6.5 CNT Source Array Geometry Calibration
7.4.2 Anthropomorphic phantom Imaging
Projections images were reconstructed into tomosynthesis slices with 4 mm spacing. Fig- ure 7.3 shows tomosynthesis slices reconstructed from linear imaging geometries with 11.6◦ and 34◦ angular coverage. Both images clearly show multiple generations of airways and de- tailed vascular structures inside the phantom. The slice with larger angular coverage shows less out-of-plane artifacts of the ribs and other structures.
Figure 7.4 compares the linear imaging geometry with 11.6◦ angular coverage and the square imaging geometry with similar 10◦ angular coverage. The images reconstructed from the square imaging geometry show more fine structures in the x direction than the linear imaging geometry. This is because the square geometry yields more isotropic MTFs and better in-plane resolution in the x-direction, as shown in table 7.2. The visibility of the out-of-plane ribs was also reduced due to better in-depth resolution.
Section 7.5: Discussion
The CNT tube operated reliably with consistent source-to-source output. System MTFs and ASF measurements show that a wider angular coverage improves the system in-depth resolution, but has little impact on the in-plane resolution in the linear imaging geometry.
(a) linear 11.6 (b) linear 34
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Figure 7.3: Comparisons of tomosynthesis reconstructed using projection images acquired with 11.6◦ and 34◦ angular coverage in linear geometry. Both slices clearly show airways and detailed vascular structures inside the phantom. The wider (34◦) angular coverage results in better reduction of out-of-plane artifacts of ribs.
The 2D source array imaging geometry shows more symmetric in-plane resolutions and better in-depth resolution compared to the 1D linear source array geometry with similar angular coverage. More geometries will be studied in the future and the optimum imaging geometry for stationary chest tomosynthesis will be determined.
The source array used in this study was not originally designed for medical imaging. Therefore, the asymmetric in-plane resolution (10% of MTFs) of the current system in both 1D linear and 2D imaging geometries was mainly due to the anisotropic focal spot size of the source array. The system in-plane resolution of the s-DCT can be further improved with a tube designed for chest tomosynthesis. The 2D source array imaging geometry has shown better in-depth resolution (FWHM of ASF) when compared to 1D linear array geometry with a similar tube footprint. This study suggests that a 2D source array would be more compact than a 1D linear design and have comparable in-depth resolution.
Figure 7.4: Comparison of tomosynthesis slices of linear imaging geometry and square imaging geometry with comparable angular coverage. The slice of square imaging geometry shows more fine structures along x direction and better reduction of artifacts of ribs from other slices.
CHAPTER 8: Prospectively Gated s-DCT Section 8.1: Overview
The electronically controlled CNT x-ray source enables physiologically gated imaging, which will minimize image blur due to the patients’ respiration motion. In this chapter, we will investigate the feasibility of prospective gated chest tomosynthesis using a prototype s-DCT system with a CNT source array, a high-speed flat panel detector and realistic patient respiratory signals captured using a pressure sensor.
Tomosynthesis images of inflated pig lungs placed inside an anthropomorphic chest phan- tom were acquired at different respiration rates, with and without gating for image quality comparison. Metal beads of 2 mm diameter were placed on the pig lung for a quantitative measure of image quality. Without gating, the beads blurred to 3.75 mm during a 3 s to- mosynthesis acquisition. When gated to the end of the inhalation and exhalation phases the detected bead size reduced to 2.25 mm, much closer to the actual bead size. With gating the observed airway edges were sharper and there were more structural details visible in the lung.
The results demonstrated the feasibility of prospective gating in s-DCT, which substan- tially reduces image blur associated with lung motion.
Section 8.2: Purpose
Commercial digital chest tomosynthesis (DCT) systems mechanically moves an X-ray source over a large distance to acquire projection images.[1, 3, 13] Typical scans last about 10 s, during which patients are required to hold their breath.[8, 12] Patient motion blur is a main source of image degradation due to long scanning time, even if projection images
are acquired within one breath hold.[33] Respiration motion during the image acquisition may cause severe artifacts.[14] A literature review by AAPM task group showed the motion of nodules due to respiration can be up to 20 mm.[32] The feasibility of stationary chest tomosynthesis system (s-DCT) using a CNT based distributed X-ray source array has been demonstrated.[85, 86, 49] The s-DCT acquires projection images by switching individually controllable CNT x-ray sources without moving the x-ray tube, saving the time to move the source. The fast switching capability of the CNT x-ray source also allows prospectively gated imaging using physiological signals, such as respiration signals.[45, 79]
The purpose of this chapter is to investigate the feasibility of physiologically gated s-DCT, and to study the effect of gating on image quality.
Section 8.3: Methods
8.3.1: System Description
Figure 8.1 shows the prototype s-DCT system with a linearly distributed CNT x-ray source array. The system consists of a CNT linear x-ray source array, a flat panel detector (Varian 3030D, Varian Medical Systems Inc., CA) and an electronic control unit. The detector has an effective imaging area of 30 cm× 30 cm with 1536×1536 pixels in full resolution. The pixel size is 194 µm×194 µm in full resolution mode. The detector can be operated up to 12.5f ps in full resolution mode and 30 f ps in 2×2 binning mode. The source to detector distance is 100 cm, and the tomosynthesis angular coverage is 16.9◦.
A switching system is used to control individual x-ray sources in the tube. Coupled with a physiological signal sensor, the x-ray beams can be gated to a specific phase of the signal. The timing graph of the gating scheme is shown in Figure 8.2. The detector was operated in continuous mode. The ready to exposure signal was used as a master signal to trigger the x-ray sources. A logical AND operation was applied between the master signal and the physiological signal. A LabVIEW program automated the image acquisition. Tomosynthesis
Figure 8.1: Stationary chest tomosynthesis system consisting of a CNT source array, a flat panel detector and an anthropomorphic phantom mounted on the same translation stage. The source array contains 75 focal spots, and has a 298 mm end-to-end length. The detector is 30 cm×30 cm, and can acquire images at up to 30 f psin the 2×2 binning mode.
images were reconstructed using an adaptive fan-beam volume reconstruction algorithm, where the 3D cone beam reconstruction was broken down into a series of 2D fan-beams to accelerate the reconstruction.[87]
Figure 8.2: Timing graph of physiologically gated s-DCT. In this study, the detector ready to exposure signal is filtered by the physiological signal captured by the Bio Vet sensor. The x-ray source only fires when the lung is in a certain respiration phase.
8.3.2: Dynamic Phantom
A dynamic phantom was constructed using porcine lungs, as shown in Figure 8.3. Pig lungs and heart block was placed inside an anthropomorphic chest phantom (Model RS-320, Radiology Support Devices, Long Beach, CA). The lungs were connected to a ventilator to simulate the respiration motion with respiration cycles and volume comparable to human. The lung motion was monitored using the Bio Vet sensor (m2m Imaging Corp., OH), the sensor measured the maximum pressure when the lung reaches maximum volume (the end of inhalation phase).