Flat-panel detectors cut radiation dose in angiography, pediatric chest x-rays

Nov 3, 2005

Two new studies have evaluated the exposure doses and performance of flat-panel detectors. While different organ systems were imaged, both studies concluded that using flat-panel detectors can reduce radiation exposure to the patient and the operator.

First, researchers in Japan compared the radiation doses to patients' skin and radiologists between a digital flat-panel system and a conventional angiographic unit. They tested this equipment during transcatheter arterial embolization (TAE) for hepatocellular carcinoma (HCC) because it is a common interventional procedure in Japan, explained lead author Dr. Shigeru Suzuki.

Suzuki and colleagues are from the department of radiology at Teikyo University School of Medicine in Tokyo. Co-author Ikuo Kobayashi is from Tokyo-based Nagase-Landauer, maker of the optically stimulated luminescence (OSL) dose badge used in this study.

An Advantx LCA unit for conventional imaging and an Innova 4100 for digital imaging (both from GE Healthcare, Chalfont St. Giles, U.K.) were used for the study. A total of 24 TAE procedures were done between April and July 2004, with a dozen patients undergoing TAE with conventional angiography and the rest with digital flat-panel angiography.

"We estimated the effective photon energy of several main modes of fluoroscopy and digital subtraction angiography in TAE with each device by means of (OSL dosimeters)," the authors wrote. "The effective photon energy was 36-42 keV with Advantx LCA and 32-43 keV with Innova 4100" (American Journal of Roentgenology, October 2005, Vol. 185:4, pp. 855-859).

The dose to patients' skin was measured with thermoluminescent dosimetry (TLD); electronic personal dosimeters placed outside the lead protector was used for the radiologists.

The results showed that the maximal skin dose to the patients was 1,068 mGy with conventional angiography. In comparison, the maximal skin dose to patients was 284 mGy with digital flat-panel detector angiography.

For conventional imaging, the dose equivalent Hp_(0.07) outside the protectors of the radiologists was 68.4 µSv and the Hp₍₁₀₎ was 64.5 µSv. For digital flat-panel, the Hp_(0.07) was 62.8 µSv and the Hp₍₁₀₎ was 57.2 µSv. The relatively small difference between the two units here may have been because of varying screening equipment and the position of the radiologist for the procedure, the authors explained.

They concluded that the "average maximal skin dose to the patients during the procedures with Innova 4100 was about one-fourth of that with Advantx LCA."

A major limitation of this study was the operators' familiarity with each system: the conventional angiography unit had been in place for seven years, while the digital flat-panel detector had been in use for one year.

However, Suzuki's group offered several reasons why the digital flat-panel detector was superior to conventional angiography, such as high detective quantum efficiency, real-time information to optimize digital image chain parameters, and a combination of high-opacity tube and filtration to enhance dose efficiency.

In the second study, German investigators compared exposure doses of a portable indirect flat-panel detector and storage phosphor radiography in a pediatric chest phantom.

Dr. Thomas Bernhardt and colleagues from the University of Muenster and Otto-von-Guericke University in Magdeburg took 50 pediatric chest templates and superimposed them over an anthropomorphic chest phantom. The templates had been divided into 12 areas including lucent lungs, obscured chest regions, catheters, and simulated patterns of interstitial lung disease. In total, 600 areas were evaluated using a portable indirect GadOX-aSi flat-panel detector (CXDI-31, Canon Medical Systems, Irvine, CA).

"Storage phosphor radiography is used as a daily routine digital detector in our intensive care unit. Thus, it was chosen for comparison with the portable flat-panel detector," explained Bernhardt's group (Radiology, November 2005, Vol. 237:2, pp. 485-491).

Images on both systems were obtained at 1 mAs, corresponding to 400 digital speed. The images of the flat-panel detector templates were exposed at 400 digital speed (0.5 mAs, 50.4 µGy surface dose, 1.4 µGy exit dose) and 800 digital speed (0.5 mAs, 25.2 µGy surface dose, 0.7 µGy exit dose). Images obtained on the conventional system were at a digital speed of 400.

Three hundred radiographs were placed in random order and read by four musculoskeletal radiologists on a standard lightbox. The readers were asked to look for a simulated pattern and rank their level of confidence on a scale of 1 (pattern definitely present) to 5 (definitely not present).

According to the results, the following showed higher areas under the ROC curve (A_z) with the flat-panel detector at 400 and 800 digital speed when compared with storage phosphor x-ray:

Catheters over obscured chest areas (A_z = 0.96 at 400, A_z = 0.69 at 800)
Nodules of 10 mm or smaller over lucent lung (A_z = 0.77 and 0.63)
Nodules of 10 mm or smaller over obscured chest areas (A_z = 0.82 and 0.61)
Miliary patterns over lucent lung (A_z = 0.91 and 0.79)
Linear patterns over lucent lung ( A_z = 0.68 and 0.75)

A_z values for reticular and ground-glass patterns with the flat-panel detector were equal to or less than those with storage phosphor x-ray, the authors reported, and did not achieve statistical significance.

However, "the depiction of almost all simulated subtle interstitial lung patterns over lucent lung, as well as catheters over obscured chest regions and nodules over peripheral and obscured chest regions with ... the flat-panel detector was increased," they wrote. This was true even when the radiation dose was reduced, they added.

The researchers concluded that the detection performance of portable flat-panel detector at 800 speed was as good as 400 speed storage phosphor radiography. Further studies under clinical conditions should be pursued, they stated.

At the 2005 RSNA meeting in Chicago, Bernhardt and colleagues will present a related study on how to optimize image processing of chest x-rays, at low tube voltage, using digital selenium radiography in a phantom model (SSA04-06).

To learn more about flat-panel systems and/or radiation dose, check out the following RSNA poster presentations:

"Radiation Dose Reduction in Uterine Fibroid Embolization Procedures with Use of a Digital Flat Panel System" (LPD14-02)
"Dosimetric Evaluation of Physician and Staff Radiation Exposure during ¹²⁵I Vicryl Mesh Implants" (LPH11-02)
"Radiation Doses from Venous Access Procedures" (LPD14-01)
"Intraarterial Hepatic 3D Angiography with a C-Arm Flat Panel Detector (FD) System: Optimizing Contrast Medium Injection Parameters" (LPL14-01)
"Comprehensive Image Quality Analysis of Amorphous Selenium-based Flat Panel X-ray Detectors for Digital Mammography" (LPR09-03)
"Screen Optics Effects on Detective Quantum Efficiency of Cesium Iodide Scintillators for Digital Radiography: Zero Frequency Effects" (LPR09-06)