DESARROLLO DE ESTRATEGIAS

Figure 3.6 shows photographs of a complete s-DBT tube, designed to be compatible with the Hologic Selenia Dimensions DBT system. The X-ray window on the tube is 1 mm thick aluminum. The tube has two ion pumps, used to maintain a vacuum of approximately 10-7 _{torr. The external collimator’s attachment points can be seen in the left image of Figure}

3.6, and the collimator is mounted below the X-ray window in the right image. There are two high voltage focusing feedthroughs on top of the tube, one for F1 and for F2. Thirty-one high voltage cathode feedthroughs are lined along the top of the tube, one per cathode. There is no gate electrode feedthrough because it is permanently grounded to the tube housing. Voltage between the gate and cathode for electron extraction is applied to the cathode, and is a negative voltage on the order of 1 kV – 2 kV.

Figure 3.6 Photographs of an s-DBT X-ray tube with key features labeled. The orientation used in breast imaging is shown in the right photograph, with the X-ray window facing downward toward the detector.

Linear focal spot configuration

The cathode-anode pairs such that the angular separation between each projection is 1°, and the total angular range is 30°. Therefore, the angular spacing of the projections is uniform, but the linear spacing of the cathodes is not. The cathode-anode pairs angle inward toward the central location to create an imaging isocenter 5 cm above the detector surface. Internal collimation is also present within the tube in order to control the beam width leaving the X-ray window. This restricts X-rays from falling too far outside of the detector area. The outermost beams are collimated to an angle of 23.4°. The external collimator restricts the beam from penetrating too far into the patient’s chest wall.

Effective focal spot size

The focal spot size can be described in two ways: the actual focal spot size, and the effective focal spot size. The actual focal spot size is the area of electron bombardment on the angled surface of the anode. The effective focal spot size is the area of the actual focal spot size projected onto the detector and is dependent on anode angle and the measurement location on the detector.

With both focusing electrodes grounded, the effective focal spot size was measured to be 0.9 mm × 0.9 mm. Due to the anode angle tilt, the real focal spot size is larger than the effective focal spot size. This only affects the focal spot dimension corresponding to the long side of the cathode, as illustrated in Figure 3.7.

In the s-DBT tube the anode is tilted by 16°. Keeping with the definitions in Figure 3.7, the relationship for the effective focal spot size is: 𝐸 = 𝑠𝑖𝑛𝜃 ∙ 𝑅. Knowing that the measured effective focal spot was 0.9 mm and the anode angle is 16°, the length 𝑅 is calculated to be 3.26 mm. To calculate the height of the electron beam, 𝐵, one would use 𝐵 = 𝑅 ∙ 𝑐𝑜𝑠𝜃. The height of the electron beam is then 3.13 mm. The original cathode dimension in that direction is 13 mm. The gate and focusing electrodes, when grounded, reduced the electron beam to approximately one-fourth of its original length. In the short cathode dimension of 2.5 mm, the anode angle does not reduce the electron beam size.

Therefore, the 0.9 mm measured size is due completely to the electrode structure focusing the electron beam to approximately one-third of its original size.

Figure 3.7 Schematic illustrating the difference in the real, or actual, focal spot size and the effective focal spot size.

The s-DBT tube is mounted onto the gantry 70 cm above the detector and is tilted to make the X-ray intensity more uniform across the detector surface. The effective focal spot size changes depending on where it is measured on the detector. A schematic illustrating how this changes the effective anode angle at the center of a 23 cm long detector is shown in Figure 3.8.

Transmission rate and anode exposure

Transmission rate (TR) is defined as the percentage of cathode current that reaches the anode: 𝑇𝑅 = _{𝐶𝑎𝑡ℎ𝑜𝑑𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡}𝐴𝑛𝑜𝑑𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 . Averaging over all 31 cathodes with grounded focusing, the s-DBT tube transmission rate is 61 ± 4 %. Transmission rate can be affected by focusing voltage and gate mesh design. It is important to know the transmission rate to extrapolate the cathode current required for a desired anode exposure.

Figure 3.8 Schematic of the geometric configuration determining effective anode angle with tube tilt and positioning on the gantry incorporated.

Anode exposure is anode current multiplied by cathode pulse width. It can be estimated if the cathode current, which can be programmed electronically, and transmission rate are known. The anode exposure for a tomosynthesis acquisition is designed to

approximately equal one 2D mammography exposure, but divided among many projections. For the Hologic Selenia Dimensions system the anode exposure, determined by AEC, ranges from 36 mAs to 72 mAs, depending on compressed breast thickness4. Assuming for a 100 mAs total exposure example with fifteen projections, each projection must produce 6.67 mAs, where the current is the anode current and the time is the pulse width.

Anode exposure is important because it is proportional to the dose that a patient will receive, as discussed in Chapter 1. The exposure produced on the anode, in units of mAs, is not the same as the exposure that leaves the X-ray tube because of filtration by the tube window. The ratio of dose to anode exposure is defined here as the dose rate. Figure 3.9 compares the dose rate of the s-DBT tube to Hologic’s Selenia Dimensions DBT system; the s-DBT system producing less dose for each anode exposure level. The difference is due to X- ray filtration differences between the systems, with s-DBT using 1 mm Al and Hologic using 0.7 mm Al plus 0.63 mm Be. The absorbed dose to the patient in mGy is proportional to exposure, as was discussed in Chapter 1. The average mean glandular dose to a patient of average breast thickness (5 cm) is approximately 2 mGy when using a Hologic Selenia Dimensions system4.

To summarize, key properties of the s-DBT tube are listed in Table 3.1.

Table 3.1 Key properties of the s-DBT tube, Argus 3.0. Tube property Value for Argus 3.0

Number of cathodes 31

CNT deposition area 2.5 mm × 13 mm

Gate electrode–to–cathode distance 280 µm

X-ray window 1 mm Al

Average TR 61 ± 4 %

Average vacuum pressure 10-7 torr Angular spacing between projections 1°

Total angular coverage 30°

Focal spot size, focusing grounded 0.9 mm × 0.9 mm

Anode angle 16°

Figure 3.9 Dose rate measurements comparing the Hologic Selenia Dimensions system to the s-DBT system. Data taken by Dr. Andrew Tucker.

3.2 System design and build challenges