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Automatic exposure control in multichannel CT with tube current modulation to achieve a constant level of image noise: Experimental assessment on pediatric phantoms
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10.1118/1.2746492
/content/aapm/journal/medphys/34/7/10.1118/1.2746492
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/34/7/10.1118/1.2746492

Figures

Image of FIG. 1.
FIG. 1.

Installation of PMMA phantoms in the CT unit. Each cylindrical phantom (10– diameters) is precisely installed on a styrofoam holder in order to precisely align the phantom center at the isocenter of the gantry.

Image of FIG. 2.
FIG. 2.

Noise measurement in PMMA phantoms. Four circular regions of interest (ROI phantom diameter) are drawn on ten reconstructed images localized in the middle of each PMMA phantom. Measured noise value is the arithmetic mean value of the standard deviations of the CT numbers.

Image of FIG. 3.
FIG. 3.

Modulated tube current profile analysis. Anthropomorphic phantom. SPR parameters: , , angle. Helical parameters: , . rotation time, SFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, standard filter, noise , mA . The tube current values provided by the AEC system (black curve) displayed in the image DICOM field are always located within the range of the measured tube current values (black points) expressed with a 95% confidence level ( 1.96 SD, ).

Image of FIG. 4.
FIG. 4.

Effect of the SPR tube angle and tube potential on the helical tube current modulation. Anthropomorphic phantom. SPR parameters: 80 and , , angle for anteroposterior SPR (“AP SPR”–white curves) and for lateral SPR (“LAT SPR”–black curves). Helical parameters: , rotation time, SFOV, head bow-tie filter, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, Standard filter, noise . The mean helical tube current values proposed by the AEC system are 17% higher when SPR is performed at (lateral) compared to (anteroposterior) at , and 18% higher at . The mean helical tube current values proposed by the AEC system are 15% higher when SPR is performed at compared to at angle, and 17% higher at angle.

Image of FIG. 5.
FIG. 5.

Comparison between anthropomorphic phantom SPR pixel value profiles and -axis modulation. Anthropomorphic phantom. SPR parameters: , , and angle for SPR. Helical parameters: , rotation time, SFOV, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, standard filter, noise . (a) Anteroposterior SPR with lines used for pixel value profile measurements. (b) Mean pixel value profiles along the axis obtained with 9, 17, or 25 lines. (c) Modulated tube current profile along the axis of the whole phantom (from head to pelvis). (d) Linear regression model with the 25-line pixel value profile (tube current logarithm assimilated to a linear function of the pixel value). A close correlation is observed between the transmission profiles of the anthropomorphic phantom and the -axis tube current modulation. With the 25-line profile, this relation can be expressed by the equation (, where is the intensity or tube current, in mA). The 25-line profile is a better predictor of tube current since the statistics decreases when the number of lines used for pixel value profile averaging decreases: for the 17-line profile and for the nine-line profile (not shown in the figure).

Image of FIG. 6.
FIG. 6.

Effect of noise index on helical tube current. PMMA phantom. SPR parameters: , , and angle. Helical parameters: 80, 100, 120, and , rotation time, SFOV, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, standard filter, noise indices range between 6 and 20. The tube current values proposed by the AEC system can be fitted with power equations: , where is the tube current intensity, and are adjusted values for each tube potential and phantom diameter combination, and NI is the noise index selected. All power values are close to .

Image of FIG. 7.
FIG. 7.

Tube current modulation response to tube potential variations. PMMA phantoms. SPR parameters: , , and angle. Helical parameters: 80, 100, 120, and , rotation-time, SFOV, DFOV, x-ray beam thickness, image thickness, 1.5 pitch, standard filter, and noise . For each noise index and phantom diameter combination, the tube current proposed by the AEC system can be expressed by a power equation: , where is the tube current intensity, is a constant, is the helical tube potential, and is a power factor. As the observed values are in agreement with the known relationship between the tube potential and the resulting absorbed dose, the AEC system accurately adjusts the tube current to the tube potential selected to keep constant the absorbed dose and image noise.

Image of FIG. 8.
FIG. 8.

Tube current modulation response to x-ray beam thickness variations. PMMA phantoms. SPR parameters: , , and angle. Helical parameters: , rotation time, SFOV, DFOV, 5– x-ray beam thickness, 2.5 or image thickness, 1.5 pitch, standard filter, and noise ). Each curve represents one x-ray beam thickness/image thickness combination (e.g., ). With constant image thickness and noise index, the tube current decreases when the x-ray beam thickness increases.

Image of FIG. 9.
FIG. 9.

Tube current modulation response to image thickness. PMMA phantoms. SPR parameters: , , and angle. Helical parameters: , rotation time, SFOV, DFOV, image thickness obtained with the or x-ray beam thickness, 1.5 pitch, standard filter, noise . Each curve represents one x-ray beam thickness/image thickness combination (e.g., ). (Data for the combination and phantom not shown because tube current reached the upper tube limit.) A twofold increase of the image thickness with a constant noise index causes the AEC system to decrease tube current values by about 35%–40%.

Image of FIG. 10.
FIG. 10.

Tube current modulation response to pitch factor. PMMA phantoms. SPR parameters: , , and angle. Helical parameters: , rotation time, SFOV, DFOV, 10 x-ray beam thickness, image thickness, 0.75 and 1.5 pitch, standard filter, noise .

Image of FIG. 11.
FIG. 11.

Tube current modulation according to tube potential with the small SFOV. Anthropomorphic phantom. SPR parameters: , , and angle. Helical parameters: 80–, rotation time, SFOV, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, standard filter, noise . The tube current provided by the AEC system decreases progressively from 80 to 100 and . Surprisingly, tube current values at (white curve) are almost the same as the values at , although they should be located between the 100 and curves.

Tables

Generic image for table
TABLE I.

Variations of tube current and absorbed dose according to the PMMA phantom diameter at a fixed noise index. Acquisition parameters: SPR settings: , , and angle; helical settings: . tube rotation time, , SFOV, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, standard filter, and AEC with .

Generic image for table
TABLE II.

Variations of effective , normalized absorbed dose and image noise according to PMMA phantom diameter and tube potential at two different noise indices. Acquisition parameters: SPR settings: , , and angle; helical settings: rotation time, SFOV, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, Standard filter, and AEC.

Generic image for table
TABLE III.

Tube current, absorbed dose and image noise variations according to AEC response to x-ray beam thickness changes.

Generic image for table
TABLE IV.

Tube current modulation response to acquisition field of view (SFOV) changes. Acquisition parameters: rotation time, SFOV, DFOV, 10 x-ray beam thickness, image thickness, 1.5 pitch, standard filter.

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/content/aapm/journal/medphys/34/7/10.1118/1.2746492
2007-06-26
2014-04-20
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Automatic exposure control in multichannel CT with tube current modulation to achieve a constant level of image noise: Experimental assessment on pediatric phantoms
http://aip.metastore.ingenta.com/content/aapm/journal/medphys/34/7/10.1118/1.2746492
10.1118/1.2746492
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