Pulmonary > V-Q exam

Evaluation of Pregnant Patients with Suspected Pulmonary Embolism

Background:

See also discussion of CT PA imaging in pregnancy

Pulmonary embolism is the most common cause of death in pre-partum patients and second only to hemorrhage as the most common cause of death (mortality up to 15%[13]) in all pregnant women in the United States [4]. Pregnancy increases the risk of venous thromboembolism by a factor of five over that of a non-pregnant woman of similar age [8,14]. The D-dimer assay has a limited role in pregnancy as levels rise above as pregnancy progresses producing false-positive results [142]. A normal D-dimer value still maintains a high negative predictive value even during pregnancy, however, PE can still occur in patients with negative D-dimer levels [14]. Treatment with anticoagulation therapy is also associated with maternal and fetal morbidity [13]. There is a general consensus that the risks related to failure to treat PE or to the unnecessary use of anticoagulation therapy outweigh the small risk to a fetus due to radiation exposure from appropriate diagnostic procedures [1,4]. Up to 73-92% of V/Q scans in pregnant patients can demonstrate normal findings [14]. In one study, V/Q scanning was found to have a higher diagnostic accuracy rate compared to CTPE imaging [15]. In another study, lung scintigraphy and CT angiography were found to have comparable performances for the diagnosis of PE during pregnancy [17]. Additionally- VQ imaging is associated with a significantly lower maternal radiation dose [17]. Click here for a more detailed discussion of the use of CT angiography for the evaluation of pulmonary embolism.

Pregnancy itself has certain inherent risks that patients should be aware of when discussing the potential effects of radiation to include: an approximately 50% chance of failed pregnancy from all conceptions in women who are not exposed to radiation, a 3-15% risk of spontaneous abortion, a 4% risk of prematurity and growth retardation, a 3% risk for major malformation, and 1% risk of mental retardation [13,19].

Prenatal radiation:

The effects of radiation can be grouped into three categories: teratogenesis (fetal malformation/birth defects), carcinogenesis (induced malignancy), and mutagenesis (alteration of genes) [3]. A patient's dose from a radiologic exam is measured in the rem (which accounts for the effect on tissue of various types of radiation), or in the older and more commonly recognized unit, the rad (radiation absorbed dose) [3]. A rem and a rad are equivalent for exposure to x-rays. The estimated background radiation exposure to the fetus is 1.1-2.5 mGy [14].

During the 9 months of a full-term pregnancy the estimated radiation dose to the mother from natural background radiation is 2.3 mSv (avergage fetal dose is 0.5 to 1 mSv because of attenuation by maternal tissue) [19]. The accepted cumulative dose of ionizing radiation during pregnancy for a non-occupationally exposed individual is 5 rad (5000 mrad or 500 mGy) [5,6]. The fetal dose for radiation workers should be less than 1 mGy during the total gestational period [19]. This dose (less than 1 mGy) is the same as that for members of the public who are incidentally exposed to radiation [19].

It is the position of the National Council on Radiation Protection and Measurements that fetal risk is considered negligible below this dose level (5 rad/50 mGy), when compared to the other risks of pregnancy [6,18]. A fetal dose of less than 100 mGy should also NOT be considered a reason to terminate pregnancy [19] (The International Commission on Radiological Protection stated that fetal doses below 100 mGy should not be considered a reason for terminating a pregnancy [12]). However, if the fetal dose reaches levels greater than 150 mGy, there is a stronger likelihood of malformations [19]. Some authors suggest that radiation exposure greater than 500 mGy (some say over 200 mGy) during neuronal development (between weeks 7 and 25 of gestation) may be associated with significantly increased risk for severe mental retardation [19]. However, other authors suggest that pregnant women exposed to less than 500 mGy display no increase in adverse pregnancy outcomes compared to observed background rates of spontaneous abortion, fetal malformation, and intrauterine growth retardation [16]. No single diagnostic radiology exam exceeds this maximum limit [3]. Therefore, fetal exposure from diagnostic radiology examinations is not an indication for pregnancy termination [3].

Teratogenic effects are referred to as deterministic or nonstochastic) effects because a threshold dose must be crossed before the effect occurs [19]. High levels of prenatal radiation exposure have been associated some specific risks for the developing fetus. A radiation dose of 5-15 rad (5-15 cGy) during the preimplantation stage (from conception to 10 days after conception) can result in prenatal death [6]. An in utero dose of 10 rad in the first 2 weeks can result in spontaneous abortion (note: the natural rate of spontaneous abortion is 25-50%) [6]. Radiation doses of 10 rads delivered to the fetus during the stage of organogenesis (through the 6th week after conception) may result in a 1% increase in the natural rate of occurrence of developmental abnormalities [6]. Radiation delivered during the 6th week to birth tends to result in diminished growth and development [6]. Radiation during this later time period has also be associated with an increased risk for mental retardation. The most sensitive time period for central nervous system teratogenesis is between 8 and 17 weeks of gestation during which time there is rapid nueoronal development and migration [3,10,19]. The risk of mental retardation is dose related and is only about 4% for a 10 rad dose (100 mGy), while it may be as high as 43% for a 100 rad dose. There may be a threshold for retardation between 20 and 40 rad, but this has not yet been proven. Other adverse effects associated with radiation exposure include small head, intellectual deficits, and induction of childhood malignancy [10].

For radiation protection purposes, a linear no-threshold model has been used to determine the relative risk for the development of childhood cancer following early in-utero radiation exposure [11,19]. The excess relative risk of developing childhood cancer has been estimated to be approximately 0.28 at 1.0 mGy in the first trimester, 0.03 at 1.0 mGy in the third trimester, and overall 0.037 at 1 mGy during pregnancy [11]. Other authors indicate that a fetal radiation dose of 100 mGy is estimated to increase the risk for childhood cancer by 0.1% [16]. Prenatal radiation exposure can produce a slight increase in the incidence of childhood leukemia [3]. Exposure to 1 to 2 rad has been associated with a slight increase in the rate of leukemia in children from the background rate of 3.6 per 10,000, to 5 per 10,000 (approximately 1 in 10,000 increased risk) [3]. With a fetal dose of 50 mGy there is an estimated twofold increase in the relative risk for fatal childhood cancer (note that the baseline risk for dying from childhood cancer is very low- 1.0-2.5 patients per 1000 [19].

There is also possibly a very small change in the frequency of genetic mutations [3]. The dosage required to double the baseline mutation rate is between 50 and 100 rads. From these numbers, it can be extrapolated that if 10,000 persons were exposed to 1 rad, 10 to 40 new genetic mutations would be induced [3].

Radiationfromventilation-perfusion scintigraphy:

In order to minimize radiation exposure, compression lower extremity US is recommended as the initial imaging study for supected cases of PE in pregnancy [13]. If negative, then additional evaluation may be required using examinations that expose the patient and fetus to ionizing radiaiton..

For radiation exposure from specific diagnostic radiology examinations (See Table 1, Table 2, Table 3, Table 4, and Table 5).

The radiation exposure to the fetus early in pregnancy from Tc99m-MAA has been calculated to be 10 mrad to the baby for each millicurie (mCi) of activity administered to the mother [5]. Therefore, for a 3 mCi dose, the baby would receive 30 mrad of radiation [5]. For Xe133, early in pregnancy, the dose to the baby from this radiopharmaceutical has been calculated to be 0.8 mrad to the baby for each mCi administered to the mother [5]. A typical administration for this exam is 30 mCi so the baby might receive an estimated dose of 24 mrad (30 mCi ?0.8 mrad/mCi) [5]. For the combined V/Q exam the fetus would receive a total radiation dose of only 54 mrad. Using a reduced dose of Tc-MAA (2 mCi) and Xe-133 (10 mCi) the fetal dose would be between 320-360 uGy (32-36 mrad) [11]. Other authors quote the fetal dose from a 3 mCi dose of Tc99m-MAA as 18 mrad, and the dose from a Xe133 ventilation exam as 19 mrad (total 37 mrad) [4]. Fetal exposure from the ventilation agent Tc99m-DTPA aerosol is also very low (see table 1) and well within safety limits. In a final publication, the fetal dose from the Tc99m-MAA exam was estimated to be 175 mrad (dose of agent administered was not recorded), and the Xe133 dose was estimated to be 40 mrad (total 215 mrad) [3]. These numbers seem high compared to other quoted exposure values.

Based upon the available literature, the radiation dose to a fetus from a combined V/Q exam would most likely be between 37 to 54 mrad (using a slightly reduced dose of 3 mCi for the perfusion exam and Xe133 as the ventilation agent), to a maximum possible dose of 215 mrad. The possible radiation dose falls well within the acceptable range for cumulative fetal radiation dose (5 rads) [3,6,9]. The dose is also well below that associated with any risk for teratogenesis, carcinogenesis, or mutagenesis. To put this into further perspective, the average exposure rate to individuals in the United States from natural background radiation is about 300 mrem per year [7] and the dose to the conceptus from natural occurring background radiation is approximately 0.5-1.0 mSv for the entire period of gestation [12].

Below are listed various conclusions from key organizations regarding the use of diagnostic radiology studies during pregnancy.

Patient Counseling:

Appropriate counseling of pregnant patients before any radiology study is critical [3]. The most important question for a pregnant patient about to undergo a radiology test is to know if the exam is safe for the baby. It is vitally important that this question be answered and that the counseling physician choose words that will help a patient understand the real, although almost negligible, risks of exposure. The general population's total risk of spontaneous abortion, major malformations, mental retardation and childhood malignancy is approximately 286 per 1,000 deliveries [3]. Exposing a fetus to a dose as large as 0.50 rad (500 mrad) adds only about 0.17 cases per 1,000 deliveries to this baseline rate, or about one additional case in 6,000 [3]. However, care must be exercised if such numbers are quoted to patients as they will only hear the words "risk," "abortion," "mental retardation" and "malignancy [3]." The bottomline is that exposure to less than 5 rads has not been shown to be associated with a difference in pregnancy outcome when compared with a control population who received only background radiation [10].

Doctors therefore face a real challenge in ensuring good communication during patient counseling [3]. It is important that the counseling physician let the patient know the exam is safe and that pregnancy is fraught with potential complications- the risk for which will not be increased by the V/Q exam. (Statistics show that among the general population some spontaneous malformation is present in 4 to 6 percent of all deliveries [3]). Most importantly, the patient should understand the benefits of appropriately diagnosing a condition such as pulmonary embolism that could harm both themselves and their baby. Nonetheless, it is important never to promise parents a perfect baby as this will likely lead to misunderstanding and anger if the baby is born with any anomaly [3].

VQ exam in pregnancy:

CT PE imaging suffers frmo a high rate of non-diagnostic exams in pregnant patients (up to 17-36%) [18]. In patients with normal CXR's, a definitive VQ result can be obtained in 94-96% of cases [18]. VQ scan is recommended by the American Thoracic Society/Society of Thoracic Radiology as the first imaging test to evaluate for PE in pregnant patients with a normal CXR [18].

Breast feeding:

Breast feeding does not need to be discontinued following administration of iodinated or gadolinium based contrast [19]. For Tc99m, breast feeding should be interrupted for 4 hours [19]. During this period, the mother should continue to pump and store milk which can be used after the radioactivity dissipates [19]. For nonurgent exams, the mother may pump and store the milk so that she may continue to feed the infant [19]. Complete cessation of breast feeding is advised after administration of Ga67 and I131 because of the agents long half-lives and more than 10% of the administered dose may be excreted in the breast milk [19].

Conclusion:

Based upon the available data, there are no apparent short or long term consequences to the fetus from the radiation received as a result of diagnostic ventilation-perfusion scintigraphy. For a V/Q scan, fetal dose would mostly come from tracer accumulating in the bladder, with some internal scatter from the lungs [2]. To minimize radiation exposure to the fetus, a smaller dose of the perfusion tracer Tc99m-MAA (2.5-3 mCi) will be used for the exam (as long as the patient can hold still for the longer imaging times) [2]. Hydration and frequent voiding will be encouraged if the patient's clinical status permits [1]. Either Xenon133 or Tc99m-DTPA aerosol can be used safely for the ventilation portion of the exam, but Xenon133 is preferred as Tc99m-DTPA is excreted via the urine [18].

REFERENCES:

(1) Radiology 1998; Boiselle PM, et al. Pulmonary embolism in pregnant patients: Survey of ventilation-perfusion imaging policies and practices. 207: 201-206

(2) Instrumentation and radiopharmaceuticals. In Practical Nuclear Medicine. Ed. Palmer EL, Scott JA, Strauss HW. 1992, W.B.Saunders, Philadelphia. 27-69 (pp. 64-65)

(3) Am Fam Physician 1999; Toppenberg KS, et al. Safety of radiographic imaging during pregnancy.  59(7):1813-8, 1820

(4) Postgrad Med 2000; Sellman JS, Holman RL. Thromboembolism during pregnancy: risks, challenges, and recommendations. 108(4):71-84

(6) J Nucl Med Technol 2001; Thompson MA. Maintaining a proper perspective of risk associated with radiation exposure. 29: 137-142

(7) J Nucl Med Technol 2001; Zeng W. Communicating radiation exposure: A simple approach. 29: 156-158

(8) Radiology 2002; Winer-Muram HT, et al. Pulmonary embolism in pregnant patients: fetal radiation dose with helical CT. 224: 487-492

(9) AJR 2003; Schuster ME, et al. Pulmonary embolism in pregnant patients: a survey of practices and polices for CT pulmonary angiography. 181: 1495-1498

(10) AJR 2004; Ratnapalan S, et al. Physicians' perceptions of teratogenic risk associated with radiography and C during early pregnancy. 182: 1107-1109

(11) AJR 2006; Hurwitz LM, et al. Radiation dose to the fetus from body MDCT during early gestation. 186: 871-876

(12) Radiographics 2007; McCollough CH, et al. Radiation exposure and pregnancy: when should we be concerned? 27: 909-918

(13) Radiographics 2007; Patel SJ, et al. Imaging the pregnant patient for nonobstetric conditions: algorithms and radiation dose considerations. 27: 1705-1722

(14) Radiographics 2009; Pahade J, et al. Imaging pregnant patients with suspected pulmonary embolism: what the radiologist needs to know. 29: 639-654

(15) AJR 2009; Ridge CA, et al. Pulmonary embolism in pregnancy: comparison of pulmonary CT angiography and lung scintigraphy. 193: 1223-1227

(16) AJR 2011; Golfberg-Stein S, et al. Body CT during pregnancy: utilization trends, examination indications, and fetal doses. 196: 146-151

(17) Radiology 2011; Revel MP, et al. Pulmonary embolism during pregnancy: diagnosis with lung scintigraphy or CT angiography? 258: 590-598

(18) Radiology 2012; Leung AN, et al. American thoracic society documents: an official American Thoracic Society/Society of Thoracic Radiology clinical practice guideline- evaluaiton of suspected pulmonary embolism in pregnancy. 262: 635-646

(19) Radiographics 2015; Tirada N, et al. Imaging pregnant and lactating patients. 35: 1751-1765

ELECTRONIC REFERENCES:

(5) Health Physics Society http:www.hps.org. An international professional scientific organization dedicated to promoting the practice of radiation safety. Ask the expert Question #411 submitted September 11, 2000. (http://www.hps.org/publicinformation/ate/q411.html)