Abdominal Computed Tomography During Pregnancy: A Review of Indications and Fetal Radiation Exposure Issues

Indications for Performing Abdominal CT in Pregnant Patients 

The most common indication for CT imaging of the pregnant abdomen is abdominal pain, and in these patients the most frequent condition being evaluated is acute appendicitis. In a study of 80 pregnant patients undergoing CT scan examination for nontraumatic abdominal pain, 13 of 29 patients (44.8%) with abnormal findings had appendicitis. CT established a diagnosis in 30% of cases with an initial negative ultrasound scan proving the accuracy of CT as well as its superiority over sonography for this indication.2

In a pregnant patient with abdominal pain, ultrasound is the preferred initial imaging modality. When inconclusive or technically limited by patient factors, use of CT and magnetic resonance imaging (MRI) becomes necessary. However MRI may not be available, and when available may not be an option for use after hours. The need for prompt and accurate diagnosis of acute appendicitis in pregnant patient necessitates the use of a modality, such as CT that has been shown to be accurate in diagnosing or excluding acute appendicitis. Acute appendicitis is a surgical condition, and prompt imaging is warranted to avoid maternal morbidity and mortality. A survey of the practice patterns in academic centers reported that in pregnant patients with suspected appendicitis and calculus disease in the second and third trimester, CT scan is still the preferred modality. For evaluation of trauma CT scanning is preferred in all 3 trimesters. In the first trimester, for evaluation of suspected appendicitis, most centers prefer to use MRI in those cases where ultrasound is inconclusive.3 In a patient suspected with appendicitis, it has been our experience and that of others that particularly in the second and third trimesters, and in a patient with a large body habitus, ultrasound is of limited value. In a few instances, at our institution we have proceeded directly to a CT evaluation on the basis of clinical urgency for a prompt diagnosis.4

A patient with abdominal pain from a suspected calculus disease is better evaluated with ultrasound. Although the diagnosis of calculus is complicated by presence of pregnancy related hydronephrosis, addition of color Doppler imaging of the bladder to identify ureteral jets and transvaginal ultrasound to detect stones in the distal third of the ureter has helped in the evaluation.5 Unlike in acute appendicitis where surgical intervention is critical to avoid maternal mortality and morbidity, urolithiasis does not carry the same urgency for diagnosis. In a majority of patients stones pass spontaneously. MR urography has been used in these group of patients but is limited by lack of consistent visualization of the ureteral calculus.6

Biological Effects of Ionizing Radiation 

The physical and chemical effects of ionizing radiation results in cell death and deoxyribonucleic acid changes, and this forms the basis of all biological effects of radiation on the developing fetus. Cell death leads to morphologic changes that cause growth retardation, congenital malformations, mental retardation, and in the extreme instance still birth. Deoxyribonucleic acid changes are responsible for genetic mutations and carcinogenesis. Morphologic changes results from the destructive effects of radiation involving many cells and is hence a threshold phenomenon unlike carcinogenesis that may be caused by injury to 1 cell. Radiation effects are hence of 2 types, deterministic and stochastic. Deterministic effects are dose related; there is a baseline threshold dose above which the severity of the radiation induced changes increases with increasing dose. Such changes result from damage to multiple cells. Deterministic effects do not occur at doses below certain threshold levels. Table 1 summarizes the deterministic effects of radiation on the developing fetus. Stochastic effects result from radiation induced damage in single cells, and form the basis for increased risk of carcinogenesis and genetic changes that result from damage to reproductive genes. These changes have no threshold but the likelihood increases as the dose increases.

Table 1. Summary of Suspected In-Utero Induced Deterministic Radiation Effects7

	Menstrual Age	Conception Age	<50 mGy	50-100 mGy	>100 mGy
	0-2 wk	Prior to conception	None	None	None
	3rd-4th wk	1st-2nd wk	None	Probably none	Possible spontaneous abortion
	5th-10th wk	3rd-8th wk	None	Potential effects are scientifically uncertain and probably too subtle to be clinically detectable
	11th-17th wk	9th-15th wk	None	Potential effects are scientifically uncertain and probably too subtle to be clinically detectable
	18th-27th wk	16th-25th wk	None	None	IQ deficits are not detectable at diagnostic dosed
	>27 wk	>25 wk	None	None	None applicable to diagnostic medicine

Among the known risks resulting from radiation exposure the thresholds for deterministic effects, such as congenital malformations, growth retardation, pregnancy loss, and neurobehavioral effects are all greater than 200 mGy.6 The threshold for mental retardation is about 180 mGy during 8-15 weeks of gestation and is higher in the later period of pregnancy.7, 8 The risk of increased incidence of cancer both leukemia and solid cancers in childhood resulting from in-utero exposure to radiation is difficult to assess, and as mentioned previously is not a threshold phenomenon. The data and findings in published reports are confusing and often conflicting. A causal relationship has been made only on the basis of case control studies; cohort studies have not shown any association, including atomic explosion survivors irradiated in utero. Leukemogenic risk seems to be more likely than other cancers but the risk is hard to quantify.9 It has been reported that the lifelong fatal cancer risk for every 10,000 infants born each year is 18%, although all 10,000 received a dose of 10 mSv, there would be 1 excess case of cancer in the population of 10,000.8, 9 The cancer risk from diagnostic radiological procedures is insignificant when compared with the spontaneous risk of cancer.10, 11

Radiation Dose Considerations 

Radiation dose and radiation risk resulting from a radiological examination is expressed as the absorbed dose that is measured in grays (Gy); the effective dose equivalent, measured in sievert (Sv), the CT dose index (CTDI) measured in mGy and dose length product (DLPI) measured in mGy.cm. The CTDI is the most commonly used dose indicator for CT examinations. It is measured and calculated in a polymethylacrylate phantom, and measured in mGy in a single slice. It's limited by not accounting for patient's size. The value applies to scanning at a pitch of 1. Dividing the dose index by pitch provides volume CTDI. DLP provides an estimate of the radiation dose of an entire examination by taking into account the total scan length or the Z factor. The CTDI and DLP provide an approximate estimate of the radiation dose resulting from CT scan examinations and are available on the console. Although not a precise estimate of the radiation dose, an idea of the variation of the dose when the technical parameters are changed provides an idea of the benefit in reduction of radiation dose after changes in the pitch and or tube current.12, 13 The best available parameter to estimate the total radiation risk to the patient is the effective dose that is expressed in Sv. This is obtained by summing up the absorbed doses of individual organs weighted for organ sensitivity. There are various computer programs that can calculate dose for individual organs using the volume CTDI and organ weighting factors from International Commission on Radiological Protection publication 60.14

It's not possible to determine the fetal radiation dose directly; existing methods are on the basis of phantom measurements and or geometrical phantom simulation methods. These assume early term pregnancy in a single-size patient model with an average nonvarying maternal anatomy. A more accurate method of estimating fetal radiation dose resulting from an abdominal and pelvic examination for a range of gestational age and patient size has been reported.15 These investigators used Monte Carlo simulations with a multidetector CT scanner to estimate fetal dose resulting from abdominal pelvic CT examinations for a range of gestational age and patient sizes. Fetal dose correlated with maternal perimeter and varied more than thought previously reported. When both maternal size and fetal depth are combined correlation improves further. Maternal perimeter was chosen due to the ease of estimation using a tape measure around the estimated center of the fetus. The fetal depth also correlated well with the radiation dose, it was measured using ultrasound or from the CT image as the distance from the skin surface to the fetus. These investigators found no correlation between fetal radiation dose and gestational age at which the examination was performed. However normalized fetal dose was reported to decrease linearly with increasing patient perimeter. Among the 24 patients used as study models, the range of fetal doses varied from 7.3 to 14.3 mGy/100 mA. Although the Monte Carlo stimulations used by the authors relate to a single set of acquisition parameters, their results can be used to estimate the radiation dose for differing technical parameters using the equation detailed in their manuscript.15

As an overview, it's useful to know that the annual background radiation dose is about 2.4 mSv/year, fetal dose from CT or fluoroscopy when fetus is not in the x-ray beam is typically less than 5 mSv. Fetal dose from abdominal radiography, lumbosacral spine examination, and limited intravenous urogram is typically less than 10 mSv. CT of the abdomen and pelvis, and interventional procedures involve fetal exposures of >10 mSv. Table 2 provides an overview of typical values for radiation doses resulting from commonly performed radiological procedures.

Table 2. Effective Doses in Computed Tomographic and Radiographic Examinationsa

	Computed Tomographic Examination	Effective Dose (mSv)	Radiographic Examination	Effective Dose (mSv)
	Head	2	Skull	0.07
	Chest	8	Chest	0.02
	Abdomen	10-20	Abdomen	1.0
	Pelvis	10-20	Pelvis	0.7
			Baswallow	1.5
			Baenema	7

a ICRP publication 87: Managing Patient Dose in Computed Tomography.

Radiation Dose Modulation Methods 

The marked increase in use of multidetector CT scanning over a period has raised great concern about the increased risk from exposure to radiation. As a result the manufacturers of CT scanners have introduced methods that help to modulate the radiation dose delivered.12 One such system is the automatic exposure control (AEC), this make use of varying methods to achieve the objective of reducing radiation dose and includes patient-size AEC, z-axis AEC, rotational or angular AEC, or a combination of 2 or more of these types. It is important for radiologists to be familiar with the appropriate use of these tools available to minimize radiation dose when optimizing image quality. The future development of AEC systems and their successful introduction into clinical practice will help to further advance in this goal of radiation dose reduction.12

The need for radiation dose reduction is of particular interest when the developing fetus is directly exposed to ionizing radiation as in an abdominal pelvic CT. Scanning only the area of clinical interest and reducing the number of phases to the absolute minimum often 1 phase only are ways of minimizing the radiation dose. The availability of automatic tube current modulation wherein the tube current is decided on the basis of patient size and density on the scanogram additionally provides the radiologist the option of choosing parameters that can be tailored to the clinical question pertaining to the patient being scanned. A higher noise index resulting from a low dose CT often provides adequate diagnostic in situation where sharpness of lesions is not critical, such as required in the evaluation and or characterization of renal or hepatic masses. For some of the common indications for scanning a pregnant abdomen namely, appendicitis, abdominal abscess, and calculus disease, an image using low dose protocol is feasible particularly because a higher noise index would still provide the clinical information desired in evaluation for appendicitis, abscess, or calculus disease.

Efforts in keeping the dose to as low as possible is ongoing, a group of investigators reported mean effective radiation dose of 2.1 mSv with a 78% mean dose reduction compared with standard dose CT by use of ultra-low-dose parameters to obtain diagnostic information in patients being evaluated for abdominal pain. They were studying the modality in assessment of patients who would have otherwise had a 3 view plain radiography of the abdomen.16

Pregnancy Imaging Policy and Consent 

The need for a radiology departmental policy to guide imaging in pregnancy has been addressed.17 Such a policy aims to create a clear understanding between radiologists, referring clinicians, and pregnant patients undergoing diagnostic radiological examinations. The pregnancy policy addresses the background information needed pertaining to known effects of diagnostic radiation exposure, as well as expected doses from commonly performed procedures. The policy puts in place steps to identify a pregnant status of every woman of child bearing age who presents for radiological investigations. The counseling of a pregnant patient would be tailored depending on the area imaged and anticipated exposure dose. Such a policy would also focus on technical aspects of reducing the radiation dose to the patient.17

The need for obtaining written consent for examination where the absorbed dose is expected to be under 100 mSv has been questioned, given the fact that deterministic effects are nonexistent and the cancer risk in later years is less.12 A survey of academic centers performing imaging studies on pregnant patients found that 68% of centers obtained informed consent before performing a CT scan examination.3 The American College of Radiology practice guideline for imaging pregnant patient women with ionizing radiation recommends obtaining consent from a patient known to be pregnant and considers it an essential component of providing comprehensive medical care.7 Furthermore, the guidelines state that it is advisable to document written or verbal consent in the patient's medical record. A written consent should be part of the patient medical record, and in those instances where only a verbal consent has been obtained it should be documented within the Radiology Information System.7 Given the uncertainty that surrounds the true risk of exposure to radiation and the often conflicting data available in the literature it's prudent to have a consent form that is simple and easy to comprehend. The consent form must be worded in a way that is easy to understand and that conveys to the patient that the risks to the fetus from typical radiation doses that result from abdominal pelvic CT is small, and the benefits resulting from the study far outweighs the minimal risk of cancer resulting from radiation.

Conclusion 

The use of diagnostic radiological procedures in pregnancy should be justified by a careful and thoughtful assessment of the benefit of prompt and early diagnosis that should substantially outweigh the small but finite risk of exposing the developing fetus to radiation particularly when the fetus is in view. The most radiation to the developing fetus from a diagnostic examination results from a CT scan of the abdomen and pelvis. Whenever feasible, initial or exclusive use of modalities that do not use ionizing radiation, such as sonography or MRI is warranted. However for reasons stated above, a CT scan of the pregnant abdomen may be indicated in clinical situations where a prompt and accurate diagnosis is needed and where modalities that do not use ionizing radiation may be unhelpful or unavailable. In these instances to provide clear and clinically pertinent information to the patient and the clinician, it is imperative for the supervising radiologist to have a thorough and adequate understanding of the issues involved in exposing the fetus to ionizing radiation resulting from radiological procedures performed.