High resolution stationary digital breast tomosynthesis using distributed carbon nanotube x-ray source array

Abstract

Purpose: The purpose of this study is to investigate the feasibility of increasing the system spatial resolution and scanning speed of Hologic Selenia Dimensions digital breast tomosynthesis (DBT) scanner by replacing the rotating mammography x-ray tube with a specially designed carbon nanotube (CNT) x-ray source array, which generates all the projection images needed for tomosynthesis reconstruction by electronically activating individual x-ray sources without any mechanical motion. The stationary digital breast tomosynthesis (s-DBT) design aims to (i) increase the system spatial resolution by eliminating image blurring due to x-ray tube motion and (ii) reduce the scanning time. Low spatial resolution and long scanning time are the two main technical limitations of current DBT technology.

Methods: A CNT x-ray source array was designed and evaluated against a set of targeted system performance parameters. Simulations were performed to determine the maximum anode heat load at the desired focal spot size and to design the electron focusing optics. Field emission current from CNT cathode was measured for an extended period of time to determine the stable life time of CNT cathode for an expected clinical operation scenario. The source array was manufactured, tested, and integrated with a Selenia scanner. An electronic control unit was developed to interface the source array with the detection system and to scan and regulate x-ray beams. The performance of the s-DBT system was evaluated using physical phantoms.

Results: The spatially distributed CNT x-ray source array comprised 31 individually addressable x-ray sources covering a 30 angular span with 1 pitch and an isotropic focal spot size of 0.6 mm at full width at half-maximum. Stable operation at 28 kV(peak) anode voltage and 38 mA tube current was demonstrated with extended lifetime and good source-to-source consistency. For the standard imaging protocol of 15 views over 14, 100 mAs dose, and 2 × 2 detector binning, the projection resolution along the scanning direction increased from 4.0 cycles/mm [at 10% modulation-transfer-function (MTF)] in DBT to 5.1 cycles/mm in s-DBT at magnification factor of 1.08. The improvement is more pronounced for faster scanning speeds, wider angular coverage, and smaller detector pixel sizes. The scanning speed depends on the detector, the number of views, and the imaging dose. With 240 ms detector readout time, the s-DBT system scanning time is 6.3 s for a 15-view, 100 mAs scan regardless of the angular coverage. The scanning speed can be reduced to less than 4 s when detectors become faster. Initial phantom studies showed good quality reconstructed images.

Conclusions: A prototype s-DBT scanner has been developed and evaluated by retrofitting the Selenia rotating gantry DBT scanner with a spatially distributed CNT x-ray source array. Preliminary results show that it improves system spatial resolution substantially by eliminating image blur due to x-ray focal spot motion. The scanner speed of s-DBT system is independent of angular coverage and can be increased with faster detector without image degration. The accelerated lifetime measurement demonstrated the long term stability of CNT x-ray source array with typical clinical operation lifetime over 3 years.

Figures

Figure 1

Geometry of the s-DBT system and the CNT x-ray source array (all dimensions are in millimeters).

Figure 2

(a) A photograph showing the Hologic Selenia Dimensions rotating gantry tomosynthesis scanner with the mammography tube in several positions. (b) A schematic of a s-DBT scanner with the CNT x-ray source array mounted on the Selenia Dimensions gantry. For illustration purposes, x-ray radiations from three sources are shown. The 31 x-ray sources were labeled from left to right as: N15, N14, …, N1, 0, P1, …, P14, P15.

Figure 3

Front and back views of the CNT x-ray source array.

Figure 4

The simulated temperature at the center of the W anode during a single x-ray exposure at three different power levels. The horizontal line is 80%Tm of W. The insert shows temperature distribution on the anode surface at the end of a 38 kV(peak), 28 mA, and 250 s exposure.

Figure 5

(a) The graph shows a lifetime measurement performed at 27 mA tube current, 250 ms pulse width, and 5% duty cycle. The inset shows one current pulse. (b) The graph shows data at 38 mA tube current, 183 ms pulse width, and 0.6% duty cycle. The extraction voltage was adjusted automatically to maintain constant tube current.

Figure 6

(a) Variation in the extraction voltages needed to obtain 27 mA tube current from each CNT cathode in the x-ray source array before any compensation. The average value is about 1.4 kV. (b) Tube current from each of the central 15 sources. The XCU automatically adjust the extraction voltage to achieve the required current.

Figure 7

(a) Experimentally measured focal spot sizes (FWHM) of all 31 sources in the x-ray source array. (b) Pinhole image of the central beam (left) and Gaussian fitting curves for intensity profiles along the x and y axes, from which the FWHM is extracted. A gold-platinum pinhole with 12° opening angle, 100 μm diameter, and 500 μm length was used for the measurement.

Figure 8

(a) The specially designed CNT source array with 31 equal angularly spaced x-ray sources mounted on the gantry of the Hologic Selenia Dimension tomosynthesis scanner, replacing the regular rotating anode mammography x-ray tube. (b) Pulsing sequence from one scan with 15 views (bottom). The upper pulse train is the TTL signal from the detector. The rising edge is used for triggering. The lower pulse train is the x-ray tube current.

Figure 9

(a) The projection MTFs of the stationary and rotating gantry DBT systems along the scanning direction. (b) The system MTF obtained using reconstructed in-focus slice.

Figure 10

(a) Fifteen projection images of the biopsy breast phantom obtained from the system were reconstructed using the BP algorithm to yield 50 slices through the phantom, three of which are shown in the first row at depths of 1, 2.5, and 4 cm from the top surface. (b) The second row shows zoomed-in views of the circled regions. The diameters of these microcalcifications are in the range of 0.2–0.5 mm. (c) The third row shows three reconstructed slices of Gammex 156 ACR phantom, which are at depths of 0.7, 1.4, and 2 cm from the top. The diameters of these microcalcifications are 0.54 mm.

Figure 11

The layout of target objects in Gammex 156 ACR phantom is shown. (a) The first row shows the focused slices of the phantom reconstructed using the s-DBT projection data. (b) The second row shows the focused slices of the phantom reconstructed using the DBT projection data. The three images from left to right in each row are the zoomed-in view of the central MC of the MC Clusters 7, 8, and 9, respectively. The diameters of these three MCs are 0.54, 0.4, and 0.32 mm, respectively. The same magnification factor and the window/level are used in displaying the two sets of data.