But as result of the well-documented high-radiation exposure associated with CT, there are serious concerns regarding radiation dose. In most protocols for spiral CT for PE, the effective dose is between 3 and 5 mSv, equivalent to 1 to 2 years of exposure to background radiation.

Women are also more sensitive to the radiation exposure of spiral CT for PE, owing to the presence of breast tissue in the radiated field. This is substantial radiation exposure, especially in children and young adults.

Scintigraphy is safe, technically easy, and widely available. Scintigraphy may be particularly important for reproductive-age female patients whose chest radiograph is likely to be normal and for whom the breast irradiation dose from CT angiography can be minimized by using a perfusion scan as the first imaging test.

Another application could be in the follow-up of patients with proven PE and undergoing anticoagulation.

Methodology
A recent study showed that patients with persistent perfusion abnormalities are at risk of chronic thromboembolic pulmonary hypertension. In patients with allergy to iodinated contrast material or impaired renal function, pulmonary scintigraphy is a useful alternative.

For reproductive-age women, breast radiation exposure is a concern. The average absorbed dose to the breast from CT angiography has been calculated as 10-70 mGy. The absorbed dose to the breast with a perfusion lung scan has been estimated to be 0.28 mGy.

In pregnant patients, besides radiation, the use of iodinated contrast medium is a cause of concern because of the reluctance to expose a fetus to any drugs. Free iodine in radiographic contrast medium given to the mother has the potential to depress fetal and neonatal thyroid function.

Reviewer’s Comments
CT angiography seems to be the Fleischner Society’s preferred method of PE detection, but V/Q scintigraphy is preferred in many circumstances, especially in patients with poor kidney function and perhaps those of baby-bearing or breast-feeding age. Our volume of V/Q scans remains substantial despite a first-rate CT section.

As to the basic assumption that CT beats scintigraphy for PE management, I am not yet willing to concede. For example, I wonder whether some tiny PEs detected by CT would not be better off left untreated.

There is good evidence that our low-probability lung scans identify patients who need not be anticoagulated–even though 5% to 19% of them actually have PE on angiogram.
Take-Home Pearl
Scintigraphy may be particularly important for reproductive-age female patients whose chest radiograph is likely to be normal and for whom the breast irradiation dose from CT angiography can be minimized by using a perfusion scan as the first imaging test.
Author: C. Richard Goldfarb, MD

Reference:
Remy-Jardin M, Pistolesi M, et al. Management of Suspected Acute Pulmonary Embolism in the Era of CT Angiography: A Statement from the Fleischner Society. Radiology; 2007; 245 (November): 315-329

Discussion
4- and 64-slice CT angiography (CTA) deliver almost 100 times the radiation to the breast than ventilation-perfusion (V/Q) scanning, which is of particular concern in young women. It is up to the radiologist to present the clinician with diagnostically equivalent options.

While CTA may help diagnose other abnormalities not diagnosable on V/Q scanning, only some of these may be related to the patient’s symptoms, such as dissecting aneurysms or pneumothorax. PIOPED II study results do not clearly support the superiority of CTA over V/Q scanning.

In PIOPED II, overall sensitivity of CTA for the diagnosis of pulmonary embolism (PE) was 83% and the specificity was 96%, with a positive-predictive value (PPV) and a negative-predictive value (NPV) of 86% and 95%, respectively.

These are comparable to V/Q statistics, where a high probability scan has a >85% likelihood of PE and a low probability result has <20% probability. A "very low probability" category correlates with <10% PPV. Best predictive values are obtained when the diagnostic test is consistent with the pretest clinical probability.

Furthermore, D-dimer assays can also be used to triage subsequent studies. A negative D-dimer in conjunction with a low-probability clinical assessment effectively rules out PE.

At Montefiore, chest x-ray is used as the initial exam for PE. If normal, it is followed by V/Q exam. If abnormal, then multi-detector CTA (MDCTA) is performed.

This approach relies on collaborating with chest radiologists to educate clinicians, including the fact that a negative CTA still has a 17% false-negative rate, very comparable to V/Q scanning. Following an aggressive educational blitz, the number of CTA studies for PE dropped by 27%, while V/Q studies increased by 82%. At Montefiore, CTA is available 24 hours a day.

Dr Freeman is able to call in technologists for V/Q in <1 hour's time. Patients could also theoretically be observed following a single dose of low-molecular-weight heparin, which will temporize for 6 to 8 hours.

Even if MDCTA is the primary method, special cases exist to perform V/Q scanning for contrast allergy, nephrotoxicity, claustrophobia, and obesity. For pregnancy, often a "rule out" scenario, Dr Freeman uses a low-dose perfusion-only study with 37 MBq of Tc-99m MAA.

Baseline studies are obtained several weeks following positive CT to establish a new baseline. This is also helpful in patients with documented DVT so that any subsequent positive lung study can be determined as acute or chronic in origin.

Reviewer’s Comments
It is our obligation to periodically take stock and become involved in counseling our referring physicians regarding the most appropriate studies for particular diagnoses and, in particular, patient populations.

We shouldn’t give up on the V/Q scan, as it achieves similar accuracy to MDCTA with approximately 100-fold less radiation to the breast.

Author: Lionel S. Zuckier, MD

Reference:
Freeman LM. Bury the V/Q Scan: It’s as Good as Multidetector CT Angiograms With a Lot Less Radiation Exposure. J Nucl Med; 2008; 49 (January): 5-8

FDG-PET/CT radiation to breasts is higher than the ACR recommendations; therefore, this modality should be limited to only necessary imaging.

Methodology
This retrospective study included 21 female patients (age range, 25 to 77 years; mean age, 50.8 +/- 16.23 years) with both breasts, who voluntarily underwent combined FDG-PET/CT imaging.
Each patient had been administered 50 mL of oral contrast diluted in 1500 mL of water at midnight before imaging. Thermoluminescence dosimeters (TLDs) were taped onto each breast and were sensitive to 100 mGy or 10 mrad to 1 Gy of radiation and were removed 24 hours after FDG-PET/CT imaging. Any nervous or shivering patient was also given 5 mg of Valium prior to FDG injection.
Each patient was administered approximately 14.0 mCi (522 MBq) of F-18 FDG intravenously with imaging (extending from the top of the head to mid-thighs) 60 minutes later. To determine the average breast radiation dose from low-dose CT scanning, an anthropomorphic dosimetric phantom, with its tissue-equivalent breast, was imaged with the same CT protocol, with TLDs placed on its surface.

Results
The mean superficial breast dose was found to be 14.42 +/- 2.41 mGy. For the phantom study, the average breast radiation dose with the low-dose CT for skin and glandular breast tissue was 9.50 mGy and 5.94 mGy, respectively. For each PET/CT study, the total effective dose was determined by summing the external transmission and internal emission components. These Results showed that this radiation exposure greatly exceeds the American College of Radiology (ACR) recommendation of 3 mGy (0.300 rad) or less for standard two-view mammography.

Reviewer’s Comments
The radiation dose to breasts during combined FDG-PET/CT imaging is higher than the ACR recommended doses; therefore, this type of imaging should be performed only when it is essential and can be justified.

The authors believed this to be particularly true in women of reproductive age.
Author: Twyla Bartel, DO

Reference:
Halac M, Yilmaz MH, et al. Female Breast Surface Radiation Exposure During FDG PET/CT Examinations.
Nucl Med Communications; 2007; 28 (December): 924-928

The greatest dose reduction is achieved by placing the breast shield after obtaining the scout image to avoid Auto mA compensation due to the density of the shield.

Methodology
An anthropomorphic phantom representing a 5-year-old child was used. This phantom was scanned using the technique clinically used for the 5-year-old age-based chest CT protocol.

The phantom was first scanned with a fixed-tube current of 65 mA without a breast shield. The phantom was next scanned using the same parameters but with the addition of a 2-ply bismuth breast shield with a 1-cm-thick foam stand-off pad.

The shield measured 9 cm in the craniocaudal dimension and was designed to cover the chest to the midaxillary line.

The phantom was then scanned using z-axis automatic tube current modulation with 65 mA maximum, 10 mA minimum, and a noise index (measured in SD of Hounsfield units) of 12.0 H (SD) with the topogram (scout) frontal and lateral radiographs obtained with the bismuth breast shield in place.

Auto mA selects tube currents on the basis of patient density, size, and shape, as reflected in the localizer radiograph, to maintain the target noise index between maximum and minimum levels. The noise index approximates noise as measured in the center of a uniform phantom.

The phantom was finally scanned with z-axis automatic tube current modulation using the same parameters, but the scout radiographs were obtained before the breast shield was placed.

Results
The shield reduced the breast dose by 26%, and shielding and automatic current modulation reduced this dose by 52%.

Conclusions
“The greatest dose reduction is achieved by placing the shield after obtaining the scout image to avoid Auto mA compensation due to density of shield. With this technique, image noise increased but remained close to the target noise index.”

Author: : Basil Hubbi, MD

Reference:
Coursey C, Frush DP, et al. Pediatric Chest MDCT Using Tube Current Modulation: Effect on Radiation Dose With Breast. AJR; 2008; 190 (January): W54-W61