November 2007, VOLUME 189
NUMBER 5

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November 2007, Volume 189, Number 5

Abdominal Imaging

Original Research

Radiation Doses from Small-Bowel Follow-Through and Abdominopelvic MDCT in Crohn's Disease

+ Affiliation:
1All authors: Department of Radiology, Duke University Medical Center, Erwin Rd., Box 3808, Durham, NC 27710.

Citation: American Journal of Roentgenology. 2007;189: 1015-1022. 10.2214/AJR.07.2427

ABSTRACT
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OBJECTIVE. The purpose of our study was to compare organ and effective doses for small-bowel follow-through (SBFT) and abdominopelvic MDCT in adults with Crohn's disease, to retrospectively evaluate the number of radiographic examinations performed for Crohn's disease indications, and to identify those patients undergoing serial examinations to better delineate the use of radiology in the diagnosis and clinical management of Crohn's disease.

MATERIALS AND METHODS. Using an anthropomorphic phantom and metal-oxide semiconductor field-effect transistor (MOSFET) dosimeters, specific organ doses were measured for 5 minutes of continuous fluoroscopy (kVp, 120; mA, 0.6) of each of the following: right lower quadrant, central abdomen, and pelvis. Effective doses were determined based on International Commission on Radiological Protection (ICRP) 60 weighting factors. Organ and effective doses were determined for abdominal and pelvic 16-MDCT: detector configuration, 16 × 0.625 mm; pitch, 1.75; 17.5 mm per rotation; rotation time, 0.5 second; 140 kVp; 340 mA. Electronic records were reviewed to determine the number of patients imaged for Crohn's disease indications and the number of studies per patient.

RESULTS. The highest fluoroscopic organ doses were as follows: in the right lower quadrant, right kidney (0.78 cGy) and marrow (0.66 cGy); in the central abdomen, kidneys (1.5 and 1.6 cGy) and marrow (0.76 cGy); and in the pelvis, marrow (0.67–0.95 cGy). Effective doses for the right lower quadrant, central abdomen, and pelvis were 1.37, 2.02, and 3.83 mSv, respectively. For MDCT, the highest organ doses were to the liver (2.95–3.33 cGy). The effective dose for abdominopelvic MDCT was 16.1 mSv. Three hundred seventy-three patients underwent imaging for Crohn's disease. The average number of SBFT and CT examinations was 1.8 and 2.3, respectively. Thirty-four (9%) of 373 patients underwent more than five CT examinations and 11 (3%) had more than 10.

CONCLUSION. Organ and effective doses are up to five times higher with MDCT than with SBFT. Crohn's disease is more frequently imaged with CT. For a subset of patients who undergo numerous CT examinations, efforts should be made to minimize the number of CT examinations, decrease the CT dose, or consider MR enterography.

Keywords: Crohn's disease, MDCT, radiation dose, small-bowel follow-through

Introduction
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Crohn's disease is an inflammatory bowel disease characterized by asymmetric mucosal and transmural inflammation of the gastrointestinal tract. The disease can affect any part of the gastrointestinal tract, with small-bowel involvement seen in 80% of patients [1]. Disease incidence and prevalence in industrial countries are estimated at five per 100,000 and 50 per 100,000, respectively [2]. Review of epidemiologic studies has shown that the incidence of the disease appears to be increasing [3]. The bimodal distribution of age at diagnosis classically refers to a peak in incidence in the second and third decades (15–25 years old), followed by a second smaller peak in the sixth or seventh decade [1, 4]. Early age at diagnosis has been associated with a family history of Crohn's disease and a more complicated disease course with greater small-bowel involvement and higher frequency of surgery [5, 6].

Crohn's disease has a relapsing, remitting course with periods of disease activity and remission within the patient's lifetime. Natural history data indicate only a small minority (10%) of patients experience prolonged remission, and up to 57% of patients require at least one surgical resection [3, 7]. Unfortunately, classification of disease activity solely on the basis of clinical and laboratory parameters has not been dependable or reproducible [8].

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Fig. 1A Anthropomorphic female phantom (Model 702-D, CIRS) used in study. Photograph shows phantom with metal-oxide semiconductor field-effect transistor (MOSFET) dosimeters in place.

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Fig. 1B Anthropomorphic female phantom (Model 702-D, CIRS) used in study. Cross-section photograph of phantom shows dosimeter locations.

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Fig. 1C Anthropomorphic female phantom (Model 702-D, CIRS) used in study. Photograph shows phantom on fluoroscopy table in preparation for fluoroscopic portion of examination.

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Fig. 1D Anthropomorphic female phantom (Model 702-D, CIRS) used in study. Fluoroscopic image of phantom shows MOSFET dosimeters.

The classic presentation of Crohn's disease is that of abdominal pain, weight loss, and diarrhea. More than half of patients with Crohn's disease have evidence of disease in the terminal ileum and one fourth of patients have disease confined to the distal ileum [4, 9]. This region is less accessible at endoscopy, especially in the case of stricturing disease, and the proximal ileum and jejunum are beyond the reach of endoscopy. For this reason, gastroenterologists and surgeons are reliant on radiographic evaluation of these regions.

Current radiographic techniques for diagnosing Crohn's disease include sonography, barium examinations, MDCT, and MRI. Sonography and MRI are less frequently used within the United States than are barium small-bowel follow-through (SBFT) and CT. Both SBFT and CT have advantages and limitations. SBFT can depict subtle intestinal mucosal disease, luminal narrowing, and fistulization between bowel loops; however, like endoscopy, it only partially evaluates extramucosal and extraluminal disease. Some small-bowel loops, specifically those inaccessible in the deep pelvis, are not well evaluated with SBFT. MDCT allows evaluation of the mural involvement of Crohn's disease (stratification, hyperenhancement, and wall thickening) and the extramucosal complications of the disease (abscess, fistula, bowel obstruction, and so forth). MDCT with 3D techniques and multiplanar reconstruction has the additional advantage of visualizing the gastrointestinal tract in multiple planes. However, because of the limited spatial resolution, CT is limited in its ability to identify early mucosal disease [1].

Both of these imaging studies involve ionizing radiation, and the long-term cumulative effect of this radiation is not known. Because Crohn's disease is typically diagnosed in young adults and the disease has a chronic relapsing nature, patients may undergo multiple imaging studies in their lifetime. To our knowledge, there has been no study comparing radiation doses from SBFT and MDCT, nor has there been an analysis of the application of these radiologic procedures in the clinical management of Crohn's disease.

Thus, the aim of our study was to determine the effective and organ doses for SBFT and 16-MDCT, to retrospectively evaluate the number of radiographic examinations performed for Crohn's disease indications, and to identify those patients undergoing serial examinations to better delineate the use of radiology in the diagnosis and clinical management of Crohn's disease.

Materials and Methods
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This Health Insurance Portability and Account-ability Act (HIPAA)-compliant study was approved by the institutional review board of our medical center. There was a waiver of informed consent for this study.

A retrospective review of the fluoroscopy records of 30 consecutive patients who underwent single-contrast SBFT from June 2005 to August 2005 was performed to determine average total fluoroscopy time. Fluoroscopy was performed by residents with 1–4 years of experience under the supervision of an attending radiologist.

Anthropomorphic Phantom

A commercially available female anthropomorphic phantom (Model 702-D, CIRS) (Fig. 1A) was used for effective and organ dose determination for both SBFT and MDCT. Phantom specifications were height, 160 cm; weight, 55 kg; and thorax dimension, 20 × 25 cm. This phantom was chosen because it most closely simulates the body habitus of a young adult with Crohn's disease [10]. The phantom is made of human tissue–equivalent material and has the same X-ray interaction as living humans [11].

Metal-Oxide Semiconductor Field-Effect Transistor Dosimeters

High-sensitivity diagnostic metal-oxide semiconductor field-effect transistors (MOSFET) (Model 1002RD, Thomson-Nielson) were used to determine effective and absorbed organ doses. The dosimeter measurements are as follows: width, 2.5 mm; thickness, 1.3 mm; and length, 8 mm. Twenty individual dosimeters were placed in defined anatomic locations in the chest, abdomen, and pelvis as designated by the phantom manufacturer (Fig. 1B). MOSFET dosimeters were calibrated with a conventional X-ray tube by simulating a fluoroscopic beam of half-value layer (HVL) ∼ 4.3 mm Al at 120 kVp and a CT beam of HVL ∼ 7 mm at 120 kVp. During calibration, the dosimeters were placed side-by-side with an ion chamber (Model 10X5-6, Radcal Corporation) and a radiation monitor control (Model 1015, Radcal Corporation) and were exposed simultaneously.

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Fig. 2 CT scout image of phantom shows metaloxide semiconductor field-effect transistor (MOSFET) dosimeters in place.

Conversion from exposure (Roentgen or coulomb) to absorbed (rad or gray) dose was computed by multiplying by the appropriate correction factors: f-factor of 0.95 at 20 kVp for both fluoroscopy and MDCT or chamber correction factor of 1.001. The Radcal Model 1015 radiation monitor automatically corrects for temperature and pressure. The MOSFET dosimeters were in the same position to calculate effective dose for both the fluoroscopic and CT portions of the study (Table 1). The dosimeters were then moved to slightly different anatomic positions, and organ doses were calculated for both fluoroscopy and CT (Table 2). For the purposes of this study, organ doses were recorded as a reflection of the measurements from the dosimeters within that organ. The liver contained three dosimeter locations.

TABLE 1: Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) Dosimeter Locations for Recording Effective Dose

TABLE 2: Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) Dosimeter Locations for Recording Specific Organ Dose

Fluoroscopy Study

The phantom was scanned with the patient in the supine position on the fluoroscopy table. Collimation and tower distance were determined by the radiologists to mimic clinical practice. To obtain the effective dose and specific organ doses, directed continuous fluoroscopy (120 kVp, 0.6 mA) was performed for 5 minutes over each of the following locations: right lower quadrant, central abdomen, and pelvis (Figs. 1C and 1D). Our tube voltage and current were similar to those noted in the literature [1214]. We directed our fluoroscopy over three locations in the abdomen to expose the regions characteristically included in a small-bowel examination for Crohn's disease: the right lower quadrant is the normal location of the terminal ileum, the central abdomen includes the majority of the jejunum and proximal ileum, and the pelvis is the typical location for mid ileal loops. We chose to measure doses during the fluoroscopic portion of the SBFT because studies have shown that fluoroscopy is the main contributor to total dose [15, 16]. The scanning was repeated twice (total, three scans) to determine the average dose for each region and the SD. Effective doses for these three fluoroscopic regions were calculated based on International Commission on Radiological Protection (ICRP) 60 weighting factors [17]. An effective dose after 5 minutes of fluoroscopy was calculated for each of the three regions separately.

CT Study

The phantom was scanned on a LightSpeed 16-MDCT scanner (GE Healthcare) using the routine clinical protocol for scanning the abdomen and pel vis: 140 kVp; 340 mA; 16 × 0.625 mm detector configuration; pitch, 1.75; table speed, 17.5 mm per rotation; gantry speed, 0.5 second per rotation (Fig. 2). The aforementioned protocol, which results in isotropic data sets, has been optimized for both image quality and radiation dose [18]. This protocol was in use at our institution before the introduction of the automated tube current modulation mode. Images were reconstructed at 5-mm-thick slices at 5-mm intervals. The scanning was repeated three times to obtain the average MDCT effective dose and specific organ doses with SDs. Volume CT dose index (CTDIvol) was also recorded.

Clinical Data Collection

Current Procedural Terminology (CPT) codes (American Medical Association) and electronic medical records (E-Browser 5.3, Pegasus Imaging) were reviewed to determine the number of adult patients (age > 17 years) imaged at this institution for Crohn's disease indications from July 1996 to April 2006. Age, sex, referring clinician, number of years of care at this institution, and occurrence of SBFTs and CTs were recorded.

Once effective dose for a single SBFT and abdominopelvic MDCT were determined, the total effective dose for multiple examinations was estimated by multiplying the effective dose for a single examination by the total number of examinations performed on a particular patient. The largest effective dose for SBFT was used to calculate the cumulative effective dose for multiple SBFTs.

Results
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Fluoroscopy and MDCT Phantom Studies

Twenty-two women and eight men underwent SBFT from June 2005 to August 2005. The average age of the patients was 52 years (age range, 23–82 years). The average total fluoroscopic time was 4.9 minutes.

Phantom effective doses for the three fluoroscopic positions (right lower quadrant, central abdomen, and pelvis) were 1.37 ∓ 0.11 (SD), 2.02 ∓ 0.23, and 3.83 ∓ 0.14 mSv, respectively. The effective dose calculated for MDCT was 16.1 ∓ 0.81 mSv.

Phantom organ doses for the fluoroscopic and MDCT examinations are included in Tables 3 and 4. The highest organ doses for the three fluoroscopic locations were as follows: right lower quadrant, right kidney (0.78 cGy) and thoracic spine bone marrow (0.66 cGy); central abdomen, kidneys (1.45 and 1.57 cGy) and lumbar spine bone marrow (0.79 cGy); and pelvis, right and left pelvic bone marrow (0.78 and 0.95 cGy, respectively). Highest organ doses with MDCT were found in the liver (2.95–3.33 cGy) and pancreas (3.14 cGy). CTDIvol was 15.35 mGy.

TABLE 3: Average Fluoroscopic Organ Dose

TABLE 4: Average CT Organ Dose

Clinical Data

From July 1996 to April 2006, 373 patients underwent imaging for Crohn's disease indications (221 women, 152 men). The average age was 43 years (age range, 18–88 years). Most of the patients were under the clinical supervision of a gastroenterologist or colorectal surgeon (345 and 18, respectively). Patients were followed at this institution for an average of 6.4 years (range, 1–13 years). The average number of SBFT examinations was 1.8 (range, 1–15), and the average number of CT examinations was 2.3 (range, 0–34) (Fig. 3). Thirty-four (9%) of 373 patients underwent more than five CT examinations and 11 (3%) of 373 patients had more than 10 CT examinations. Of the patients receiving more than 10 CT examinations, 45% (5/11) were followed primarily by the emergency department physicians.

Cumulative effective doses for SBFT and CT delivered to most patients were low because most patients underwent only one to three examinations. Multiple SBFT examinations contributed little to accumulating effective dose; however, those patients who underwent three or more CT examinations were exposed to effective doses ranging from 39.9 to 133 mSv (Fig. 4).

Discussion
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Radiographic evaluation of the small bowel in Crohn's disease has changed significantly in the past 20 years. Before the mid 1980s, fluoroscopy was the imaging choice for diagnosis of small-bowel inflammatory bowel disease. SBFT is well tolerated, nearly universally available, and relatively simple to perform [4]. Classic teaching suggested that SBFT with “careful fluoroscopy” of the small bowel with “vigorous manual compression” was a sensitive method of detecting Crohn's disease activity [19]. A prospective comparison of SBFT and small-bowel enteroclysis found SBFT the preferred test for imaging Crohn's disease, with an increased accuracy of SBFT for mucosal detail and fistulous disease, lower radiation dose, and patient preference [9].

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Fig. 3 Bar graph shows distribution of imaging of 373 patients with Crohn's disease referred for small-bowel follow-through (SBFT) or CT in 10-year period. One patient underwent 34 CT examinations. Gray bars = SBFT, black bars = CT.

Since the introduction of CT scanners in the 1970s, rapid technologic advances have altered the application of this technique in clinical use. Even early studies of the impact of CT in the imaging of Crohn's disease noted that in at least 25% of cases, CT identified unexpected findings that changed clinical management of the disease [20]. With advances in technology, we have seen a flood of research in MDCT imaging of Crohn's disease [1, 4, 8, 2031].

To our knowledge, to date, no one has compared effective and organ radiation doses for SBFT and abdominal MDCT. Common sense would suggest that CT exposes patients to much higher doses than does SBFT. The advent of MOSFET dosimeters markedly improved the ease of acquisition of radiation dose measurements because doses can be acquired in real time and the dosimeters have shown comparable results to single-use thermoluminescence dosimeters (TLD) [3235]. MOSFET dosimeters have a linear response with dose and are extremely small [32]. These dosimeters can easily be placed within the anthropomorphic phantom using the manufacturer's guide to organ location. This allows for both effective dose calculation and direct organ dose measurements. As noted previously, for the purposes of this study, organ dose measurements reflect the dose registered by the dosimeter within that organ.

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Fig. 4 Bar graph shows estimated effective dose from both small-bowel follow-through (SBFT) and CT examinations in patients with Crohn's disease. Cumulative effective dose for 10 CT examinations reaches 140 mSv. Gray bars = SBFT, black bars = CT.

Until now, fluoroscopic effective dose has been estimated by applying conversion coefficients to measurements of surface dose or dose–area product [36, 37]. Our total fluoroscopic time was within the range previously noted in the U.S. literature [38, 39]; however, we chose to perform 5 minutes of fluoroscopy over a directed quadrant of the abdomen or pelvis to maximize potential exposure. Our calculated effective doses approximate or fall below other reported effective doses for SBFT [4042]. This suggests that a clinically modified examination for Crohn's disease indications would, in fact, have lower effective doses than those calculated here.

Effective dose values for CT have been typically estimated by using Monte Carlo methods to simulate CT of a mathematic patient model or by multiplying dose–length product or measured CT dose index (CTDI) by conversion factors for general anatomic regions [43]. We have previously shown that MOSFET dosimeters and phantom dose data with ICRP 60 weighting factors are statistically similar to that predicted by the CT scanner manufacturer's dose simulator program [18]. There is a wide range of effective doses for abdominal and pelvic MDCT reported in the literature, with numbers ranging from 6 to 28 mSv, depending on the type of scanner and scanning protocol [18, 4446]. Our effective dose of 16.1 mSv is well within this range.

We chose to use a 16 × 0.625 mm detector configuration for routine scanning of the abdomen and pelvis after studying the optimal scanning parameters for 16-MDCT scanners. The 16 × 0.625 mm detector configuration results in isotropic data sets without a compromise in image quality or a significant increase in dose [18]. It has been previously shown that CTDI measurements used in many MDCT scanners underestimate dose by up to 30% compared with TLD and MOSFET technology [44, 47, 48]. Our method of calculating effective dose is a more accurate representation than the dose–length product method with conversion factors.

Our institution has historically used higher CT tube voltage coupled with a decrease in tube current to maintain radiation dose. The scanning parameters given here (140 kVp, 340 mA) were those adopted for our clinical practice at the time of the study and are similar to those in the literature [46, 49]. It is true that when leaving all other imaging parameters constant, an elevation of tube voltage (140 vs 120 kVp) increases radiation dose [50]; however, MDCT allows us to increase pitch and table speed, thus offsetting some of the increase in dose [18]. The application of the automated tube current modulation mode with a set noise index (z-axis modulation) further decreases radiation dose, with a range of 15–68% reported in the literature [51]. In a study by Kalra et al. [49], scanning of the abdomen and pelvis using z-axis modulation (with a tube current range set from 10 to 380 mA) and a noise index of either 10 or 12.5 H resulted in decreases in mean tube current–time product of 31.9%. Given these findings, we have adopted the automated tube current modulation mode in our clinical abdominal and pelvic protocols to decrease total radiation dose.

For both the fluoroscopic and CT components of this study, we calculated organ doses based on the average of MOSFET dosimeter readings. The highest organ doses were seen in the kidneys and bone marrow for all three fluoroscopic regions. Although the bone marrow doses were comparable to those previously reported in the literature, our doses for the right and left kidneys were higher than the dose reported by Calzado et al. [13] (0.3 cGy) but similar to that reported by Ruiz-Cruces et al. [15] (0.8 cGy). Our higher fluoroscopic renal doses are likely due to directed prolonged fluoroscopy over each of the three quadrants (5 minutes each) instead of target-specific fluoroscopy of the abdomen. For our abdominal and pelvic 16-MDCT protocol, the highest organ doses were found in the central abdomen (liver and pancreas), with doses reaching 3 cGy, up to five times higher than those seen in SBFT. Brenner and Elliston [52] reported estimated organ doses that are lower than these reported here; however, their technique used lower kV and mAs values, and their doses were approximated and not directly measured. Again, our direct measurements should more accurately reflect the direct dose to specific organs.

Given that MDCT results in both effective and organ doses up to five times greater than SBFT, radiologists and referring clinicians should take caution in routinely transferring all Crohn's imaging to CT. It is critical to recognize the clinical question before undertaking imaging in these patients to accurately diagnose the disease and its complications without exposing the patients to too much risk.

Our review of the clinical experience with Crohn's imaging at our institution has revealed several interesting trends. Most of the adult patients referred for radiologic imaging of Crohn's manifestations underwent fewer than three SBFT and CT examinations, suggesting that much of this disease is clinically managed without radiologic correlation. There is, however, a small subset of this patient population who underwent more than five CT examinations and an even smaller, but important, subset with more than 10 CT examinations. In the latter group, almost half of the patients received the majority of their care from emergency department physicians and were not followed by either a gastroenterologist or a colorectal surgeon. This would suggest a disproportionate use of CT in Crohn's patients seen in the emergency department for abdominal pain. Our findings are similar to those of Katz et al. [53], who, in the management of renal colic, found a subset of patients who were estimated to receive between 20 and 154 mSv from the repetitive use of CT. Our effective dose totals are similar to those reported by Katz et al.

At this point, the biologic impact of this type of radiation exposure is not yet known. Brenner and Elliston [52] determined the estimated lifetime attributable cancer mortality risk at around 0.08% for full-body MDCT, and Dixon and Dendy [54] noted that an effective dose of 10 mSv corresponds to an excess risk of fatal cancer of 1 in 2,000. Data from survivors of the atomic bombings of Nagasaki and Hiroshima have shown that survivors exposed to radiation doses ranging from 5 to 200 mSv were found to have a statistically significant increased risk for developing fatal cancers [55]. In addition, data suggest that cumulative exposure of lower-dose radiation may have a similar effect as a single acute dose [52, 56]. The patient population that undergoes repeated MDCT is at a greater risk for developing a fatal malignancy.

Katz et al. [53] suggested alternative means of management of renal colic in those patients with chronic nephrolithiasis, including combined unenhanced abdominal radiography and sonography as first-line screening and MR urography as indicated. For patients with Crohn's disease who required repeated radiologic imaging, alternatives to CT should include low-dose MDCT and MR enterography. Although the MR experience in imaging Crohn's disease is well established in the literature [5761], to our knowledge, there are no studies to date evaluating low-dose MDCT of bowel disease. Kalra et al. [46] showed that a 50% reduction in tube current at a constant voltage yields acceptable images and a significant decrease in radiation dose, with limitations seen in only larger patients. Because patients with active Crohn's diseases are typically smaller than average, the artifacts seen with low-dose CT should be less evident. Further study of low-dose MDCT in this patient population appears warranted.

Sonography has been widely used in imaging of Crohn's disease outside of the United States, and, although operator-dependent, sonography has been shown reliable in identifying the disease and its complications [6265]. In facilities familiar with this imaging technique, sonography serves as another imaging alternative to MDCT.

There were several limitations to this project. One, our decision to evaluate effective and organ doses for specific fluoroscopic regions overestimates normal fluoroscopic practice in SBFT. Indeed, most patients would experience less directed fluoroscopy for a typical Crohn's SBFT, which in turn would yield smaller effective and organ doses. It is also more likely that in clinical practice, pulsed fluoroscopy would yield smaller doses than those described here. In addition, when only a portion of the organ was in the radiographic field, the dose recorded reflects the measurement from the dosimeter exposed in that organ. For instance, if only a portion of liver was in fact in the field of fluoroscopy, the dose recorded here reflects the MOSFET dosimeter dose, not the whole organ dose. This is, therefore, likely an overestimation of organ dose when the organ is only partially within the field of radiation.

Two, we did not establish the dose incurred by overhead or spot radiographs because they likely contributed little to the total doses for SBFT. We designed our protocol to optimize for MOSFET dosimeter function, and we were willing to overestimate fluoroscopic doses derived from this portion of the project. Three, we have potentially underestimated radiation exposure to this patient population by only taking into account patients in the database who have been referred for imaging of Crohn's indications on the basis of CPT coding. Our institution is in a university setting and college-age patients are transient members of our patient population, so total numbers of CT and SBFT examinations may be higher than stated here. In addition, CT examinations performed outside the 10-year time frame or performed at other institutions or for other clinical indications were not included in this tally.

In conclusion, we found directly measured organ and effective doses to be up to five times higher for MDCT than SBFT. Crohn's disease is more frequently imaged with CT than SBFT. Most patients have limited ionizing radiation exposure for the management of Crohn's disease. However, a small subset of the population experiences a significant radiation exposure, which increases the lifetime risk of developing a fatal malignancy. In these patients, all care should be made to limit radiation dose either by modifying MDCT protocols or performing MR enterography.

Address correspondence to T. A. Jaffe.

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