appear either black (low attenuation value) or white (higher attenuation value).
• It is only possible to visualize subject contrast between the structures and tissues whose image attenuation values lie within the window width.
• Altering the window level and width (‘windowing the image’) allows the contrast and brightness of the image to be optimized within the tissues of interest. For example, when reviewing bone structures the window level will be set at higher values than when reviewing fat and soft tissues.
• Slice thickness. The thickness of the slices can be varied; thinner slices provide a better resolution (ability to distinguish two separate points) but the signal to noise ratio (the amount of signal forming the image compared to the background) appears less.
• These can be used orally or intravenously.
• Iodinated contrast and barium are of high attenuation.
• Oral contrast agents include iodinated compounds and barium sulphate. They are used to opacify bowel to aid discrimination between lymphadenopathy and masses within the abdomen.
• Intravenous contrast is used to improve visualization of the vascularity of different tissues.
• In cases of inflammation and infection there is an increase in uptake of involved tissue. An abscess will show peripheral rim enhancement.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) does not use ionizing radiation, but utilizes the interaction of magnetic fields and radio waves to create an image. MRI scanners use very strong magnets, that can vary in strength from 0.25 to 3 Teslas. A Tesla is a measurement of magnetic strength. The earth's magnetic strength is 25 gauss and 1 gauss is 1/1000 of a Tesla.
• Relatively expensive.
• There may be a need for sedation or general anaesthesia.
• Cannot put pacing wires or patients with metallic fragments near the magnet.
There are an immense number of sequences that can used and their use depends on the body part under examination and the suspected pathological process.
The commonest sequences are spin echo (SE) sequences. There are essentially three basic SE sequences; T1-weighted, T2-weighted and proton density (or intermediate weighted). They vary in their repetition time (TR) and echo time (TE). Fast-spin echo (FSE) sequences are an adaptation of SE sequences.
Gradient echo (GRE) sequences decrease acquisition time and reduce movement artefact, but there is reduced signal to noise ratio.
Signal intensity and Imaging sequences
Tissue that returns a lot of signal will appear bright on an image (high signal intensity). The signal intensity of tissue is partly a reflection of its physical properties but dependent on the imaging sequence used. Some tissue will be of high signal intensity on one sequence but of low signal intensity on another.
Signal intensity of tissue on SE sequences
• Low signal (dark) on T1-weighted images:
• Increased free water (oedema, tumour, infarction, inflammation, infection.
• Haemorrhage (hyperacute or chronic).
• Low density of protons (calcification, fibrous tissue, bone cortex).
• Flow void.
• High signal (bright) on T1-weighted images:
• Subacute haemorrhage.
• Protein-rich fluid.
• Slowly flowing blood.
• Paramagnetic substances: gadolinium, manganese, copper.
• Calcification (rarely).
• Low signal (dark) on T2-weighted image:
• Low density of protons (calcification, fibrous tissue).
• Paramagnetic substances: deoxyhaemoglobin, methaemoglobin (intracellular), iron, ferritin, haemosiderin, melanin.
• Protein-rich fluid.
• Flow void.
• High signal (bright) on T2-weighted image:
• increased water (oedema, CSF, tumour, infarction, inflammation, infection).
• Methaemoglobin (extracellular) in subacute hemorrhage.
Signal to noise ratio (SNR)
The signal is the image pattern produced by the structures in the body. The noise is the random information produced as result of quantum mottle (the inherent random nature of molecules within the body). The higher the signal to noise ratio the better the quality of the images.
MRI contrast agents
In MR imaging, gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) is the commonest used contrast agent. This is a paramagnetic agent that appears bright on T1 weighted image (T1WI). It is water-soluble and can increase the contrast between normal and pathological tissue. It does not cross the normal blood–brain barrier and so can be used to detect a breakdown in this barrier.
MRI sequence selection
Sequence selection will depend on the body part under investigation and its tissue characteristics, the clinical condition, presentation and the pathological changes suspected. MR machine availability and experience will also be determining factors.
Suggested MR imaging sequences
• Sagittal T1WI (SE).
• Axial T1WI (SE).
• Axial T2 WI(SE).
• Coronal T2WI (SE).
• Axial fluid-attenuated inversion recovery (FLAIR) for children over 3 months of age.
Additional sequences as necessary
• Neonates: inversion recovery.
• Acute changes in neurological status: diffusion imaging.
• Developmental delay: T1 volume.
• Suspected haemorrhage: gradient echo imaging.
• Epilepsy: fine coronal T2 slices through the hippocampus.
• Tumour: post-gadolinium and spinal imaging.
• Axial turbo SE T2 WI (with fat saturation).
• Coronal SE T1WI.
• Sagittal short tau inversion recovery (STIR).
• Coronal turbo SE PD-weighted (with fat saturation).
• Coronal STIR.
• Post-contrast if needed.
• Coronal and sagittal SE T1WI (with fat saturation).
• Axial turbo SE T2WI (with fat saturation).
• Coronal SE T1WI.
• Sagittal SE Proton density (PD) WI.
• Sagittal turbo T2WI (with fat saturation).
• Coronal turbo SE PD weighted.
• Coronal turbo SE T2WI (with fat saturation).
The choice of sequences used depends on the indication. The upper abdominal organs are susceptible to respiratory artefact and either breathhold techniques or respiratory gating is used to minimize this effect.
When imaging the biliary tree with magnetic resonance cholangiopancreatography (MRCP) T2 weighted sequences are used which show fluid as very bright signal, making it easier to distinguish any filling defects such as stones or areas of stricture formation.
For oncology staging or follow-up a combination of pre-contrast T1 and T2 sequences are used with at least 2 planes performed as post-contrast T1 fat saturated images.
There are a variety of liver-specific contrast agents which can be used to classify intra-hepatic lesions but more detailed description is beyond the scope of this text.
Nuclear medicine (scintigraphy)
Images are created from the gamma radiation emitted from radioactive nucleotides (or radioisotopes) which are attached to different protein labels. It is these labels that enable the isotope to be taken up by the specific tissue under investigation and allow an assessment of both anatomy and physiological activity. The labelled isotope is usually administered intravenously: after a variable time it is taken up by the tissue/organ under examination and the patient is then imaged using a Gamma camera.
A Gamma camera detects the emitted radiation, converts it into light signal using NaI crystals, this emitted light is then subsequently converted into a digital image. This conversion process creates images of relatively low resolution compared with all other imaging modalities.
Nuclear scintigraphy does involve a significant radiation burden and its use in paediatric medicine should be closely monitored and investigations appropriately indicated.
The most commonly used radioisotope in paediatric nuclear medicine is Technetium (99mTc). This is 140keV gamma-emitting radioisotope with a half-life of 6hrs.
Indications and labels
Static renal imaging
The radiopharmaceutical is 99mTc 2,3 dimercaptosuccinic acid (DMSA), which binds to plasma proteins, that are cleared by tubular excretion and retained in the renal cortex, with no excretion into the renal collecting system during the imaging period.
Dynamic renal imaging
The radiopharmaceutical commonly used is 99mTc-MAG 3 (mercaptoacetyltriglycine), which is cleared by tubular secretion. Often a diuretic is administered at the same time as the isotope to aid excretion.
99mTc-methylene diphosphonate (MDP) or other diphosphonate compounds. These are phosphate analogues and will assess bone turnover.
• Staging and assessment of malignant metastatic disease.
• Detection of primary bone tumours.
• Trauma (occult fractures and NAI).
• Infection: three phase studies. The initial blood flow to an area, the blood pooling in an area of interest and uptake within the bone on delayed images (all increased in acute infection).
The use of bone scintigraphy is becoming less common with the increased use of MR imaging, which does not involve radiation and is often more specific. The use of bone scintigraphy should be closely monitored.
• The radiopharmaceutical is 123 I Metaiodobenzylguandine (MIBG), taken up by neuroblastoma, phaeochromocytomas, for metastases, response to treatment.
• This uses iodine as the radioisotope which will be taken up by the thyroid gland; it is important children given are given iodine prior to study to block thyroid uptake.
SPECT (single photoemission computed tomography)
These are tomographic images which are acquired in multiple planes to allow improved anatomical localization and resolution.
Positron emission tomography
Positron emission tomography (PET) uses isotopes of basic biological elements that are positive β emitters (positrons). As the isotopes decay, energy is released as β and α radiation. This radiation is detected, magnified and converted to electrical signals, which are then processed to generate images. A commonly used isotope is Fluorine 18 (F-18) with a label fluorodeoxyglucose (FDG), so called FDG-PET. This tracer is a glucose analogue and is taken up by glucose-using cells. Uptake is often elevated in rapidly growing malignant tumours and can be used to detect malignant disease.