Physics: CT 3

small elements along the z-axis (patient table) direction.

yes

30 seconds

near isotropic resolution

Figure 1. Sample CT image. A CT image is com-
posed of pixels (picture elements). Each pixel on the
image represents the average x-ray attenuation in a
small volume (voxel) that extends through the tissue
section. (In this example, the pixel size is exaggerated.
In addition, in a real CT image, all tissues within a
single pixel would be the same shade of gray.) w

width

because of the elimination of scatter

Fundamentally, a CT scanner makes many mea-
surements of attenuation through the plane of a
finite-thickness cross section of the body. The
system uses these data to reconstruct a digital im-
age of the cross section, with each pixel in the im-
age representing a measurement of the mean at-
tenuation of a boxlike element (a voxel) that ex-
tends through the thickness of the section

An attenuation measurement quantifies the
fraction of radiation removed in passing through a
given amount of a specific material of thickness

straight-line path through the patient from
the tube focal spot to a detector is the sum of the
individual attenuations of all materials along the
path. If it is assumed that the ray path through the tissue is broken up into incremental voxel thick-
nesses

The image reconstruction process derives the
average attenuation coefficient () values for each
voxel in the cross section by using many rays from
many different rotational angles around the cross
section

voxel attenuation increases with the density and the atomic num-
bers of tissues averaged through the volume of the
voxel and declines with increasing x-ray energy.

Be-
cause this process generates a blurred image, the
data from each projection are mathematically al-
tered (filtered) prior to back-projection, eliminat-
ing the intrinsic blurring effect.

by numbers

, one
early design used three detectors each displaced
by 1°. Since each detector viewed the x-ray tube
at a different angle, a single translation produced
three projections. Hence, the system could rotate
3° to the next projection rather than 1° and had to
make only 60 translations instead of 180 to ac-
quire a complete section (Fig 5).

A single projection
was acquired by moving the tube and detector in
a straight-line motion (translation) on opposite
sides of the patient (Fig 3). To acquire the next
projection, the frame rotated 1°, then translated
in the other direction. This process of translation
and rotation was repeated until 180 projections
were obtained

The gantry is a moveable frame that contains the x-ray tube including
collimators and filters, detectors, data acquisition system (DAS), rotational
components including slip ring systems and all associated electronics such as
gantry angulation motors and positioning laser lights.

Slip ring technology eliminated the
need for cables and allows continuous rotation of the gantry components. The
inclusion of slip ring technology into a CT system allows for continuous
scanning without interference of cables for power

the opening through which the patient passes

The development of spiral/helical CT allows
continuous scanning while the patient table or couch moves through the gantry
aperture.

it will not operate until it cools down.

yes, CT x-ray tubes must possess a high heat capacity which is the amount
of heat that a tube can store without operational damage to the tube.

The x-ray
tube must be designed to absorb high heat levels generated from the high speed
rotation of the anode and the bombardment of electrons upon the anode surface.

Modern CT systems
utilize x-ray tubes that have a heat capacity of approximately 3.5 to 5 million
heat units(MHU).

oil and air cooling system

A CT x-ray
tube anode has a large diameter with a graphite backing. The large diameter
backed with graphite allows the anode to absorb and dissipate large amounts of
heat.

The focal spot size of an x-ray tube is determined by the size of the filament
and cathode which is determined by the manufacturer.

yes

The fundamental principle behind computed tomography is to acquire multiple views of an object over a range of angular orientations. By this means, additional dimensional data are obtained in comparison to conventional X-radiography, in which there is only one view.

The focal spot track is the surface area of the anode that is bombarded by the electron beam. In stationary anode tubes, it is a small area with dimensions of a few millimeters. In the rotating anode tube, the focal spot track is much larger because of the movement of the anode with respect to the electron beam. The figure below shows a small section of a rotating anode.

From the standpoint of producing x-ray images with minimum blur, a small focal spot is desired.

a small focal spot tends to concentrate heat and give the focal spot track a lower heat capacity

you use low power

Notice that the actual focal spot, the area bombarded by the electron beam, is always larger than the projected, or effective, focal spot.

For a given anode angle, the width of the focal spot track is directly proportional to the size of the projected spot

he actual relationship between focal spot width (and heat capacity) and the size of the projected focal spot is determined by the anode angle

Anode angles generally range from about 7° to 20

yes

Although anodes with small angles give maximum heat capacity, they have specific limitations with respect to the area that can be covered by the x-ray beam

X-ray intensity usually drops off significantly toward the anode end

http://www.sprawls.org/ppmi2/XRAYHEAT/#Focal Spot Size

Although anodes with small angles give maximum heat capacity, they have specific limitations with respect to the area that can be covered by the x-ray beam

X-ray intensity usually drops off significantly toward the anode end

http://www.sprawls.org/ppmi2/XRAYHEAT/#Focal Spot Size

In tubes with small angles, this is more pronounced and limits the size of the useful beam.

Yes, The effective length of the focal spot track is proportional to the speed of rotation for a given exposure time.

yes

High-speed rotation simply spreads the heat over a longer track, especially in short exposure times

yes

The maximum heat capacity of the focal spot area, or track, is the major limiting factor with single exposures

If the quantity of heat delivered during an individual exposure exceeds the track capacity, the anode surface can melt, as shown below.

Because of the pulsating nature of single-phase power, some points on the anode surface are raised to higher temperatures than others and exceed the heat capacity

yes

All settings be the same thinner slices on a 64 slice machine will result in less radtion (0.625 vs 5mm), but the signal to noise decreases. Therefore to obtain high quality images with thinner slices there must be increase in Kvp or mA to compenstate and there fore the radiation does go up.