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Skin and Thyroid Dosimetry in Cervical Spine Screening: Two Methods for Evaluation and a Comparison Between a Helical CT and Radiographic Trauma Series

¹ All authors: Department of Radiology, Brigham and Women's Hospital, 75 Francis St., Boston, MA 02115.

Received January 30, 2002; accepted after revision March 27, 2002.

 
Address correspondence to F. Rybicki.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. The first objective of this study was to test the hypothesis that estimates of radiation dose from an ionization chamber correspond to thermoluminescent dosimeter measurements in patients with suspected cervical spine injury. The second objective was to compare the radiation dose of a protocol using helical CT of the entire cervical spine with that of a protocol using radiography alone.

SUBJECTS AND METHODS. Thermoluminescent dosimeter measurements of radiation dose to the skin over the thyroid were made in two patient groups: six patients evaluated with CT of the cervical spine and six patients evaluated with radiography. The skin dose for both groups was estimated with an ionization chamber, and the thermoluminescent dosimeter measurements and ionization chamber estimates of skin dose were compared for both groups. Using the ionization chamber, we estimated the radiation dose to the thyroid for all 12 patients. With these estimates, we computed the ratios of skin dose and thyroid dose (CT / radiography).

RESULTS. Thermoluminescent dosimeter measurements correlated with ionization chamber estimates of skin dose in both patient groups. Using the ionization chamber estimates, we found that CT delivered 26.0 mGy to the thyroid. In the patients evaluated with radiography, the mean thyroid dose was 1.80 mGy (95% confidence interval, 1.05-2.55 mGy). Ionization chamber dose ratios (CT / radiography) for the skin and thyroid were 9.69 and 14.4 mGy, respectively.

CONCLUSION. The correlation between the ionization chamber estimates and the thermoluminescent dosimeter measurements supports the use of ionization chamber estimates in future research. Although helical CT of the entire cervical spine is cost-effective in patients at high risk for fracture, the greater than 14-fold increase in the radiation dose to the thyroid emphasizes the importance of clinical stratification to identify patients at high risk for fracture and the judicious use of CT in patients with suspected cervical spine injury.

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Obtaining a minimally adequate radiographic series of the cervical spine (lateral, anteroposterior, and open-mouth odontoid) in patients who have sustained trauma can be challenging and, at times, even impossible [1,2,3]. Consequently, CT has been incorporated into the imaging protocol for evaluating patients with suspected cervical spine injury. Because CT has a superior sensitivity for fracture detection, its use has broadened from a problem-solving tool [4] to a primary modality for screening [5,6,7], creating a spectrum of screening examinations ranging from radiography to helical CT performed in two acquisitions from the occiput to T4 [8]. Paralleling the spectrum of examinations is a spectrum of costs including the cost of missing a fracture: the cost of lengthened imaging time during which trauma surgeons could potentially intervene, the direct medical cost, and the cost associated with an increased risk of malignancy from thyroid irradiation.

The optimal screening examination depends largely on the probability that a patient has a cervical spine fracture before imaging. Consequently, mechanism of injury and clinical parameters are used to separate patients into groups on the basis of risk for fracture [8]. When the cost of missing a fracture is compared with the direct medical cost, helical CT of the cervical spine is cost-effective for patients who are at high risk and scheduled for contemporaneous CT of the head [9]. However, for patients who do not meet the highrisk criteria, the cost of the imaging time [5, 10] and the radiation dose to the thyroid are factors that influence the choice of imaging protocol. The initial step in determining the radiation cost is to estimate the radiation dose to the thyroid gland for cervical spine protocols. In addition to contributing to a cost-effectiveness analysis, thyroid dosimetric data for a radiographic trauma series and for helical CT of the cervical spine can contribute to the development and implementation of low-dose CT protocols for excluding cervical spine fracture.

This study reports the results from two related experiments. The first experiment tested the hypothesis that two methods for determining radiation dose would yield similar results for patients with suspected cervical spine injury. The first method, for which a thermoluminescent dosimeter was used, required a small strip of dosimeters to be placed as close as possible to the organ of interest (the thyroid) and to subsequently be evaluated by the vendor. Although placement of the thermoluminescent dosimeters is usually a simple task, it can be cumbersome in the multitrauma setting. In the second method, for which an ionization chamber was used, radiation dose was estimated from the CT dose index measured in a phantom [11,12,13,14,15,16,17,18]. We hope that the results of the first experiment will prove useful as the optimal parameters for CT of the entire cervical spine are explored. Because patients who have sustained trauma are routinely triaged to undergo either helical CT of the entire cervical spine or radiography alone, the first experiment compares thermoluminescent dosimeter measurements and ionization chamber estimates for both screening examinations. The second experiment quantifies the difference in radiation dose to the skin and thyroid between the two screening methods, and the results are discussed as a ratio (helical CT / radiography).

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Skin Dose Measurements with a Thermoluminescent Dosimeter
Institutional review board approval was obtained, as was patient informed consent. A radiologist positioned a linear strip of six 3 x 3 x 1 mm thermoluminescent dosimeter chips (TLD100; Harshaw Bicron, Solon, OH) with no spacing between them and as close to the thyroid gland as possible in 12 adults (six men and six women; age range, 25-85 years; mean age, 48 years) who had sustained trauma. The strip was oriented along the long axis of the cervical spine.

CT of the entire cervical spine.—Six of the 12 patients were considered at high risk for cervical spine fracture [8]. For these patients, a radiologist who was present for the entire study placed a strip of thermoluminescent dosimeter chips on the immobilization collar over the thyroid. The patients were then evaluated using a single-detector helical CT protocol (Somotom Plus 4; Siemens, Iselin, NJ) for the entire cervical spine (one 24-sec acquisition to T1, 3-mm slice thickness, pitch of 1.5:1, 120 kVp, 240 mA) after one portable in-collar lateral radiograph (75 kVp, 40 mAs, 72-inch [183 cm] focal-film distance) had been obtained. Completion radiographs were not obtained [19]. The attenuation of the collar was determined to be negligible (<2%) via direct X-ray beam transmission measurements.

Radiography.—The six patients who sustained trauma but did not meet the criteria to be considered at high risk for cervical spine fracture [8] were evaluated using a radiographic series that included a portable in-collar lateral radiograph, out-of-collar lateral radiograph, anteroposterior radiograph, and open-mouth odontoid radiograph. In these patients, a radiologist who was present for the entire study placed a strip of thermoluminescent dosimeter chips on the immobilization collar for the in-collar lateral radiograph. When the collar was removed, the radiologist moved the thermoluminescent dosimeter chips to the skin overlying the thyroid gland. The parameters for each image as well as additional views (e.g., swimmer's) were documented by the radiologist.

Analysis of thermoluminescent dosimeter chips.—The thermoluminescent dosimeter chips were sent to the vendor (Landauer, Glenwood, IL) for dose determination. For both helical CT scans and radiographs, the mean of the chips was used to represent the radiation dose to the skin over the thyroid gland, and 95% confidence intervals (CI) around the mean skin dose were computed. In two patients evaluated with CT, the anatomically inferior two (of the six) thermoluminescent dosimeter chips measured markedly less radiation than the four superior thermoluminescent dosimeter chips. Specifically, radiation dose for each of the four superior chips was interpreted as greater than 13 mGy, whereas radiation dose for each of the two inferior chips was interpreted as less than 7 mGy. In these two patients, the radiologist who placed the thermoluminescent dosimeter strip documented that the chips were placed low on the collar because placement closer to the thyroid gland was precluded by monitoring, access, and life-support equipment. For these two patients, the inferior two measurements were discarded, and the mean of the chips was obtained from the superior four chips.

Skin Dose Estimates with an Ionization Chamber
CT of the entire cervical spine.—The CT dose index was estimated from a single-section ionization measurement obtained using a pencil ionization chamber (model PC 4P; Capintec, Ramsey, NJ) that was 10 cm in length and 7 mm in diameter. The pencil ionization chamber was placed in a 2-cm-deep hole in a 20-cm-diameter acrylic dosimetry phantom. The ionization chamber was calibrated as an exposure meter and a conversion factor of 9.5 mGy/R was used to convert the exposure measurement to a dose—length product. The CT dose index was estimated by dividing the dose—length product by section thickness. A comparison of ionization measurements on the surface and a depth of 2 cm indicated that ionization on the surface was 4% higher. Consequently, to estimate the average dose to the surface of the phantom, we multiplied the CT dose index estimate by 1.04. Incorporating the protocol for helical CT as described earlier, the skin dose was estimated using the CT dose index phantom measurement.

Radiography.—A diagnostic ionization chamber (model PM-30 [30-mL diagnostic chamber with National Institute of Standards and Technology—traceable calibration]; Capintec) was used to measure tube output. To accurately compare the ionization chamber estimates with the thermoluminescent dosimeter measurements, we measured the tube output using the identical radiographic technique (including repeated radiographs) that was used for each of the six patients.

Comparison of Thermoluminescent Dosimeter Measurements and Ionization Chamber Estimates
CT of the entire cervical spine.—For the ionization chamber, the estimate of skin dose from CT (a single value) was compared with the mean and 95% CI of the thermoluminescent dosimeter measurements of skin dose from CT.

Radiography.—The mean (95% CI) skin dose based on thermoluminescent dosimeter measurements was compared with the mean (95% CI) estimate of skin dose from the ionization chamber. Further evaluation between thermoluminescent dosimeter measurements and ionization chamber estimates of skin dose was performed by calculating the absolute value of the percentage difference between the two methods for each patient and computing the mean and 95% CI around the mean. The following formula was used to calculate the absolute value of the percentage difference between the two methods:

Thyroid Dose Estimate with an Ionization Chamber
CT of the entire cervical spine.—Using a 20-cm-diameter acrylic dosimetry phantom, we measured CT dose index at a depth of 2 cm in the phantom. The dose to the thyroid was estimated from comparing the CT dose index measurement at the surface with the CT dose index measurement 2 cm in the phantom.

Radiography.—From the tube output, the entrance skin exposure was calculated, and the thyroid dose was estimated from the entrance skin exposure using organ dose tables [20].

Comparison of Doses from Helical CT and from Radiography
Three ratios (CT of the cervical spine / radiography) were computed: thermoluminescent dosimeter measurements of skin dose, ionization chamber estimates of skin dose, and ionization chamber estimates of thyroid dose.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Skin Dose
Thermoluminescent dosimeter measurements.—For CT of the cervical spine, the six mean thermoluminescent dosimeter skin dose measurements (21.5, 35.0, 31.3, 19.6, 30.8, 24.9 mGy) had a mean of 27.2 mGy (95% CI, 20.8-33.6 mGy) (Table 1).

For radiography, the mean of the six mean thermoluminescent dosimeter skin dose measurements was 2.75 mGy (95% CI, 2.23-3.27 mGy) (Table 2).

Ionization chamber estimates.—For CT of the cervical spine, the estimated skin dose was 28.0 mGy (Table 1).

For radiographs alone, the mean skin dose was 2.89 mGy (95% CI, 1.97-3.81 mGy) (Table 2).

Comparison of Thermoluminescent Dosimeter Measurements and Ionization Chamber Estimates of Skin Dose
CT of the entire cervical spine.—For CT of the cervical spine, the ionization chamber skin dose estimate (28.0 mGy) was within the 95% CI of the mean thermoluminescent dosimeter measurement (20.8-33.6 mGy) (Table 1).

Radiography.—For radiography, the mean skin dose measurement using the thermoluminescent dosimeter (2.75 mGy) was not statistically different from the mean skin dose estimate using the ionization chamber (2.89 mGy) at the 0.05 confidence level (Table 2). The mean absolute value of the percentage difference between the two methods was 21% with 95% confidence that the value was between 8% and 34% (Table 2).

Thyroid Dose Estimates
CT of the entire cervical spine.—For CT of the cervical spine, the ionization chamber estimated thyroid dose to be 26.0 mGy (Table 1).

Radiography.—For radiography, the mean ionization chamber estimate of thyroid dose was 1.80 mGy (95% CI, 1.05-2.55 mGy) (Table 2).

Comparison of Doses from Helical CT and Radiography
Ratios (CT/radiography) of skin and thyroid dose are given in Table 1. The ionization chamber estimates of thyroid dose indicate that CT of the entire cervical spine delivered more than 14 times as much radiation to the thyroid as radiography.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
For patients who meet the high-risk criteria for cervical spine fracture and are scheduled for contemporaneous CT of the head, the improved fracture detection and speed of CT plus the high cost of failing to rapidly diagnose a fracture overshadow the impact from thyroid irradiation. Although recent work suggests that the portable lateral radiograph is unnecessary when helical CT is scheduled [21], at the time of data collection in this study, the portable lateral radiograph was obtained to rapidly assess the prevertebral soft tissue and vertebral alignment and to determine whether a foreign body was present. The thyroid dose from obtaining a portable lateral radiograph is minimal (<0.10 mGy) compared with that of CT (26.0 mGy).

Combined with the data for patients at high risk for cervical spine fracture, the increased availability of helical acquisition of CT data may prompt the more liberal use of CT of the cervical spine for routine screening, even in patients who do not meet the high-risk criteria. The two experiments reported in this study address issues that are outgrowths of the increased use of CT: methods for determining radiation dose and the actual ratio of the radiation doses between helical CT of the entire cervical spine and radiography.

The correlation between the thermoluminescent dosimeter measurements and the ionization chamber dosimetry measurements argues that ionization chamber data are valid for estimating the thyroid dose in future work. Although placement of the thermoluminescent dosimeter strip is usually a simple task, in the multitrauma setting it can be cumbersome. Thus, the incorporation of ionization chamber measurements could simplify future studies in the trauma setting. For example, CT images obtained at a lower milliampere setting (and thus lower thyroid dose) may be technically adequate for evaluating the cervical spine (Fig. 1A,1B). However, images obtained at a lower milliampere setting have more noise, and for a given patient, there is a critical milliampere setting at which images become technically inadequate to exclude fracture. As future work addresses the trade-off between organ dose and image noise, ionization chamber data rather than thermoluminescent dosimeter measurements could be used to quantify the difference in organ doses between protocols under investigation.

View larger version (136K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —50-year-old woman with nondisplaced fracture of left C6 transverse process. Both axial helical CT scans were obtained at same level with 120 kVp, 3-mm slice thickness, and pitch of 1.5 and filmed in bone window settings (width, 2000 H; length, 500 H). Milliampere setting was 130 mA for A and 240 mA for B. Ionization chamber estimates indicate protocol for A delivered 14.1 mGy to thyroid, whereas protocol for B delivered 26.0 mGy to thyroid.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —50-year-old woman with nondisplaced fracture of left C6 transverse process. Both axial helical CT scans were obtained at same level with 120 kVp, 3-mm slice thickness, and pitch of 1.5 and filmed in bone window settings (width, 2000 H; length, 500 H). Milliampere setting was 130 mA for A and 240 mA for B. Ionization chamber estimates indicate protocol for A delivered 14.1 mGy to thyroid, whereas protocol for B delivered 26.0 mGy to thyroid.

Using the radiographic trauma series protocol at our institution (in-collar lateral, out-of-collar lateral with swimmer's view as needed, anteroposterior, and open-mouth odontoid radiograph) and accounting for repeated radiographs, we found that radiographs delivered less than one-fourteenth the radiation dose to the thyroid than helical CT. Table 3 shows representative thyroid doses as a function of radiographic projection and thus allows dose to be estimated for a patient with any combination of radiographs.

The difference between the two protocols in thyroid dose underscores the importance of accurate clinical decision-making rules and careful attention to the mechanism of injury and clinical parameters in patients with cervical spine trauma. For patients with a relatively low probability of fracture, the cost of thyroid irradiation may be comparable to the overall costs, particularly among the subset of patients who are young and otherwise healthy. These patients have not only the highest theoretic cost from radiation dose, but also the highest likelihood that a fracture can be confidently excluded using a radiographic trauma series.

The main limitation of this study is the fact that determining the theoretic cost of a radiation-induced neoplasm [22] is difficult because the radiation dose is small and the population is heterogeneous. Consequently, a cost-effectiveness analysis that incorporates radiation dosimetry is not currently available. However, thyroid dosimetry is the initial step in the development of more complex cost-effectiveness analyses that account for radiation risks. Along with the difference in imaging time [5, 10], the significant difference in dose to the thyroid could influence the radiologist who must choose between performing CT of the cervical spine and a radiographic trauma series for a patient who does not meet high-risk criteria.

In summary, this project addresses two issues that are the consequence of the increased use of CT in the evaluation of the cervical spine. The results of the first experiment verified our hypothesis that radiation doses estimated with an ionization chamber are similar to those measured with a thermoluminescent dosimeter. Consequently, the use of an ionization chamber for estimating radiation dose may supplant placement of a strip of thermoluminescent dosimeter chips in future studies. The second experiment quantified the increase in the radiation dose to the thyroid when CT of the cervical spine is performed. The 14-fold increase over the dose associated with a radiographic trauma series emphasizes the importance of implementing accurate clinical decision-making rules to determine those patients at high risk for cervical spine fracture. For patients who do not meet the high-risk criteria, the radiation dose to the thyroid should be factored into the judicious use of helical CT in routine screening for suspected cervical spine injury.

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