Correspondence
Diagnosis of Pulmonary Embolism with Magnetic Resonance Angiography
N Engl J Med 1997; 337:859-860September 18, 1997
- Article
To the Editor:
Meaney et al. (May 15 issue)1 describe encouraging results with the use of magnetic resonance angiography to confirm the diagnosis of pulmonary embolism. However, their inference that magnetic resonance angiography may provide a substitute for scintigraphy for noninvasive screening ignores the cost-effective scintigraphy-based algorithms in widespread use. Although the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study has popularized the concern about a higher-than-optimal prevalence of scans indicating an intermediate probability of pulmonary embolism,2 over half the patients still had scans indicating a low, very low, or high probability, which would typically indicate a therapeutic strategy. Combining the clinical probability before testing with the objective result of lung scanning significantly improved the accuracy of the post-test diagnosis, 2 a strategy commonly and appropriately applied in other diagnostic settings. A very good overall correlation was observed between the PIOPED readers' quantitative estimates of likelihood and the observed prevalence of pulmonary embolism.3 The use of revised PIOPED criteria further improves the accuracy and discriminative power of the lung scan.4,5
Finally, the authors' reference to the often-touted disadvantage of ionizing radiation with scintigraphy is misleading. The total-effective-dose equivalent for a xenon-133 ventilation scan with 20 mCi (148 MBq) followed by a technetium-99m macroaggregated-albumin perfusion scan with 4 mCi (148 MBq) is 0.2 rem (2 mSv),6 which is approximately equivalent to the amount of normal background radiation one would be exposed to if one lived in Denver for eight months. Lung scanning is noninvasive, inexpensive, readily available in the acute care setting, and sufficiently sensitive to merit its ongoing use in screening for pulmonary embolism.
6 ReferencesStephen K. Gerard, M.D., Ph.D.
Veterans Affairs Medical Center, San Francisco, CA 941211
Meaney JFM ,Weg JG ,Chenevert TL ,Stafford-Johnson D ,Hamilton BH ,Prince M . Diagnosis of pulmonary embolism with magnetic resonance angiography. N Engl J Med 1997;336:1422-1427
Full Text | Web of Science | Medline2
The PIOPED Investigators
CrossRef | Web of Science3
Worsley DF ,Alavi A . Comprehensive analysis of the results of the PIOPED study. J Nucl Med 1995;36:2380-2387
Web of Science | Medline4
Freitas JE ,Sarosi MG ,Nagle CC ,Yeomans ME ,Freitas AE ,Juni JE . Modified PIOPED criteria used in clinical practice. J Nucl Med 1995;36:1573-1578
Web of Science | Medline5
Sostman HD ,Coleman RE ,Delong DM ,Newman GE ,Paine S . Evaluation of revised criteria for ventilation-perfusion scintigraphy in patients with suspected pulmonary embolism. Radiology 1994;193:103-107
Web of Science | Medline6
Stabin MG, Stubbs JB, Toohey RE. Radiation dose estimates for radiopharmaceuticals. Oak Ridge, Tenn.: Oak Ridge Institute of Science and Education, April 1996:36, 74.
To the Editor:
The preliminary data reported by Meaney et al. demonstrate the potential efficacy of magnetic resonance angiography in diagnosing pulmonary embolism. The authors suggest that magnetic resonance angiography is a cost-effective imaging technique for the diagnosis of pulmonary embolism. Unfortunately, they fail to provide explicit data on the cost as well as on the effectiveness of this strategy, leaving their statement on cost effectiveness completely unsubstantiated. At present, the cost-effectiveness ratio for the use of magnetic resonance imaging in diagnosing pulmonary embolism is simply unknown and should be determined by studies using appropriate methods of cost evaluation once the efficacy has been confirmed.1 The results will have to be compared with the cost-effectiveness ratio for standard clinical practice.
More than 10 years after Doubilet et al.2 pointed out the widespread misuse of the term “cost effective” in the medical literature, this term continues to be used incorrectly.
2 ReferencesHans-Rudolf Frey, M.D.
University Hospital Basel, CH-4031 Basel, Switzerland1
Udvarhelyi IS ,Colditz GA ,Rai A ,Epstein AM . Cost-effectiveness and cost-benefit analyses in the medical literature. Ann Intern Med 1992;116:238-244
Web of Science | Medline2
Doubilet P ,Weinstein MC ,McNeil BJ . Use and misuse of the term “cost effective“ in medicine. N Engl J Med 1986;314:253-256
Full Text | Web of Science | Medline
Author/Editor Response
The authors reply:
To the Editor: Calculating the cost effectiveness of new diagnostic procedures is difficult. This is especially true for magnetic resonance procedures, because the costs of magnetic resonance scanning have been plummeting over the past several years. In many states, the charge for magnetic resonance scans is as low as $300. Presumably, the costs are lower than the charges. The fast pace of advances in magnetic resonance technology is likely to help maintain the trend toward lower cost and increased effectiveness. We agree that more research in this area is warranted.
We also agree that the radiation dose for a ventilation–perfusion scan is well within the current guidelines for an acceptable level of exposure during a medical procedure. Suggesting that exposure to 0.2 rem of radiation with a ventilation–perfusion scan is similar to exposure to 0.2 rem from living for eight months at high altitude is misleading because of the vast difference in exposure rates. An eight-month exposure provides an opportunity for DNA repair between ionizing events. This argument also ignores radiation exposure among health care workers and the general population in the vicinity where the radiation is released. In any case, physicians should strive to prevent unnecessary exposure to radiation whenever possible.
As we indicated, only 27 percent of the 931 patients in the PIOPED study had interpretations permitting definitive clinical decision making: 13 percent had high-probability scans, and 14 percent had nearly normal or normal scans. An additional 9 percent had low-probability scans and a low clinical probability. In the group of patients with low-probability scans and discordant clinical findings, the incidence of pulmonary embolism was 16 percent for those with an intermediate clinical probability and 40 percent for those with a high clinical probability; these are unacceptable error rates.1 In patients with chronic obstructive pulmonary disease, only 10 percent had high-probability scans or nearly normal or normal scans, and 9 percent had low-probability scans with a low clinical probability. No patient with a forced expiratory volume in one second of less than 65 percent had a nearly normal or normal scan.2
2 ReferencesJames F.M. Meaney, M.D.
Leeds General Infirmary, Leeds LS2 9NS, United KingdomJohn G. Weg, M.D.
Martin R. Prince, M.D., Ph.D.
University of Michigan Hospitals, Ann Arbor, MI 481091
The PIOPED Investigators
CrossRef | Web of Science2
Lesser BA ,Leeper KV Jr ,Stein PD , et al. The diagnosis of acute pulmonary embolism in patients with chronic obstructive pulmonary disease. Chest 1992;102:17-22
CrossRef | Web of Science | Medline
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