BME 4701/6701 MEDICAL IMAGING FALL 2015
LAB 4: EXPERIMENT B
CONTRAST
DUE: 11:59 PM, WEDNESDAY, 11 NOVEMBER 2015
PURPOSE: Contrast, noise and beam energy are important variables in radiographic imaging. Ideally, we want a high contrast image with minimal noise. To achieve high contrast, low beam energy is necessary. However, noise is reduced by increasing the beam energy. Therefore, depending upon the imaging situation (information desired, patient body part, patient size, etc.), a balance between high contrast and low noise may be achieved by adjusting the beam energy. In this experiment, the relationship between contrast, optical density, and beam energy will be examined.
PROCEDURE: The film generated in Lab 4, Experiment A will be used in this experiment. Measure and record the optical density of the film both at Step # 5 of the aluminum step wedge and at a representative point of the Plexiglas block under each of three different exposure conditions:
TABLE 2: Exposure conditions for optical density measurements used in Lab 4 Experiment B.
X-Ray Unit 1
X-Ray Unit 2
1
50 kVp
2.3 mA
14 min
Condition 3 in Table 1
1
50 kVp
3.0 mA
4 min
Condition 3 in Table 1
2
70 kVp
2.3 mA
4 s
Condition 1 in Table 1
2
70 kVp
2.5 mA
3 s
Condition 1 in Table 1
3
90 kVp
0.5 mA
4 s
Condition 4 in Table 1
3
90 kVp
0.5 mA
1 s
Condition 4 in Table 1
REPORT REQUIREMENTS:
NOTE: When reporting results from a film measurement, be sure to clearly indicate all known exposure conditions (film type, focus-to-film distance, tube voltage, current, exposure time and attenuation present).
INCLUDE THE FOLLOWING IN YOUR INTRODUCTION (THEORY) SECTION:
1. Define contrast and latitude in reference to radiographic imaging.
2. Define noise and mottle within radiographic imaging and the effect of increasing beam energy on noise.
3. Describe the difference between low and high contrast X-ray films. What are these films typically used for?
4. Mention the technical specifications of the Nuclear Associates Densitometer used in measuring optical density.
INCLUDE THE FOLLOWING IN YOUR METHODS SECTION:
1. The thickness of aluminum and Plexiglas at which the film optical density was measured.
BME 4701/6701 MEDICAL IMAGING FALL 2015
INCLUDE THE FOLLOWING IN YOUR RESULTS SECTION:
1. Tabulated optical density measurements at Step # 5 of the aluminum wedge and at the Plexiglas under each exposure condition.
2. A plot of the optical densities of each material as a function of beam energy (one plot, two curves, distinguished by different markers or line types.)
3. A plot of the ratio of optical densities between Step # 5 and the Plexiglas right beside it as a function of beam energy (one plot, one curve). This is a measure of contrast.
ADDRESS THE FOLLOWING QUESTIONS IN YOUR DISCUSSION SECTION:
1. Why is contrast enhanced at lower beam energies? What type of photon interaction is predominately involved at lower beam energies? What material property is important in enhancing image contrast?
2. Discuss the effects of fog level and gamma on film contrast. Based on the plots generated in Part A of this lab, would you describe this film as low-contrast or high-contrast? Explain.
3. As the lead engineer for a development team, you have to make a decision on what components to use for a new x-ray imaging system, which is going to be sold to the military for field use. The system is to be transportable and will provide digital x-ray images of injured body parts. Your team has evaluated the following components, and they all fulfill the military ruggedness specifications:
Xray tube with generator
Voltage Range
Max. Anode Current
Cost
Model A
20-80 kVp
25 mA
$15,000
Model B
30-120 kVp
40 mA
$20,000
Digital Image Receptor
Peak Sensitivity
Required Exposure
Cost
Imager 1
60 keV
250 mR
$20,000
Imager 2
80 keV
150 mR
$30,000
Imager 3
100 keV
70 mR
$40,000
Considering all you know about the energy dependence of x-ray attenuation, image contrast and image noise, and also considering the patient welfare and marketability of your final product, identify the x-ray-tube/generator and imager that will be part of the final system and justify your choice. Provide your answer in typed form, double spaced, 150-200 words.
Please turn in your film with your reports. Do not staple the film.