Direct cooling by conduction or convection, as in the Coolidge tube, is difficult.
In the Coolidge tube, the electrons are produced by accelerated, and then hit the anode.
Figure 2.3 compares the X-ray beams from the transmission target and from the Coolidge tube.
This idea was quickly applied to X-ray tubes, and hence heated-cathode X-ray tubes, called " Coolidge tubes ", completely replaced the troublesome cold cathode tubes by about 1920.
A conventional X-ray tube for mammography consumes 4.5-6 kilowatts, and calls for a Coolidge tube with a rotational anode in order to spread the thermal load from a single spot to the edge area of a spinning anode disc.
"' Solid-anode microfocus X-ray tubes "'are in principle very similar to the Coolidge tube, but with the important distinction that care has been taken to be able to focus the electron beam into a very small spot on the anode.
In relativistic transformation the bremsstrahlung trajectories are forward-leaning, moving along the e-beam; if the X-ray fluence is collected from the transmission target in the forward direction and integrated over the azimuth angles, fluence could be several-hundredfold enhanced compared with the Coolidge tube.
Also note the kVp could be increased several folds with virtually no change in the emission spectrum in the tube using a transmission X-ray target whose brightness increases to the kVp2.1 ( Figures 2.4 and 2.5 ), as compared to the conventional Coolidge tube being proportional to kVp1.7 with a shifted photon spectrum.
The transmission target emission angle whose total X-ray fluence, mostly in line-emission, could be 250 times that of the Coolidge tube with kVp reaching 100kV . Most importantly, by making use of a greatly enhanced X-ray generating efficiency, the thermal load of a medical imaging unit such as mammography operating at 100kV without a spectrum shift, can be reduced from 6 kW to under 100 Watts, for example, thus providing the X-ray tube to deliver a sharper focal spot with much improved line-pair formation in the imager.