Diagnostic Radiology: Paediatric Imaging Niranjan Khandelwal, Veena Chowdhury, Arun Kumar Gupta, Ashu Seith Bhalla, Sanjay Thulkar
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GENERAL

Technical Considerations in Pediatric ImagingCHAPTER 1

Ashu Seith Bhalla,
Arun Kumar Gupta,
Amar Mukund
Technical factors such as the ability to position the patient and expose the radiograph with the patient immobile, which are often taken for granted in adult radiography, may appear as crippling problems in pediatric radiography. For this reason and because of the importance of minimizing radiation to the child, special attention must be paid to certain technical points. These will be discussed under the following five broad groups:
  1. Minimizing heat loss
  2. Immobilization
  3. Sedation
  4. Reduction of radiation dosage
  5. Use of contrast media
 
MINIMIZING HEAT LOSS
Neonates and infants lose body heat rapidly, the risk of hypothermia being greatest in the premature baby with little subcutaneous fat. Local warmth may be obtained by special table-top heating cradles, but the most convenient way of avoiding heat loss is to maintain a room temperature of about 27°C in room. A room thermometer is an important piece of equipment. Enhanced humidity is not normally required for the duration of an X-ray examination.1
 
IMMOBILIZATION
Atraumatic immobilization is essential in order to ensure proper positioning and to minimize patient motion. Newborn babies and small infants need only soft sandbags and adhesive tape to stop movement. Towels and sheets can also be used to tightly wrap babies. Older, stronger children require wrapping on immobilization boards in addition to tape and sandbags. Special X-ray equipment is available which is designed specifically for the examination of infants and children. The essential factors for an immobilization device are (1) absence of artifacts, (2) safety, (3) no disturbance of the patient's sedation, and (4) ease of handling. It is not always possible to accommodate older children on such apparatus. Cradle holding devices are provided which enable the infant to be rotated in relation to the tabletop. When this type of equipment is not available, views such as the prone shoot through swallow for tracheoesophageal fistula may be obtained by using a device such as the Charteris baby holder inverted on the step of an up-right adult screening table.13
 
SEDATION
One fundamental technical component for imaging is the need for the child to remain motionless during the duration of imaging. Babies and infants under 6 months of age will often sleep after a feed and may not need sedation unless they are known to be restless, or the procedure is painful. Adjunctive measures such as sleep deprivation can also be useful. In older children verbal reassurance may be sufficient. Venous access produces less disturbance if a cannula is put in place after the area has been treated with a topical gel. Two possible area can be prepared about 30 mins before injection. Small gage needles are used, often 22 to 25 gage. Warming of contrast medium makes injection through fine needles easier and less painful.1, 4, 5
Sedation, however, is often necessary, especially for procedures like MRI due to its long duration, and for interventions due to the pain involved. Once it is deemed that both the procedure and sedation are necessary, every effort must be made to provide safety for the child. Even if the radiologist is not directly responsible for the sedation procedure, he or she must expedite the procedure in order to minimize the length of sedation. The timing of sedation and of the procedure need careful coordination.1, 5
Monitoring the patient in the radiology suite is not an easy task for the clinicians. Observing children from the CT or MRI control room through a glass window is much more difficult for the clinician than direct observation at the bedside and increases their reliance on monitoring devices. Hence, adequate monitoring devices should be available. Pulse oximetry is commonly used during sedation. Monitoring is further complicated during MRI because the scanner generates strong static, radio-frequency and time varied magnetic fields which interfere with the monitoring devices. New nonferromagnetic monitors and cables have been deviced which are safe and reliable within the scanning suite. Standard ferromagnetic monitors if used need to be placed outside the magnetic field or carefully shielded.4, 5
The radiology suite is a less than ideal environment for dealing with respiratory arrest or cardiovascular collapse. Hence, the most important prerequisite to sedation is the availability of adequate equipment for resuscitation and personnel experienced in managing sedation complications. If the radiology suite is not equipped with wall outlets for oxygen and suction, portable oxygen cylinders and 2suction apparatus should be available. Also, a cart with resuscitation drugs, defibrillator, and age and size appropriate equipment for different age groups and body sizes for purpose of oxygen administration and intubation are absolutely essential.4, 5
Risk factors must be taken into consideration before planning sedation. If the child's condition is tenuous enough for sedation to be a significant risk, precautionary measures like securing the airway should be taken before the child arrives in the radiology department. Knowledge of both the past and the present medical history is equally important.
 
NIL PER ORALLY (NPO) STATUS
Aspiration is a significant concern in sedated children and NPO guidelines should be as stringent as those in children undergoing general anaesthesia. Guidelines recommended by American Academy of Pediatrics Committee on Drugs (AAPCOD) are as follows: clear liquids are allowable up to 2 hours before the procedure for any age; semisolid liquid (including breast milk) and solid foods are acceptable for up to 4 hours for children less than 6 months, 6 hours for children 6 to 36 months old and 8 hours for older children. Whenever possible these recommendations must be followed. Bowel obstruction or ileus are other factors that increase the risk of aspiration because they delay gastric emptying. In these patients, nasogastric suction of gastric contents should be performed and agents given that promote gastric emptying, such as metoclopramide. The actual risk of aspiration in children undergoing diagnostic imaging, is unknown, but it is probably quite low. In one recent report, aspiration of gastric contrast was present in no more than 4 percent of children undergoing CT scan examination in a setting of trauma.4, 6
The practice of administering oral contrast material in children before sedation for abdominal CT is controversial. At some institutions, the practice of administering an enteric contrast material before sedation is being discouraged because it violates the “nothing by mouth” status that is otherwise strictly enforced before sedation. However, recent studies have indicated that oral contrast appears to be safe when using the sedation drugs like chloral hydrate and propofol. Further study of the safety of this practice is required.7
 
Pharmacological Agents
Several different agents have been successfully used for sedation of children for imaging studies. The choice of the agent depends on availability, local expertise and patient risk factors. The route of administration could be oral or parenteral. Intravenous route has the advantage of faster onset and reliable titration of dose. Other non-parenteral routes include the intranasal or rectal route.
The most commonly used sedative agents belong to one of the three classes of drugs:
  1. Barbiturates,
  2. Benzodiazepines, or
  3. Narcotics.
The most often used barbiturate is pentobarbital. Others being methohexital and thiopental sodium. Pentobarbital and Quinalbarbitone are safe, effective oral agents in children under the age of 5 years. The benzodiazepines include diazepam and midazolam. Diazepam is not used routinely as a sedative for diagnostic imaging in children. Respiratory depression is the most important concern with barbiturates while vomiting is often seen with midazolam. Narcotics are commonly used as an adjunct to other sedative agents in situations where pain control is desirable in addition to sedation.4, 5
Besides these three groups of drugs, other commonly used agents include triclofos, chloral hydrate, propofol, ketamine and a combination of meperidine (Demerol), chlorpromazine (Phenargan) and promethazine (Thorazine) {also known as DPT, or the “lytic” cocktail}, injected intramuscularly. Triclofos (pedicloryl) is a good sedative agent which can be used orally to sedate infants and children <5 year undergoing procedure or imaging. Maximum dose of 70–100 mg/kg body weight may be given.8 Years of experience, ease of administration, and an excellent safety record have made chloral hydrate the most widely used sedative for children undergoing radiologic imaging. It is most effective in children under 2 years of age. Use of propofol by radiologists is not widespread due to its anaesthetic properties. Ketamine is a safe and effective agent for pediatric outpatient sedation and analgesia. Ketamine can be given by multiple routes and is one of only a few agents that are extremely predictable when administered intramuscularly. Furthermore, unlike benzodiazepines, barbiturates, and sedative/hypnotic agents; ketamine seldom causes respiratory depression. Ketamine, however, may cause raised intracranial pressure and should not be used when this is an issue. DPT has been popular as an analgesic and sedative to facilitate a variety of painful or anxiety-provoking procedures in children, but prolonged sedation times and the possibility of respiratory depression argue against the use of DPT in any setting. Table 1.1 enlists the routes and doses of frequently used sedatives.4, 9
 
REDUCTION OF RADIATION DOSAGE
It is essential to limit the dose of ionizing radiation to children as much as possible. Based on information concerning the effects of low-dose radiation to atomic bomb survivors who were irradiated as children with doses that are comparable to those received by children in helical CT, it is now known that there is a statistically significant, albeit, small individual risk for excess cancer in patients with doses used in CT. Children are more sensitive than adults by a factor of 10 because firstly, they have more time to express cancers than do adults and also because they have more dividing cells. Also, girls are more sensitive than boys. Hence, applying the ALARA principle in imaging becomes especially important in children. Gonad protection especially is essential.1012 Reduction of dose during an examination can be done at several levels, i.e. rational use of referral criteria, modification of equipment and technique. The most important prerequisite is the evaluation of need for an examination and the choice of most suitable modality accordingly. Specific pediatric adaptation of equipment such as restriction devices, tubes for immobilization and a removable grid are very useful for dose saving. Besides modification of equipment, operator dependent factors, i.e. the technique is equally important.12, 13
3
Table 1.1   Sedative agents and antagonists for pediatric imaging4, 8
Agent
Class and Route *
Effect and Onset
Dose
Duration
Chloral hydrate
PO (PR)
Sedative
50–100 mg/kg; up to 120 mg/kg. reported. Maximum single dose 2 g
30–90 min
Pentobarbital sodium
Barbiturate IV (PO, IM)
Sedative 5–10 min
2–3 mg/kg dose titrated over 5–7 min until sedated, maximum cumulative dose of 8 mg/kg or 150–200 mg
40–60 min
Fentanyl citrate
Narcotic IV
Analgesic with sedative properties 1–2 min
1 mg/kg slowly IV over 5 min; adult-size paitents 25–50 mg/dose; maximum cumulative dose 4 mg/kg
30–60 min
Midazolam
Benzodiazepine IV (PO)
Sedative, anxiolytic, amnestic 1–5 min (IV)
0.02–0.05 mg/kg; titrate using half of original dose (over 2–4 min) based on effect and oxygen saturation; maximum of 1 mg
20–30 min
IV—Intravenous, IM—Intramuscular, PO—Per os, PR—Per rectum. Preferred route listed first with alternate routes in brackets
Two major areas where reduction of radiation dose are critical are:
  1. Radiography and fluoroscopy
  2. Computed tomography
 
Radiography and Fluoroscopy
In procedures involving radiography and fluoroscopy, radiation dose reduction can be achieved at two broad levels:
  1. Radiographic equipment factors
  2. Operator dependent techniques
 
Radiographic Equipment Factors
  • Use of increased film-screen sensitivity
  • Use of digital radiography
  • Addition of filtration
  • Use of carbon fiber materials
  • In fluoroscopy
    • Modern image intensifiers
    • Pulsed fluoroscopy
    • Last image hold up system
    • Dynamic recording on videotape for screening procedures.
Potential reduction using these changes have been studied by many investigators like Gozalez et al (1995), Martin et al (1994), Mooney et al (1998), etc. who found dose reduction ranging from 30 to 85 percent.1416
 
Screen-Film Combinations
The choice of the optimal screen-film combination has the greatest impact on dose reduction. The higher the sensitivity of the screen-film combination, the lower the patient dose. A dose reduction by a factor of 8 to 10 in comparison to universal screens is possible when rare earth screens with a high speed are used. Generally speaking, for routine examinations (with the exception of some bone disease like osteomyelitis, battered child), screens with a speed of at least 400 should be used. Some authors recommend systems with a speed of 600 because the radiation doses are minimum and their use permits very short exposure times, which also prevents motion blurring artifacts. These advantages outweigh the slightly lower resolution.12,17
 
Digital Radiography
Phosphor plates are now frequently used for intensive care radiography and even portable DR systems are available. Most modern CR and DR systems now effectively offer substantial patient dose reduction compared to screen-film radiography, however in pediatric radiography this may not be true due to lack of standardization in exposure factors due to lack of understanding of fundamentals of CR and DR technology.18,19 Moreover, other advantages of digital radiography are the fact that images can be stored and transferred electronically. Repeated exposures are no longer necessary because the contrast resolution is sufficient over a wider range than with conventional screen-film combinations; in particular, fine catheters and tubes are clearly seen. Also modern equipment have integrated ‘diagnostic reference dose levels'. This is defined as dose levels for typical examinations for groups of standard sized patients. Using these presets, exposure can be adjusted as per the patient's size and hence dose may be reduced. Despite these advantages, these may be counterbalanced by dose increases due to the radiographer's unawareness of possible overexposure, since the visible film blackening is standardized by the reading mechanism of the laser imaging device. So, in order to reduce the radiation dose all guidelines implemented for conventional radiography, including appropriate collimation, appropriate source-to-image distance (SID), focal spot size, and patient positioning should be practiced in digital techniques as well.18,19 It should be emphasized that while doing various portable examinations and pediatric applications requiring manual tube settings, special care should be taken as there may be unnoticed increase in exposure.
 
Additional Filtration
Additional filtration can reduce the entrance surface dose considerably up to about 50 percent, depending on the material used. Its use has the disadvantage that the image contrast is 4deteriorated and the tube load is increased. Adjustable additional filtration should be available for all X-ray tubes, which are used for pediatric exposures (Bucky tables, fluoroscopic equipment and mobile X-ray units). The recommended materials for additional filtration are 1.0 mm aluminum and 0.1–0.2 mm copper. Steel filtration (0.7 mm) can also be used.12,20,21
 
Use of Carbon Materials
The use of carbon materials for patient support, in anti-scatter grids and for the radiographic cassette face, allows transmission of a larger portion of the X-ray beam. The overall reduction in the absorbed dose due to this measure is in the range of about 30 percent to more than 50 percent.13
 
Fluoroscopy
Image intensifiers: Direct dark room fluoroscopy delivers higher radiation dose to the patient than fluoroscopy with image intensification, and produces images of lower quality. The use of direct fluoroscopy should hence be discontinued.
Also, the size of the image intensifier determines the receptor dose rate. The rule is that because of the need for constancy of brightness at the image intensifier input, smaller sizes of the image intensifier require higher dose rates. A similar increase in dose occurs if electronic magnification is used. Two different dose rates should be available in order to select the lower dose for simple follow-through contrast studies, such as the barium enema, and to switch to the higher dose rate if a high contrast examine is needed, such as the tracheoesophageal fistula. Electronic magnification during fluoroscopy should be restricted to rare cases.12,13
Pulsed fluoroscopy: Pulsed fluoroscopy with and without grid controlled tubes reduces the effective screening time and hence, the dose can be dramatically reduced.12,13
Last image hold up system, Dynamic recording on videotape for screening procedures: Use of these techniques essentially contributes to a decrease in screening time by the operator and thus the dose.12,13
Flat panel detector units: Recently flat panel detector units have been introduced for fluoroscopic and angiographic use. These units offer high quality images. However the available studies do not show these units to be superior to conventional units with image intensifiers in reducing the radiation dose.22,23 Hence, the guidelines implemented for conventional fluoroscopy should be followed to reduce patients dose.
 
Operator Dependent Techniques
  • Field size
  • Focus film distance
  • Use of high voltage
  • Shielding of sensitive organs
  • Beam direction
  • Avoid use of anti-scatter grid
  • Minimizing fluoroscopic time
  • Decrease in number of films
Studies in the previous decade have shown that operator dependent changes could lead to dose reductions of about 30–50 percent with no increase in the cost.
 
Field Size
This is the most important and most variable factor in the amount of radiation dose imparted to the patient and hence accurate field collimation should be meticulously followed. This is especially important in children because an increase in the field size in pediatric patients will cause a proportionally greater increase in individual exposure as compared to adults. This relatively higher increase is due to the smaller anatomical size of young patients. Compared to adults a similar edge length increase in pediatric patients will lead to a larger percentage of the body surface area being irradiated. Also because, upto 35 percent of the red bone marrow of infants is in the long bones of the arms and the legs, correct patient positioning and collimation in the transverse axis is also important when chest and/or abdominal films are taken.12,13
The actual field size also depends on a correctly functioning collimating system. In most machines, an automatic setting prevents collimation of radiographic exposure even if fluoroscopic field is collimated. This can be identified and switched off12,24 (Figs 1.1A and B).
Five frequent reasons for bad collimation in daily practice, and consequently for oversized field areas, are:
  • Lack of knowledge of age dependent anatomy
  • No information on pathology
  • Difficulty in patient positioning
  • Difficulty in patient immobilization
  • Difficulty in handling of the X-ray equipment
Permanent training and supervision of the technicians and young radiologists is needed to optimize collimation, especially in neonates. In a European survey on neonatal chest radiography only 15 percent of the films had an acceptable field size.25 In chest X-ray inadvertent exposure of both the skull and the abdomen, and in abdominal films of both the chest and the legs is too high, thereby increasing the red bone marrow dose. Up to 40 percent of the red bone marrow of infants and toddlers is in the skull, and 25 percent in the femora of premature babies.12
 
Focus Film Distance
The radiographer should always try to select the largest possible focus-patient distance during radiography. For example the longest distances are used for the upright anteroposterior spine for scoliosis (up to 300 cm). On the other hand, the focus-film distances (FFD) for X-rays performed with mobile units in neonatal wards (mainly chest and abdominal films) are often too short (less than 80 cm, sometimes even less than 60 cm). Ideally, the focus-film distance should be at least 100 cm. Incorrect and varying focus-film distances are the most important factor responsible for over-exposure of patients in the intensive care units.12,13
 
Use of High Voltage
In terms of radiation protection, there is an important relationship between the absorbed dose and the voltage used. A reduction in the kV causes a steep increase in the relative dose. A basic rule of radiation protection is that voltage values below 60 kV should not be used for X-rays of the body, trunk and head. The highest possible kV should be used.
5
zoom view
Figs 1.1A and B: MCU, AP view bladder area (A) without automatic setting switched off showing unsatisfactory collimation, (B) With the setting is switched off, desired collimation is achieved
An increase in the voltage up to 120 kV can diminish the dose slightly further, but is not at all useful in infants and young children because the image contrast is significantly degraded. Many generators cannot cope with the short switching times needed for high kV exposures. When voltage settings above 50 kV are used for small patients, one should use additional filtration to counterbalance the very small mAs-product, and thus allow for longer switching times.12,13,24
An increase in patient thickness causes an enormous increase in the required dose. The anteroposterior diameter in children up to one year of age is only 10–12 cm and is about 15 cm in older children up to five. In patients with a body diameter above 15 cm the dose increase is very steep. In order to deal with these relatively higher doses in school children, one should at first increase the kV-setting to avoid an undesirable concomitant increase in exposure time. This simple relationship is also important for fluoroscopy when lateral views have to be performed during voiding cystourethrography. Increasing the kV not only reduces the dose but also shortens the exposure time and thereby reduces motion blurring artifacts. However, the scatter will be increased with the use of higher kV. Scattered radiation can be diminished if the irradiated volume can be kept small by good collimation.12
zoom view
Figs 1.2A and B: Pelvis AP view: (A) Taken at 48 kV and 8 mA, (B) Taken at 58.5 kV and 3.6 mA. No significant difference in image quality, dose reduction by 28 percent in high kV technique
In a study in our department at AIIMS, the use of high kV and low mA in radiographic exposures was studied during micturating cystourethrograms, under fluoroscopic guidance. The kV used was about 25 percent higher and mA about 50 percent lower than the routinely used parameters. It was found that the resultant dose area product (DAP) value per radiograph during an exposure was 25–28 percent lower than the DAP value per radiograph at the routinely used parameters when the same degree of collimation was used, and comparable image quality was obtained23 (Figs 1.2A and B).
6
Low kV settings for the bones are only rarely needed for special indications, i.e. mostly for special skeletal disorders (osteogenesis imperfecta, etc).12
 
Shielding of Sensitive Organs
The best gonadal protection is tight collimation to exclude the gonads from direct exposure. If this is not possible, gonadal shielding should be used. It is recommended that the gonads should be shielded when they are directly in the X-ray beam or within 5 cms of it, unless such shielding excludes or degrades important diagnostic information. Various gonadal shields are available. Every pediatric radiology department must have different size shields for the various age groups. Contrary to general opinion, it has been shown that shields like lead capsules, can be used in over 90 percent of cases for a voiding micturition cystourethrography without overshadowing the urethra, and even for pelvic examinations.
Effective protection of the ovaries is more difficult because these generally lie within the pelvis.13 Ovarial masks (fixation of the shadowing material at the collimator) should be preferred over contact shielding which can be easily displaced during the examination, thereby possibly shadowing the hip joints causing a need for retakes.3, 25, 26, 27
 
Beam Direction
Another important aspect for patient exposure is the beam direction {anteroposterior (AP)/posteroanterior (PA)}. The standard AP projection can be replaced by prone positions for a large number of examinations (Table 1.2). For example, the gonadal and breast tissue dose in gastrointestinal or urographic examinations can greatly be decreased using PA instead of AP projection. In addition patient thickness is decreased by compression of the belly in prone positioning, thereby decreasing the scattered dose.12
 
Anti-scatter Grid
Scattered radiation plays no major role in pediatric patients. Use of anti-scatter grid increases patient dose by a factor of 3 to 5. No grid is needed if the object thickness is less than 12 cm. No anti-scatter grid is needed for chest X-ray in patients upto 8 years of age, for the infant hip, for abdominal films in infants or for most of the fluoroscopic examinations (with the exception of double-contrast examinations of the gastrointestinal tract studies in older children). However, a grid is needed for imaging of fat patients (diameter over 15 cm) and for high kV exposures (over 90 kV). A grid ratio of 8:1 is sufficient for all pediatric examinations.12,13,29
In nearly all pediatric fluoroscopic examinations, grids are not needed, because no significant scatter is produced, especially when collimation is good. A removable grid is essential for pediatric fluoroscopy equipment. Gridless screening may reduce the radiation dose by as much as 50 percent. Infants under 1 year of age are always examined with the explorator grid removed since there is little scatter and no appreciable loss of detail. When fine detail such as mucosal pattern is not required, gridless screening may be used in children upto 5 years of age.1, 12, 13, 29
Table 1.2   The possible use of posterior-anterior beam projection for radiographic examinations in pediatric patients11
Radiographic Examinations
Clinical Setting
Critical Organ
Skull
Trauma, ventriculoperitoneal shunt
Lens
Chest in lateral decubitus
Foreign body aspiration, empyema
Breast
Abdomen in lateral decubitus
Bowel perforation, ileus
Breast
Spine
Scoliosis
Breast, gonads
Abdomen post-contrastfilms
Intravenous urography
Breast
Abdomen
Enteroclysis
Breast
 
Minimizing Fluoroscopic Time
The most important factor for dose reduction in fluoroscopy is the limitation of screening time. This depends mostly on the clear definition of the clinical questions, on the patient's disease and on the radiologist's experience. Reduction of fluoroscopic time can be achieved by modifying some practices as a habit. Techniques like collimating fluoroscopic field before placing the child on the table and use of fluoroscopy only after satisfactory positioning of the child and the explorator are useful.1,13,24
 
Decrease in Number of Films
Tailoring an examination based on clinical problem and minimizing the number of films can significantly reduce the radiation dose.13,24
 
Computed Tomography (CT)
Various studies have established that CT scans performed in children result in a significantly increased life time radiation risk over adult CT, both because of the increased dose per milliampere second as well as the increased life time unit dose. This underlines the importance of dose reduction in pediatric CT examinations. In older helical CT scanners parameters were not adjusted on the basis of examination type or age of the child so that most pediatric CT scans were performed using the same parameters as for adult CT. Donnelly et al suggested a number of techniques for minimizing radiation dose in pediatric helical CT. Most important of these include reduction in mA and increase in pitch.30,31
Dose is directly proportional to the product of scan time and tube current. Hence, keeping other parameters constant, absorbed dose shows a linear relationship with mA. This has been shown by numerous investigators including Fearon et al and Donnelly et al in phantom studies.30,31 Selection of most appropriate mA is a compromise between image quality and radiation dose. Phantom experiments have shown that an increase in mAs will always result in a decrease in image noise and thus an improvement in image quality. But at high tube current settings the gain in image quality will not be significant. Tube current should be adjusted to provide the lowest dose consistent with adequate diagnostic quality. A technique chart that relates current to patient's weight is appropriate. Scan time can be shortened by more rapid gantry 7rotation or by decreasing beam rotation to less than a full 360°. In general, the fastest scan time that uses full rotation should be used.31,33
Similarly it has been seen that radiation dose in helical CT is inversely proportional to the pitch used. When pitch is doubled, radiation dose gets halved.31
The dose increase caused by increasing kilovoltage is not linear, and is greater than often appreciated. An increase from 120 to 140 kilovoltage peak increase dose by approximately 40 percent. Pediatric patients are rarely large enough to warrant the use of increased kilovoltage peak.33
Newer multidetector CT scanners are equipped with automatic exposure control system (AEC) under different names (Care dose 4D, Dose right, AutomA etc.). These dose modulation systems work in various ways, so as to adjust the radiation dose according to the patient's body size and attenuation. AEC reduces the patient's dose without compromising image quality.34,35 The radiation dose is essentially reduced by controlling the tube current which is performed by three methods. These methods are based on (a) patient size, (b) z axis and (c) angular or rotational AEC. Most of the scanners use combination of all these methods.35 The scanner uses the projection radiograph data (topogram/scanogram) to assess size and attenuation of patient and accordingly dose is modulated using patient size and z axis. In angular AEC, the dose is modulated so as to equalize the photon flux to the detector while the tube is rotating. This is needed because the human body is non circular and hence the attenuation of the beam varies at different projections. Generally lateral projections are more attenuating than antero-posterior projections. By using AEC there is considerable reduction in the magnitude exposure dose, in the range of 35–60.36 AEC thus is helpful in reducing the patient's dose, especially in pediatric cases.
Despite these developments following facts should always be taken into account while performing a CT in pediatric cases. In a neonate approximately 30 percent of the marrow is contained in the skull and the marrow absorbed dose for a CT brain examination in a 6 years old patient phantom has been reported to be even higher than that for a CT chest or abdomen examination. Therefore, high priority should be given to dose reduction measures for head scans in children. Results of a study in our department at AIIMS also showed that a reduction of mAs from 115 or 141 mAs to 77 or 94 mA, which represents a 53–65 percent reduction in dose did not result in any significant difference in diagnostic accuracy although there was a slight reduction in image quality which was also not statistically significant,37 (Figs 1.3A and B). Chest is a naturally high contrast area because of large attenuation differences that result from the presence of air in lungs and fat in mediastinum. Scans of objects with large differences in attenuation values such as lungs are less likely to be sensitive to image noise as image noise mainly affects low-contrast resolution. So, low doses may be used. Low radiation dose technique was used by Rogalla et al in their study of chest CT. Rogalla etal had found that although there is no consensus regarding which mA setting may be regarded as ideal low dose technique for spiral CT scanning of pediatric chest, 25–75 mAs is sufficient for lung window and 50–75 mAs for mediastinal window. A mA of 77 (57.5 mAs) time represents a nearly 67 percent reduction in dose as compared to mA of 240 (180 mAs) with no significant loss of diagnostic information.38 Our departmental study also showed similar results37 (Figs 1.4A and B). Now-a-days most of the modern scanners provide CT dose index (CTDI) which is the most commonly used dose indicator. It does not provide the precise dose, rather it is an index of dose measured using a phantom. However, CTDI may greatly help in comparing radiation dose at different scanning parameters.35
zoom view
Figs 1.3A and B: CT head at 94 mA (A) and at 206 mA (B). No significant difference in overall image quality. Radiation dose in A is 50 percent of B
Various manufacturers provide multiple protocols for different examination and as per the age of patient. Hence when imaging for children, pediatric protocols should be followed. Ideally, all institutions must set their own scanning protocol, involving various parameters (tube voltage, tube current, slice thickness, collimation and pitch) optimized as per the use.
8
zoom view
Figs 1.4A and B: Chest CT at 240mA (A) and at 77mA (B). Image quality is comparable while radiation dose in B is less than one-third of that in A
The important point to remember is that different manufacturers use different techniques for dose modulation so the user should know about the system's characteristics before trying to attempt any change in scanning parameters.39 Any changes should be performed using appropriate (weight range) phantoms.
Recently dual energy CT scanners have been introduced which are faster and have ability to provide greater information about tissue composition than obtained by single energy scanners. Although not much is known about its use in pediatric cases, however with dual energy scanners non contrast CT scans are not needed as contrast media can be subtracted, and the patient is spared the radiation dose of a second scan.
 
USE OF CONTRAST MEDIA
Contrast media available for intravenous (IV) use in radiography are categorized as high-osmolality contrast media (HOCM), low-osmolality contrast media (LOCM) and isosmolar contrast media (IOCM). Considerations in choice amongst these are the concentration of iodine achieved within plasma and urine, economic factors, and safety factors.
Table 1.3   Suggested tube current (mA) by weight of paediatric patients for single-detector Helical CT38
Weight
mA
kg
Chest
Abdomen or pelvis
4.5–8.9
40
60
9.0–17.9
50
70
18.0–26.9
60
80
27.0–35.9
70
100
36.0–45.0
80
120
45.1–70.0
100–120
140–150
> 70
> 140
> 170
 
High Osmolality Contrast Media
HOCM have an iodine content ranging from 280 to 480 mg/mL and an osmolality range from 1400 to 2500 mOsm/kg. Dosage of contrast material is based upon grams of iodine administered in relation to body mass. It is appropriate to use a dosage of approximately 300 mg of iodine per kilogram. This represents approximately 1.0 mL/kg in the most commonly used forms of diatrizoate or iothalamate. The total dose for excretory urography or for CT is usually 2.0 mL/kg in children or 3.0 mL/kg in the newborn.
Speed of injection is important for the resultant plasma concentration of contrast material. After rapid injection there is an increase in serum osmolality within 3 minutes, a decrease in serum sodium concentration, and an increase in heart rate. The osmotic effect is particularly significant in young infants. A mean increase of 3 percent in serum osmolality is observed in adults. Excretion occurs rapidly by renal glomerular filtration. Because of a high osmotic load, these contrast media also produce diuresis, opposing tubular resorption.38
 
Low Osmolality Contrast Agents
LOCM have an iodine content ranging from 128 to 320 mg/mL and an osmolality range from 290 to 702 mOsm/kg. Agents with low iodine content are most suitable for intra-arterial digital subtraction arteriography. Those with iodine content of 240 to 300 mg/mL are used for excretory urography, venography, venous injection digital subtraction arteriography, and bolus IV enhancement for CT scans. The contrast media with high iodine content, 320 to 370 mg/mL, are used for aortography and selective arteriography.
 
Iso Osmolar Contrast Agents
IOCM have an iodine content ranging from 270 to 320 mg/mL and an osmolality of 290 mOsm/kg.
Initial reports showed that the IOCM reduces the risk of contrast induced nephropathy (CIN) in patients with deranged renal parameters. However recently various meta-analysis of randomized control trials have shown that there is no statistically significant 9reduction in CIN associated with iodixanol as compared to LOCM.39,40 Hence with this equivocal kind of reports IOCM offers no significant advantage over LOCM.
Unlike HOCM, LOCM and IOCM have little or no effect on serum osmolality, serum sodium, vasodilation, haemodilution, red blood cell morphology, or vascular permeability. There is little or no effect on the blood-brain barrier, fewer electrocardiographic changes, and fewer alterations in myocardial contractility, cardiac output, and left ventricular, pulmonary artery, and aortic pressures. There is less endothelial damage, and lower release or activation of vasoactive substances including complement activation, histamine release, and acetylcholinesterase inhibition. Diminished effects on coagulation pathways have been demonstrated. These effects are attributed to the lower osmolality and the reduced chemotactic effect of the molecules. Of importance is reduction in the nephrotoxic effect noted with HOCM. Hence, there are definite advantages to adoption of LOCM. A major consideration is degradation of the resulting examination resulting from pain, heat or vomiting with HOCM.38
Performing a multiphasic CT scans in neonate and infants may be challenging, as the IV cannula is of smaller gage limiting the injection rate of power injector, moreover only small amount of IV contrast can be used depending on the weight of the child. These situations may be handled by (i) using bolus tracking and saline chasing technique and (ii) large bore cannula.41,42 An injection rate of 2–3 ml/sec is safe and provides good results.40 Although there is no consensus, but contrast may be administered using central venous line with a maximum injection rate of 2 ml/sec.41
 
MR Contrast Agents
The most commonly used contrast agents are paramagnetic substances and amongst these Gadolinium diethylenetriamine penta-acetic acid (Gd DTPA) dimeglumine is most frequently used. Gd DTPA is excreted by glomerular filtration with 90 percent excreted within 24 hours. Rapid renal clearance, and low toxicity are important features of this contrast material. The clinical dose of Gd DTPA is 0.1 mmol/kg. It has an osmolality of 1,900 mOsm/kg. However, the high osmolality is of little importance because of the small volume administered.38,43
Although gadolinium-enhanced MR imaging was once considered one of the safer imaging procedures, but recently there has been a significant concern regarding nephrogenic systemic fibrosis (NSF) associated with gadolinium based contrast agents. The identified risk factors associated with development of NSF include - administration of a high dose of gadolinium-based contrast agent, acute or chronic renal failure, venous thrombosis and coagulopathy and vascular surgery.44,45
Some additional guiding principles for use of contrast in the neonates are:
  1. Use warm contrast for maintenance of body temperature.
  2. Use iso-osmolar+ve, non-ionic contrast in most instances.
  3. When giving oral or rectal contrast, use low-osmolar, non-ionic agents instead of barium to avoid barium contamination of the peritoneal cavity.
  4. Do not give contrast blindly; oral contrast may be aspirated, and rectal contrast may get into peritoneal cavity via a perforation. Even in the intact bowel, the contrast may not progress distally as quickly as predicted and therefore may lead to unnecessary radiographs.
  5. Be judicious in the volume of contrast administered. Renal function in neonates is less than in babies over one month and age, therefore excretion of contrast may be delayed.
  6. Gadolinium is the preferred contrast agent for magnetic resonance imaging. However, it should be used with a caution.43,44,45
In conclusion the aim of all departments and radiologists dealing with pediatric imaging should be to achieve a diagnostically adequate radiograph or examination, with minimum radiation exposure and discomfort to the child. This goal can only be achieved if the radiologists and technicians in charge are committed to quality control programs, and are aware of the necessity for radiation protection in children.
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