Radiation Dose and Diagnostic Accuracy in Pediatric Computed Tomography

Since its inception in the 1970's, computed tomography (CT) has revolutionized the practice of medicine and evolved into an essential tool for diagnosing numerous diseases not only in adults but also in children. The clinical utility of CT examinations has led to a rapid expansion in CT use and a corresponding increase in the radiation burden to patients. CT radiation is of particular concern to children, whose rapidly growing tissues are more susceptible to radiation-induced cancer and who have longer life spans during which cancerous changes might occur. In recent years, the increasing awareness of CT radiation risk to children has brought about growing efforts to reduce CT dose to the pediatric population. The key element of all dose reduction efforts is to reduce radiation dose while maintaining diagnostic accuracy. Substantiating the tradeoff between the two is the motivation behind this dissertation work.

The first part of this dissertation involved the development of an accurate method for estimating patient-specific radiation dose and potential cancer risk from CT examinations. A Monte Carlo program was developed and validated for dose simulation in a state-of-the-art CT system. Combined with realistic computer models of patients created from clinical CT data, the program was applied to estimate patient-specific dose from pediatric chest and abdomen-pelvic CT examinations and to investigate the dose variation across patients due to the variability of patient anatomy and body habitus. The Monte Carlo method was further employed to investigate the effects of patient size and scan parameters on dose and risk for the entire pediatric population.

The second part of this dissertation involved the development of tools needed to study the diagnostic accuracy of small lung nodules on pediatric CT images. A prior method for modeling two-dimensional symmetric liver/lung lesions was extended to create three-dimensional nodules with asymmetric shapes and diffused margins. A method was also developed to estimate quantum noise in the lung region of a CT image based on patient size.

The last part of this dissertation involved assessment of diagnostic accuracy using receiver operating characteristic (ROC) observer experiments. A pilot study of 13 pediatric patients (1-7 years old) was first conducted to evaluate the effect of tube current on diagnostic accuracy, as measured by the area under the ROC curve (Az). A study of 30 pediatric patients (0-15 years old) was then conducted to assess protocol- and scanner-independent relationships between image quality (nodule detectability and noise) and diagnostic accuracy. The relationships between diagnostic accuracy and nodule detectability, between noise and scan parameters, and between dose/risk and scan parameters were lastly combined to yield the relationship between diagnostic accuracy and dose/risk.

For pediatric patients in the same weight/protocol group, organ dose variation across patients was found to be generally small for large organs in the scan coverage (< 10%), larger for small organs in the scan coverage (1-18%), and the largest for organs partially or completely outside the scan coverage (6-77%). Across the entire pediatric population, dose and risk associated with a chest scan protocol decreased exponentially with increasing patient size. The average chest diameter was found to be a stronger predictor of dose and risk than weight and total scan length.

The effects of bowtie filter and beam collimation on dose and risk were small compared to the effects of helical pitch and tube potential. The effects of any scan parameter were patient size-dependent, which could not be reflected by the difference in volume-weighted CT dose index (CTDIvol).

Over a nodule detectability (product of nodule peak contrast and display diameter to noise ratio) range of approximately 52-374 mm with an average of 143 mm, tube current or dose had a weak effect on the diagnostic accuracy of lung nodules. The effect of 75% dose reduction was comparable to inter-observer variability, suggesting a potential for dose reduction.

Diagnostic accuracy increased with increasing nodule detectability over the range of 25-374 mm, but reached a plateau beyond a threshold of ~ 99 mm. The trend was analogous to the relationship between Az and signal-to-noise ratio and suggested that the performance of the radiologists saturates (or increases slowly) beyond a threshold nodule detectability level; further reducing noise or increasing contrast to improve nodule detectability beyond the threshold yields little gain in diagnostic accuracy.

For a typical product of nodule contrast and physical diameter (1400 HU·mm) and a set of most commonly used scan parameters (tube potential of 120 kVp, helical pitch of 1.375, slice thickness of 5 mm, gantry rotation period of 0.4 second, image pixel size of 0.48 mm), diagnostic accuracy increased with effective dose and effective risk for a given patient size, but reached a plateau beyond a threshold dose/risk level. At a given effective dose, Az increased with decreasing patient size, i.e., the dose needed to achieve the same noise and hence diagnostic accuracy increased with patient size. To achieve an Az of 0.90, the dose needed for a 22-cm diameter (male) patient was about quadruple of that for a 10-cm diameter patient. While the effective risk associated with achieving the same diagnostic accuracy also increased with patient size, the risk associated with an Az of 0.90 was only twice as high for a 22-cm diameter (male) patient than for a 10-cm diameter patient due to the older age of the larger patient.

The research in this dissertation has two important clinical implications. First, the quantitative relationships between patient dose/risk and patient size, between patient dose/risk and scan parameters, between diagnostic accuracy and image quality, and between diagnostic accuracy and radiation dose can guide the design of pediatric CT protocols to achieve the desired diagnostic accuracy at the minimum radiation dose. Second, patient-specific dose and risk information, when included in a patient's dosimetry and medical records, can inform healthcare providers of prior radiation exposure and aid in decisions for image utilization, including the situation where multiple examinations are being considered.