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1
Department of Radiology, Royal Belfast Hospital for Sick Children, 180 Falls
Rd., Belfast BT12 6BE, Northern Ireland.
2
Department of Radiology, Division of Pediatric Radiology, Rm. 1905,
McGovern-Davison Children's Health Center, Duke University Medical Center,
Erwin Rd., Durham, NC 27710.
3
Department of Radiology, Children's Hospital Medical Center, 3333 Burnet Ave.,
Cincinnati, OH 45229.
Received July 3, 2000;
accepted after revision October 11, 2000.
Address correspondence to D. P. Frush.
Abstract
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SUBJECTS AND METHODS. This prospective investigation included all body (chest and abdomen) helical CT examinations (n = 58) of neonates, infants, and children (n = 32) referred from outside institutions for whom radiologic consultation was requested. Information recorded included tube current, kilovoltage, collimation, and pitch. Examinations were arbitrarily grouped on the basis of the individual's age: group A, 0-4 years; group B, 5-8 years; group C, 9-12 years; and group D, 13-16 years old.
RESULTS. Thirty-one percent (18/58) of the CT examinations were of the chest and 69% (40/58) were of the abdomen. Sixteen percent (9/58) of the CT examinations were combined chest and abdomen. In 22% (2/9) of these combined examinations, tube current was adjusted between the chest and abdomen CT; in one (11%) of these examinations, the tube current was higher for the chest than for the abdomen portion of the CT examination. The mean tube current setting for chest was 213 mA and was 206 mA for the abdomen, with no evident adjustment in tube current based on the age of the patient. Fifty-six percent of the examinations of neonates, infants, or children 8 years old or younger were performed at a collimation of greater than 5 mm and 53% of these examinations were performed using a pitch of 1.0.
CONCLUSION. Pediatric helical CT parameters are not adjusted on the basis of the examination type or the age of the child. In particular, these results suggest that pediatric patients may be exposed to an unnecessarily high radiation dose during body CT.
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There are several methods by which information about the common practice of helical CT techniques in pediatric patients can be obtained. One method is by a survey [7, 8]. Among the difficulties with this kind of sampling is assessing the validity of responses. We approached the issue of the practice of pediatric helical CT by another method in part to minimize this potential reporting bias. We collected and reviewed all chest and abdomen helical CT examinations in children 16 years old or younger performed at referring institutions or practices and submitted to our department for review. The objective of this type of survey was to determine what, if any, adjustments were made for CT examinations of children compared with adults and for CT examinations of pediatric patients of different ages. We focused on the adjustable scan parameters that primarily contribute to the amount of radiation the patient receives.
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For each helical CT examination, data were recorded from the CT scans. These data included the individual's age and sex, clinical indications for the examination, and the body region or regions imaged. Body regions consisted of the chest and abdomen. Pelvic CT examinations, if performed, were considered part of the abdominal examinations for the purposes of this investigation (because pelvic CT examinations were always performed in conjunction with an abdominal study). If CT of the chest and abdomen were performed during the same examination time, these were counted as separate examinations. Follow-up CT examinations, all of which were performed at intervals of at least 2 weeks, were recorded as separate examinations.
Multiphase abdominal CT examinations were also recorded; the different phases were analyzed not only as a single examination, but also as separate examinations because examination parameters could be changed between phases and because grouping multiple phases underestimates radiation exposure. Multiphase examinations were considered present when additional scans were obtained within seconds or minutes of the preceding series. These examinations were performed only with abdominal imaging, and phases were recorded on the basis of recognized criteria for hepatic scanning [1, 9] as unenhanced and contrast-enhanced (arterial, portal venous, equilibrium, or delayed) IV phases.
The scanning parameters for each body region wre also recorded. These parameters included tube current, kilovoltage, collimation, pitch, and reconstruction interval.
No data other than the parameters on filmed images were recorded. In addition, we had no knowledge of, nor did we pursue the rationale for, specific techniques or protocols for any of the examinations. In particular, we had no knowledge of possible manufacturers' preset CT protocols or radiologists' CT protocols.
Examinations were included if the individual was 16 years old or younger. The patients were arbitrarily divided into the following four groups by age: group A, 0-4 years; group B, 5-8 years; group C, 9-12 years; and group D, 13-16 years old. Weight data for the patients were not available. Because preliminary digital (scout) radiographs were not always available, we elected to not consider body size for grouping purposes. Because CT examinations were separated on the basis of the two regions (the chest or abdomen), there were eight different patient groups (Tables 1 and 2).
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Thirty-one percent (18/58) of the examinations were of the chest, and 69% (40/58) were of the abdomen. Indications for CT included malignancy (32/58 [55%]), inflammatory conditions (7/58 [12%]), trauma (6/58 [10%]), and other (13/58 [22%]).
Thirty-two children underwent a total of 58 CT examinations. Nine individuals (28%) underwent one chest CT examination, and seven (22%) underwent one single-phase abdominal CT examination (n = 16 examinations). Ten additional individuals (31%) underwent multiphase abdominal examinations (n = 24 examinations). Five individuals (16%) underwent a single combined chest and abdomen (single-phase) CT examination (n = 10 examinations). Two additional individuals (6%) underwent a single combined chest and abdomen (single-phase) CT examination on two different days (n = 8 examinations). One of these children had a multiphase examination obtained on a third day, and this CT examination and child were counted in the multiphase group.
The 22% (7/32) of the infants and children who underwent combined chest and abdomen CT underwent a total of nine examinations. Two combined chest and abdomen examinations were in groups A, B, and C, and three examinations were in group D.
Ten (31%) of the 32 infants and children underwent multiphase abdominal CT examinations (24 total examinations). Of these 10 children, one was in group A; one, in group B; four, in group C; and four, in group D. Dualphase combinations included unenhanced and enhanced phases (n = 2), arterial and equilibrium phases (n = 1), or portal venous and equilibrium phases (n = 1). Three additional children underwent repeated abdomen CT examinations (delayed phase) within hours of the initial CT examination. Two children in group C underwent triple-phase CT examinations of the abdomen (unenhanced, portal venous, and equilibrium phases). The final child, in group C, underwent a triple-phase examination (unenhanced, portal venous, and equilibrium phases) with an additional delayed CT scan obtained 2 hr later for unknown reasons.
Tube Current and Kilovoltage
Table 1 lists the calculated
mean, range, and median or mode values for the recorded tube current setting
(not adjusted for gantry rotation cycle speed) and kilovoltage. When
considering all the patients, the mean tube current for helical CT of the
chest was 213 mA, and the mean tube current for examinations of the abdomen
was 206 mA. The highest tube current settings were found in the groups with
the youngest and those with the oldest children. The maximum recorded tube
current used for examination of both the chest and abdomen was 280 mA in group
A children, and it was 260 mA for the chest and 300 mA for the abdomen in
group D children. Of note, none of the children in group B underwent a study
performed at less than 200 mA (Table
1).
When the data are grouped for examinations of neonates, infants, or children 8 years old or younger versus children ranging in age from 9 to 16 years, 63% (5/8) of the chest CT examinations performed in the younger age group were performed at 200 mA or greater, and 90% (9/10) of the abdominal CT examinations were performed at 200 mA or greater. In the older patient group (age range, 9-16 years), 90% (9/10) of the chest CT examinations were performed at 200 mA or greater, and 57% (17/30) of the abdominal—pelvic CT examinations were performed at 200 mA or greater.
Of the nine examinations of the chest and abdomen, two (22%) were performed at different tube current settings for the chest and abdomen. The first of these two examinations was of a group A patient; notably, the tube current was higher for the chest CT examination (200 mA) than for the abdominal CT examination (170 mA). The second examination was of a group D patient. In this instance, the tube current was 160 mA for chest CT and 180 mA for abdominal CT. No other CT parameters were changed between the chest and abdomen CT examinations.
The mean tube current setting for all children varied little for chest CT performed in single versus combined studies. The mean tube current was 218 mA when chest CT was a single examination versus 211 mA when chest CT was performed as part of a combined CT examination of the chest and abdomen.
Although most examinations were performed at community-based imaging centers, it is worth noting that the tube current for each of the four examinations performed at either university (n = 1) or children's (n = 3) hospitals was 200 mA, almost the mean value for all the examinations combined (Table 1).
The peak kilovoltage used for different body regions or age groups varied little. The majority (37/58 [64%]) of examinations in all groups were performed at 120 kVp.
Collimation and Pitch
Table 2 shows the range and
mode of slice collimation used for each age group and body region imaged.
Fifty-six percent (10/18) of the examinations (4/8 chest CT; 6/10 abdomen CT)
in children 8 years old or younger were performed at a collimation greater
than 5 mm. In all individuals, the reconstruction interval was equal to the
slice collimation.
Although the pitch (or table speed) was available on films for helical examinations performed on some scanners (i.e., the scanners manufactured by General Electric Medical Systems [Milwaukee, WI]), pitch (or table speed) was not consistently identifiable on examinations performed on other scanners. These examinations consisted of one abdominal examination of a group B patient, seven abdominal examinations of group C patients, and three examinations (one chest and two abdominal examinations) of group D patients. Therefore, the figures quoted for pitch are based on the remaining examinations (n = 47) (Table 2). A pitch of 1.0 was used in 53% of the examinations. Forty-one percent (7/17) of the chest CT examinations and 60% (18/30) of the abdominal CT examinations were performed at a pitch of 1.0. For all children, the mean collimation varied just over 1.0 mm and was 7.1 mm for single chest, 8.5 mm for single abdomen, and 7.6 mm for combined CT examinations of the chest and abdomen. For all children, the mean pitch varied little and was 1.3 for single chest, 1.1 for single abdomen, and 1.4 for combined examinations of the chest and abdomen.
If the data were grouped slightly differently so that any multiphase examination (regardless of the number of phases) was considered as a single CT examination, there were a total of 26 abdominal examinations in 23 children. Because this grouping affected only the abdominal examinations, the data for chest CT do not change (Tables 1 and 2). The mean tube current for the 26 examinations was 206 mA. Age-specific data for mean tube current were as follows: group A, 205 mA (n = 4); group B, 226 mA (n = 4); group C, 194 mA (n = 6); and group D, 205 mA (n = 12). These values are only slightly different than when phases in multiphase examinations were counted as individual examinations (Table 1).
In two patients, the parameters were changed between phases. In the first child, the tube current was increased from 200 to 280 mA during a dual-phase examination (mean for the examination, 240 mA). In the second child, the collimation was increased during the dual-phase examination from 7 to 10 mm.
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Radiation exposure has been an important issue in CT since the technique was introduced three decades ago. For example, 10 years ago, CT examinations accounted for 2% of radiography in the United Kingdom, but 20% of the radiation dose for the population was from the medical use of ionizing radiation [11]. More recently, reports suggest that the medical radiation dose for the population is now 30-50% [12, 13]. Because CT is a major source of this radiation, an effort to minimize dose is critically important [14].
The principal long-term disadvantage of CT is radiation exposure. This is especially important in children because the younger the patient is at the time of exposure to radiation the greater is this risk [1, 15]. In addition, organ radiosensitivity and the effective radiation dose from an individual CT examination are higher in children than in adults [16,17,18,19]. With the advent of helical CT in the early 1990s, radiologists gained the ability to control several new facets of radiation exposure. In addition to tube current and kilovoltage, table speed (hence, pitch) became a selectable parameter.
Settings for CT should be selected to optimize pertinent diagnostic information. This goal can be partly achieved by maximizing both spatial resolution and contrast resolution. Increasing contrast resolution is based on intrinsic tissue attenuation and is improved by use of both oral and IV contrast material. Spatial resolution is, in part, determined by tube current, collimation, table speed, display field of view, and reconstruction algorithm. These are the recognized parameters that radiology personnel control in CT. In contrast, another goal should be to minimize the amount of radiation exposure by judicious adjustments to these parameters. Image quality must be balanced with excessive radiation exposure.
Despite the differences in terms of actual radiation dose and tube current for different manufacturers' CT scanners [10], radiation dose is directly proportional to the tube current (for a given CT scanner and kilovoltage). In conventional radiography, an increase in the tube current results in loss of information (i.e., overexposure), but the converse is true for the digital acquisition of CT images; increasing the tube current improves quality. Although increased image quality is a desirable effect, the cost is an increase in radiation. Reducing the tube current results in an increase in image noise and decreased spatial resolution and image quality. There is an increasing body of literature that provides guidelines for tube current settings for helical CT of pediatric patients. For example, studies related to children have shown that it is possible to reduce tube current to less than 100 mA for general abdominal CT (phantom) [20], chest CT [13, 21], and pelvic CT [22]. The images obtained at a lower tube current may be less appealing aesthetically, but these images are sufficient for diagnostic purposes [23]. Data from adults also indicate that the infants and small children in our study population were imaged using mean tube currents exceeding the recommendations for children [6] and approaching the tube current recommendations for adults [3, 24].
In this investigation, we have shown that in a limited geographic region and a small population of children no appreciable adjustments in tube current were made for pediatric patients. In addition, no adjustment on the basis of patient age in tube current was made, with the youngest infants and children being scanned at identical mA values used for teenaged patients, exceeding recommendations for tube current in pediatric patients, and approaching the dose recommendations for adults [3, 24]. Indeed, many infants were being imaged at a tube current (280 mA) greater than that used for adolescent patients (160 mA) for both chest and abdominal CT. Finally, we found that no reduction in tube current was made in 89% of the examinations of children when the chest portion of a combined chest and abdominal CT examination was performed [24].
Our data also indicate that there is little difference in peak kilovoltage used in helical CT of pediatric patients because most studies (64%) are performed at 120 kVp. Although there are no data, to our knowledge, that show the effect that reducing the kilovoltage has on image quality and disease detection in children, decreasing the kilovoltage from 120 to 80 kVp can reduce the radiation dose by 65% [16, 17]. Alternatively, increasing the kilovoltage to 130 or 140 kVp allows the tube current to be decreased without any loss of information. The overall radiation dose to the patient can be decreased if the peak kilovoltage is increased and the tube current is reduced [10].
If kilovoltage and tube current are kept constant, the radiation dose for two different CT examinations is also affected by the collimation and pitch. Pitch depends on collimation, table movement, and gantry rotation time. Although the exact definition of pitch varies among manufacturers of scanners (i.e., subsecond CT scanners and dual and multisection scanners), the concept of pitch simplifies the discussion of examination parameters. For example, using a scanner with a single array of detectors and a 1.0-sec gantry rotation cycle and increasing the pitch from 1.0 to 1.5 leads to a 33% decrease in radiation dose. A 50% dose reduction is achieved by changing the pitch from 1.0 to 2.0. In one investigation of pediatric patients, CT examinations performed at a pitch of 1.5 did not result in any reduction in diagnostic accuracy when compared with those performed at a pitch of 1.0 [25]. This finding is in agreement with other investigations in both children and adults [26,27,28,29,30,31]. Despite these general recommendations, the majority (53%) of CT examinations in infants and children in our investigation were obtained at a pitch of 1.0. Notably, no examination (or examination phase) in a child less than 13 years old was performed at a pitch of greater than 1.5 (Table 2).
Collimation is often not adjusted for examinations of children; 56% of the children 8 years old or younger were imaged with a collimation of greater than 5 mm (the value recommended for CT of adults [8]). This collimation is used despite the fact that the length of an infant is substantially less than that of an adult. Choosing a scaled collimation for the spectrum of sizes of children makes more sense in terms of the number of sections relative to section width. Choosing collimation that is unnecessarily narrow will increase the radiation dose. Conversely, collimation that is too wide means that small abnormalities might be missed. The appropriate collimation depends on the CT indication but should also be adjusted for the size of the child. Collimation generally varies from 3 to 5 mm in infants and from 7 to 10 mm in adults for general scanning [3]. Relative adjustments, therefore, should be made for patients of ages or sizes in between.
There are several limitations to this investigation. First, a relatively small number of helical CT examinations were analyzed. In addition, we were unable to calculate the actual radiation dose an individual patient received. Tube current values do not necessarily convey as equal values among different models and manufacturers of CT scanners. However, tube current is an approximation of dose and a factor that is commonly used as a gauge of technique. Another limitation of our study is that the results quoted reflect local radiology practices only within a limited geographic region of the United States. Most of the examinations studied originated at community hospitals, so comparison among various types of institutions is not possible. Finally, we are not establishing ranges of parameters for helical CT in children. Our comments on the appropriateness of CT parameters are based on comparing our data with those available in the helical CT literature.
In conclusion, these preliminary survey results show that those technical parameters that influence radiation dose for helical CT are not adjusted for infants, children, or adolescents, despite the tremendous variability in body size among these individuals. This method of performing helical CT examinations in the pediatric population may compromise diagnostic ability (i.e., use of a collimation that is too wide) or result in radiation exposure that is unnecessarily and inappropriately high.
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