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How does magnification affect image quality and patient dose in digital subtraction angiography? Nikolaos A Gkanatsios*a, Walter Hudab, Keith R Petersc a b c Medical Physics (H-539), Baylor University Medical Center 3500 Gaston Avenue, Dallas, TX 75246 Department of Radiology, SUNY Upstate Medical University, Syracuse, NY 13210 Department of Radiology, University of Florida, PO Box 100374, Gainesville, FL 32610 ABSTRACT Digital subtraction angiography (DSA) images were obtained of a phantom containing 1 mm diameter vessels. The iodine concentrations ranged from 5 to 50 mg/cc, which permitted the determination of the detection threshold concentration of iodine. The source to image receptor distance was 105 cm, and image magnification was varied between 1.15 and 2.0. One experiment was performed at an input exposure of 1 µGy per frame, and a second experiment was performed at 4 µGy per frame. Surface (skin) doses were measured using an ionization chamber, and the corresponding values of energy imparted were determined from the exposure area product. Increasing the radiation exposure by a factor of four reduced the threshold iodine concentration by ~30%. The average detection threshold iodine concentration for a magnification of 1.15 was 13.6 mg/cc. Detection performance improved linearly with magnification, with an average value of 7.4 mg/cc at a magnification of 2.0. Values of energy imparted were essentially independent of image magnification, whereas surface doses increased by a factor of four when image magnification was increased from 1.15 to 2.0. Magnification offers improved imaging performance at no additional patient risk provided that surface doses do not exceed the dose threshold for deterministic effects such as skin burns and epilation. Keywords: magnification, dosimetry, image quality, neuroradiology, digital subtraction angiography. 1. INTRODUCTION Diagnostic and therapeutic interventional neuroradiologic procedures involve imaging of catheter manipulation and vascular anomalies of the brain.1,2 These interventional neuroradiologic procedures often involve long fluoroscopic exposure times and the acquisition of a large number of radiographic images.3 As a result, there is a possibility of induction of deterministic radiation effects such as skin erythema and epilation, as well as producing a relatively high stochastic risk.4,5 Magnification and its effects on image quality have been studied in conventional radiography.6,7,8 In general, magnification improves visibility of small, low contrast objects. As the magnification increases, the effective noise in the image detector is reduced improving the signal-to-noise ratio, and visibility of small structures improves.9 Scatter radiation is also reduced with increased magnification, which improved contrast detectability.10 Geometric magnification can also improve detection of small vessels during interventional neuroradiologic procedures using digital subtraction angiography (DSA). Modification of geometric magnification will impact on image quality and patient doses during interventional neuroradiologic procedures. Quantifying the effect of magnification on image quality and patient doses will help optimize these types of radiologic examinations. In this study, we investigated the relationship between geometric magnification, image quality and the corresponding patient doses. * Correspondence: Email: [email protected]; WWW: nersp.nerdc.ufl.edu/~nikos; Telephone: 214 820 7143; Fax: 720 221 4942. 326 2. METHOD 2.1. Phantom Figure 1 illustrates the phantom used to simulate 1.0 mm diameter vessels for the purpose of evaluating image quality in neuroradiology. The phantom consists of stacked acrylic blocks with dimensions of 30 cm × 30 cm × 1.3 cm. An insert holder made of acrylic with a thickness of 1.3 cm is positioned at the center of the phantom to accommodate a vessel insert. A blank and a vessel insert measuring 30 cm × 9.0 cm × 1.3 cm were made out of acrylic. The blank insert was used to acquire mask images during digital subtraction angiography. The vessel insert had thirty cylindrical vessels 1.0 mm in diameter and 35 mm in length drilled along its midplane at intervals of 8.0 mm apart. The total phantom thickness was 16.5 cm of acrylic, equivalent to 20 cm of water. The vessels on the acrylic insert were filled with iodinated contrast with iodine concentrations ranging from 50 mg/cc to 5.0 mg/cc. The iodine concentration in each vessel was 92% of the previous concentration. 2.2. DSA Acquisitions The experimental setup used an x-ray source to image receptor distance (SID) of 105 cm (maximum SID). The 23 cm diameter image intensifier mode was used for all image acquisitions. DSA acquisitions were performed using the 0.6 mm focal spot size and 70 kVp. A 10×5-60 ionization chamber of a MDH 1015C exposure meter was attached to the surface of the image intensifier behind the grid to record the input exposure to the image receptor. The effect of image magnification was studied by varying the distance of the phantom relative to the image intensifier entrance surface. The position of the phantom was varied to achieve a range of geometric object magnification from 1.15 to 2.0. During all digital subtraction acquisitions, a mask of a single frame was acquired at 3 frames/sec acquisition rate using the blank phantom insert. The blank insert was then replaced by the vessel insert and twenty additional frames were acquired with no frame integration at the same acquisition rate of 3 frames/sec. The image intensifier input exposure was first set to 1 µGy/frame and the experiment was repeated at 4 µGy/frame. The mAs was adjusted accordingly to produce a mean pixel value between 2000 and 2100 as shown in Table 1. TABLE 1. Imaging Techniques During Geometric Object Magnification Experiments Mag 1.15 1.20 1.40 1.60 1.80 2.00 1 µGy/frame Video mAs Level 4.40 2100 4.60 2068 4.80 2008 5.00 2036 5.40 2000 5.80 2072 4 µGy/frame Video mAs Level 16.3 2100 17.0 2018 17.8 2000 18.3 2010 18.8 2000 19.3 2000 2.3. Dosimetry The exposure to the phantom was measured at the x-ray beam entrance plane using a 10×5-6 ionization chamber of an MDH 1015C. The measured exposure included backscatter radiation coming from the acrylic phantom. The entrance exposure per frame was obtained from the integral exposure measured for each image acquisition sequence divided by the number of acquired frames. A conversion factor of 2.58×10-4 C/kg (1R) corresponding to an absorbed dose of 9.3 mGy for muscle tissue was used to convert the entrance exposure to surface dose. The energy imparted was computed using the method described by Gkanatsios and Huda11 for the applied tube voltage and corresponding half-value layer. The exposure was obtained from direct exposure measurements during each image acquisition sequence and included backscatter. The backscatter radiation fraction was measured for the corresponding phantom and applied tube voltages and was subtracted from the measured exposures to obtain the free-in-air exposure. The exposure area at the beam entrance plane of the phantom was computed from geometry assuming that the beam area at the image intensifier plane was a circle with a diameter of 23 cm. 327 2.4. Image Quality Evaluation The threshold iodine contrast concentration was determined from the subtracted images of the vessel insert using a single (blinded) observer. A five-point scale was used to determine the visibility of any vessel containing iodine contrast ranging from 1 (vessel invisible) to 5 (vessel perfectly visible). A rank of 3 corresponded to a vessel between these two extremes, where the observer was 50% confident of seeing the vessel. The observer was trained on the criteria of scoring each iodine concentration using ten subtracted images similar to those asked to evaluate, and requested to be consistent in the application of the scoring scheme. All the images were then presented in a random order to the trained observer who selected the concentration that corresponded to the rank value of 3. The window and level of the displayed images were adjusted to optimize signal detection during each DSA image acquisition, so that the resulting images would not be contrast limited. 3. RESULTS AND DISCUSSION 3.1. Imaging Performance. Figure 2 shows the threshold iodine concentration as a function of geometric object magnification. The precision of the threshold contrast determination was ~12% when the detection threshold was ~8 gm/cc. The difference in threshold concentration between the 1 µGy/frame and 4 µGy/frame input exposure was ~30% across the magnification range shown in Figure 2. The average detection threshold iodine concentration for a magnification of 1.15 was 13.6 mg/cc. Detection performance improved linearly with magnification, with an average value of 7.4 mg/cc at a magnification of 2.0. The relationship between magnification and threshold iodine contrast was approximately linear with the coefficient of determination of the least squares fit (r2) equal with 0.53 for the 1 µGy/frame an 0.92 for the 4 µGy/frame. An increase by a factor of two to geometric magnification decreased threshold concentration by approximately one half. Image quality is very critical in interventional neuroradiologic procedures. The ability to visualize small and low contrast objects is of paramount importance, where neurovascular instruments may be ~200 µm and vessel sizes may be ~100 µm. Any dose reduction strategy must always ensure that image quality is not compromised and patients do not suffer any adverse clinical consequences as a result of inadequate visualization of catheters or vasculature. 3.2. Patient Dose. Figure 3 shows the surface dose and energy imparted as a function of geometric object magnification at 1 µGy/frame and 4 µGy/frame. The measured precision of the surface dose measurements was ~0.7%. Surface dose increased as the square of increase in geometric magnification due to its inverse proportionality to the square of the source-to-surface distance. On the other hand, energy imparted remained almost constant with geometric magnification. The 10% increase in energy imparted across the magnification range shown in Figure 3 was caused by the increased in imaging techniques to maintain a constant video level. The increase in radiographic techniques was caused by the reduced scatter reaching the image intensifier by moving the phantom farther away from the image intensifier to increase magnification. The surface dose predicts the possibility of inducing deterministic injuries from high dose interventional radiologic procedures (i.e., cardiac catheterization, abdominal interventional or neurointerventional procedures). Deterministic injuries associated with interventional neuroradiologic procedures primarily consist of injuries induced to the skin of the patient, including skin erythema and epilation. Below the threshold dose for the induction of deterministic effects, the patient risk is primarily from the stochastic processes of carcinogenesis and the induction of genetic effects.12 The energy imparted to the head may be taken to be proportional to the stochastic radiation risk associated with this type of x-ray examination for any given patient. In neurointerventional radiology, magnification is often used as a tool to visualize small vasculature. Care should be taken, however, when magnification is used, since the surface dose to the patient increases significantly with magnification. Energy imparted, on the other hand, is independent of magnification as distance from the x-ray source and area of exposure decrease equally as magnification is employed. The results of this study clearly demonstrate that magnification offers improved imaging performance at no additional patient risk provided that surface doses do not exceed the dose threshold for deterministic effects. 328 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. F. Vinuela, Interventional Neuroradiology, Raven Press, New York, 1992. A. G. Osborn, Diagnostic Cerebral Angiography, Second Edition, Lippincott, Philadelphia, 1999. N. A. Gkanatsios, W. Huda, K. R. Peters, and J. A. Freeman, “Evaluation of an on-line patient exposure meter in neuroradiology,” Radiology 203, pp. 837-842, 1997. W. Huda, and K. R. Peters, “Radiation-induced temporary epilation after a neuroradiologically guided catheterization procedure,” Radiology 193, pp. 642-644, 1994. L. K. Wagner, P. J. Eifel, and R. A. Geise, “Potential biological effects following high x-ray dose interventional procedures,” J of Vasc Int Rad 5, pp. 71-84, 1994. K. Doi, and K. 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Digital Subtraction Angiography Phantom Interchangeable FIGURE 1. Schematic diagram of the acrylic phantom with the vessel and blank inserts used to simulate small vessels for the purpose of evaluating image quality in neuroradiology. 329 Iodine Concentration (mg/cc) 24 24 18 18 1 µG/frame 12 12 6 6 4 µG/frame 0 0 1.0 1.2 1.4 1.6 1.8 2.0 Geometric Object Magnification FIGURE 2: Threshold iodine concentration as a function of geometric object magnification at 70 kVp. 1.6 1.6 1.2 1.2 0.8 0.8 0.4 0.4 Energy Imparted Surface Dose 0.0 Energy Imparted (mJ/frame) Surface Dose (mGy/frame) 1 µG/frame 0.0 1 1.3 1.6 1.9 2.2 Geometric Object Magnification FIGURE 3: Surface dose and energy imparted as a function of image intensifier input exposure at 70 kVp. 330