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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Med Phys. Author manuscript; available in PMC Jun 25, 2009.
Published in final edited form as:
Med Phys. Feb 2008; 35(2): 764–768.
PMCID: PMC2701968
NIHMSID: NIHMS84437

Measurement of Temporal Resolution in Dual Source CT Using a Visually Intuitive Technique

Abstract

This work assesses the temporal resolution of dual-source CT in a visually intuitive manner. Exploiting the principles of ring artifact creation, a phantom containing a highly attenuating delta function signal (a wire) was rotated at the same speed as the gantry, creating a partial ring artifact where the angular extent of the artifact provides a direct indication of the temporal resolution. A demonstration of the effect of the evaluated 165 and 83 ms nominal temporal resolutions on motion artifacts in cardiac CT is provided using patient data.

Introduction

This work describes the assessment of temporal resolution and motion artifact of a dual-source CT system (Somatom Definition, Siemens Medical Solutions, Forchheim, Germany) 13. This system has two x-ray sources and two detector arrays mounted orthogonally on the gantry. The primary tube (Tube A) and detector array images a 50-cm field of view (FOV), while the secondary system (Tube B) images a 26-cm FOV. Each tube/detector array pair has an 80-kW generator, a 32 × 0.6 mm detector array and uses double z sampling to acquire 64 overlapping projections along the z axis at every tube position 4.

When the system is operated using only a single x-ray tube and detector array and a gantry rotation time of 330 ms, the nominal temporal resolution near isocenter for partial scan reconstructions is 165 ms 5. This is because, at isocenter, 180° of fan beam data are sufficient to provide 180° of rebinned parallel data. If redundant data from the fan beam are ignored, the temporal resolution at isocenter can be as good as one half of the gantry rotation time. When both tube/detector systems are used, the nominal temporal resolution for partial scan reconstructions (180 deg) is 83 ms because each tube provides one half of the required data, or one quarter of the gantry rotation time 1, 3.

In this work, we describe a visually intuitive manner by which to measure temporal resolution. The method exploits the principles responsible for ring artifacts in 3rd generation CT geometry (where the x-ray tube and detector assembly rotate in synchrony) 5. This principle is demonstrated in Fig. 1. In Fig. 1a, the sinogram corresponding to a simulated uniform cylinder containing a highly attenuating ”delta function” is shown. When the simulated phantom is static, the sinusoidal curve in the sinogram represents the projection of the attenuating point onto different detector elements as the tube is rotated about the object. In Fig. 1b, we have simulated the condition when the same phantom is rotated at the same angular velocity of the x-ray tube. In this case, the projected attenuation from the small bright object is mapped to a vertical line in the sinogram. The vertical line in the simulated sinogram is similar to that which occurs when a specific ray sum is affected in the same way at every projection angle, such as would occur in the case of one bad detector element. After image reconstruction, the result is a circular or ring artifact. If, for some reason, less than 360° of data are used for the image reconstruction process, the ring would be an incomplete circle, covering only the angular range included in the image reconstruction (Fig. 2). In the case of DSCT, the use of two sources, each acquiring half of the required projections, creates two arcs, each one reflecting the temporal resolution associated with the image. The purpose of this work was to assess the temporal resolution of dual-source CT (DSCT) using the described visually intuitive method and to demonstrate the reduction in motion artifact between 83 and 165 ms images using patient data.

Figure 1Figure 1Figure 1Figure 1
Simulation of sinograms (a, c) and corresponding reconstructed images (b,d) of a highly-attenuating “delta function” object in a uniformly dark cylindrical background for a stationary phantom and full-scan (360°) reconstruction ...
Figure 2Figure 2Figure 2Figure 2
Simulations of sinograms (a, c) and corresponding reconstructed images (b, d) of the same phantom shown in Figure 1. (a, b) The phantom was rotated at the same speed and direction as the gantry. The image was reconstructed from half-scan (180° ...

Materials and Methods

Measurement of temporal resolution using a rotating phantom

The phantom used to attenuate the ray sums corresponding to a small number of detectors was comprised of a narrow gauge needle inserted at a radius of 4 cm into a 15-cm diameter acrylic cylinder. The needle essentially creates a highly-attenuating “delta function” signal in the image FOV. The acrylic cylinder was mounted to a computer-controlled drive and imaged at rest and while spinning at 3 rotations/s (Fig. 3). Verification of the angular velocity of the rotating cylinder was performed by video recording the spinning phantom using the fast frame rate mode (60 fps) and playing back the data in slow motion to count the number of rotations per second of the needle (Handycam DCR-TRV460, Sony Electronics, Inc, San Diego, CA; iMovie 7.0, Apple Computers, Cupertino, CA). Because both the phantom and the gantry rotated with the same speed, a partial “ring” artifact was created. Thus, the angular extent of the ring artifact provided a direct visual indication of temporal resolution near isocenter.

Figure 3Figure 3Figure 3
(a) Photograph of the motion apparatus within the gantry of the CT system and a close-up of the phantom (b). The 22 gauge hypodermic needle inserted into the 15 cm acrylic cylinder is indicated by the narrow black arrow. The large arrow indicates the ...

Five images of the rotating cylinder were acquired using each of the following modes: 1) axial single-source, 2) axial dual-source, and 3) helical dual-source. Partial (cardiac) reconstructions were performed on both axial data sets to produce 83 (dual-source) and 165 ms (single-source) images. Partial (cardiac) reconstructions were also performed on the dual-source helical data set to produce 83 and 165 ms images, using approximately 90 and 180° of projection data for each x-ray tube, respectively. This resulted in a total of 20 images.

The angular extent of each partial ring artifact was measured by using a circle to fit the radius of curvature of each arc. The diameter and position of a circle were manually varied to achieve the best fit to the partial ring artifact, where the best fit was determined by visual inspection. The CT numbers along the path of the circle were sampled using 1440 points to generate a one-dimensional profile of CT number vs. angular position. The data were background corrected by subtracting the mean CT number from a background region distant from the wire (120 HU) from each data point. The resultant profile was smoothed using a 101-point window width, and the full-width-at-half-maximum (FWHM) of the smoothed profile used to estimate the corresponding temporal resolution. The method used to quantify temporal resolution, as represented by the partial ring artifact, is demonstrated in Fig. 5. The mean and standard deviation were calculated for the 5 images from each of the 4 scan modes (axial and helical 83 ms and axial and helical 165 ms).

Figure 5Figure 5
Demonstration of the method used to measure the angular extent of the partial ring artifact. a) The image shown in Figure 4a with a circle manually fit to the radius of curvature of the outermost arc. b) Plot of the background-corrected CT number versus ...

Demonstration of the effect of temporal resolution on motion artifact for cardiac CT

Data from a patient having a mean heart rate of 90 bpm were reconstructed using either one or both x-ray tube(s). First, the data from both tubes were used for the commercially implemented dual source reconstruction (nominal 83 ms). Second, with assistance from the manufacturer, the reconstruction algorithm was modified to disregard data from tube B, and a single-source reconstruction was performed using only data from tube A (nominal 165 ms). This approach was required in order to reconstruct 165 ms images from the identical helical dataset (i.e. without rescanning the patient) and to ensure equivalent image noise between the 83 and 165 ms images.

Results and Discussion

The angular velocity of the rotating phantom was measured using video photography to be 2.994 rotations/sec, which is in excellent agreement with the programmed speed (3 rotations/sec).

In Fig. 4a and 4c, the angular extent of the arcs appear to be approximately 90°, which is consistent with a temporal resolution of 83 ms for a gantry rotation time of 330 ms (90°/360° × 330 ms= 83 ms). In Fig 4b and 4d, the angular extent of the arcs appear to be approximately 180°, which is consistent with a temporal resolution of 165 ms for a gantry rotation time of 330 ms (180°/360° × 330 ms= 165 ms). Figs. 4a, 4c, and 4d each were reconstructed using data from both tubes, hence the presence of two arcs, each of which independently represents the temporal resolution of the image.

Figure 4Figure 4Figure 4Figure 4
(a) Partial scan reconstruction performed using data from both Tube A and Tube B (i.e. a cardiac dual-source reconstruction) for the axial scan acquisition. The mean angular extent of the partial ring artifacts (n=5) was 87.6°, which corresponds ...

The FWHM (mean±standard deviation) of the angular ranges associated with the images in Fig. 4a-d were 87.6°±0.9°, 177.2°±2.8°, 87.4°±1.1° and 182.2°±3.3°, respectively. These correspond to temporal resolutions of 80.3±0.8, 162.4±2.5, 80.1±1.0, and 167±3.0 ms, respectively.

Fig. 6 demonstrates the differences in motion artifact between 83 and 165 ms nominal temporal resolutions for a patient having a mean heart rate of 90 bpm. The decrease in motion artifact with the dual-source reconstruction (83 ms temporal resolution) is evident.

Figure 6Figure 6
Patient data for a patient with a heart rate of 90 bpm were reconstructed using (a) tubes A and B or (b) tube A only. The 3D rendering and oblique reformatted images from patient with a mean heart rate of 90 bpm demonstrate the improved temporal resolution ...

Conclusions

We have demonstrated the utility of a rotating motion phantom for a visually intuitive assessment of temporal resolution in CT. For the dual-source CT system, the partial “ring artifact” covered approximately 87.5° of angular extent, corresponding to an 80.2 ms temporal resolution near isocenter. Using only one of the sources, the partial reconstruction demonstrated a partial “ring artifact” covering approximately 179.7°, corresponding to a temporal resolution of 164.7 ms near isocenter. These estimations of temporal resolution correspond well to the nominal values of 83 and 165 ms, respectively. The standard deviation of the measurements (n=5 for each scan mode) ranged from 0.8 to 3.0 ms.

Patient data reconstructed at the two temporal resolutions demonstrate improved sharpness and decreased motion artifact using dual-source CT for a heart rate of 90 bpm. Clinical experience with this technology has thus far supported the premise that the use of heart rate control is not required for a temporal resolution of 83 msec 1, 2, 68.

Acknowledgments

The authors would like to thank Mr. William Ollhoff for his assistance with phantom construction and Ms. Kristina Nunez for assistance with manuscript preparation.

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