X-Ray Tube Lecture Notes PDF

X-Ray tube Dr. Nahla Atallah  TABLE OF CONTENTS 1.1 X-Ray tube  Objective:  To familiarize the student with the X- Ray tube and how it works  Focal Spot The focal spot is the area of the target from which x-rays are emitted.  Radiology requires small focal spots...

X-Ray tube Dr. Nahla Atallah  TABLE OF CONTENTS 1.1 X-Ray tube  Objective:  To familiarize the student with the X- Ray tube and how it works  Focal Spot The focal spot is the area of the target from which x-rays are emitted.  Radiology requires small focal spots because the smaller the focal spot, the better the spatial resolution of the image.  Before the rotating anode was developed, another design was incorporated into x-ray tube targets to allow a large area or heating while maintaining a small focal spot.  This design is known as the line-focus principle.  By angling the target (Figure 6-17), one makes the effective area of the target much smaller than the actual area of electron interaction The advantage of the line- focus principle is that it simultaneously improves spatial resolution and heat capacity. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  Effective Focal Spot The effective target area, or effective focal spot size, is the area projected onto the patient and the image receptor. When the target angle is made smaller, the effective focal spot size also is made smaller. Diagnostic x-ray tubes have target angles that vary from approximately 5 to 20 degrees. Biangular targets are available that produce two focal spot sizes because of two different target angles on the anode (Figure 6-18). Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  A circular effective focal spot is preferred.  Usually, however, it has a shape characterized as a double banana (Figure 6-19). These differences in x-ray intensity across the focal spot are controlled principally by the design of the filament and focusing cup and by the voltage on the focusing cup. Round focal spots are particularly important for high-resolution magnification radiography and mammography. One unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x- ray field is greater than that on the anode side. Electrons interact with target atoms at various depths into the target. The x-rays that constitute the useful beam emitted toward the anode side must traverse a greater thickness of target material than the x-rays emitted toward the cathode direction (Figure 6-20).  Heel Effect The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target and therefore increased absorption. This is the heel effect. The heel effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density. In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor. In chest radiography, for example, the cathode should be inferior.  The lower thorax in the region of the diaphragm is considerably thicker than the upper thorax and there ore requires higher radiation intensity if x-ray exposure of the image receptor is to be uniform. Figure 6-21 shows two posteroanterior chest images—one taken with the cathode down and the other with the cathode up. Can you tell the difference? Which do you think represents better radiographic quality? Resolve the difference be ore looking at the figure legend. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1. In abdominal imaging, on the other hand, the cathode should be superior. The upper abdomen is thicker than the lower abdomen and pelvis and requires greater x-ray intensity or uniform x-ray exposure  Another important consequence of the heel effect is changing focal spot size.  The effective focal spot is smaller on the anode side of the x-ray field than on the cathode side (Figure 6-22). Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  X-ray tubes are designed so that projectile electrons from the cathode interact with the target only at the focal spot.  However, some of the electrons bounce of the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot (Figure 6-23).  These x-rays are called off-focus radiation. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1. Off - focus radiation is undesirable because:  It extends the size of the focal spot.  The additional x-ray beam area increases skin dose modestly but unnecessarily.  Off - focus radiation can significantly reduce image contrast.  The rotating anode x-ray tube allows the electron beam to interact with a much larger target area; therefore, the heating of the anode is not confined to one small spot, as in a stationary anode tube.  Figure 6-13 compares the target areas of typical stationary anode (4 mm2) and rotating anode (1800 mm2) x-ray tubes with 1-mm focal spots. Thus the rotating anode tube provides nearly 500 times more area to interact with the electron beam than is provided by a stationary anode tube. The stem of the anode is the shaft between the anode and the rotor. 1. It is narrow so as to reduce its thermal conductivity. 2. The stem usually is made of molybdenum because molybdenum is a poor heat conductor. Occasionally, the rotor mechanism of a rotating anode tube fails. When this happens, the anode becomes overheated and pits or cracks, causing tube failure (Figure 6-14).