The exposure and the exposure radiography

The exposition in rentgenotechnika conditionally called the amount of electrical energy that has passed through the x-ray tube for the shooting time, measured in millisecond or milliamps seconds (see).
Exposure is a time interval during which the x-ray film is exposed to continuous action of irradiation. In medical rentgenotechnika shutter speed is usually measured in seconds or fractions of seconds.
Since the exposition represents the product of the current, expressed in millimetres, restraint, expressed in seconds, we can assume that under other equal conditions rentgenologicsky the effect will be the same as when the current through the tube to 10 mA for 10 sec., and at the current 100 mA for 1 sec., because in both cases, the exposure is equal to 100 mA/sec. Practically the same in the latter case, especially for low-power mains, due to the increase in current will be a voltage drop more than in the first case, which will cause the reduction of rigidity radiation, and exposure is insufficient. Therefore, when working in low-power electrical networks and low-power x-ray machines should avoid heavy loads on the x-ray tube, compensating for the small currents corresponding increase stress and aging.
A significant factor influencing the formation of the image in radiography, is the rigidity of radiation, i.e. its penetrating power, practically defined by the voltage on the x-ray tube, measured in kilovolts.
Many authors (Asbah, jancker and others) instead of absolute values of the exposition are the so-called coefficients exhibitions (multipliers), based on the exposure needed to obtain a good picture of the wrist joint in the Palmar projection, taking the exposure per unit. For calculating the exposure in radiography other objects under the same technical conditions to use the factors listed in table 1.
The above factors are not exhaustive, as in fact we have to take pictures on the films of different sensitivity, with different modes of rigidity of radiation, with the use of screening lattices and without it, and also at different focal lengths.


Fig. 2. The scheme of the projection x-ray image with conventional radiography (1) and teleroentgenography (2): With - heart.

In radiography with small focal length of the image on the x-ray enlarged due to the divergence of rays. To get the dimensions of the image that are close to true, produce x-rays with larger focal distances of the order of 150-200 cm - teleroentgenography (Fig. 2). The direction of the rays parallel approaches and the dimensions of the image correspond to the dimensions of the object of research. This is especially important when x-ray of the heart, when you know its true dimensions. In radiography limited parts in special projections produce so-called sighting shots.
Given that the exposure level is proportional to the square of the focal length, you should change the focal length can be used as a correction factor (table. 2).
The correction factor when you change the sensitivity of the film, expressed in reverse roentgen (PR), specified in table. 3.

Table 1
Objects radiography The coefficients of expositions
The fingers of the hand.
Toes
Wrist joint lateral projection
Stop and elbow joint in direct projection
Stop and elbow joint in the lateral projection
The lower leg and ankle joint
Knee joint
Hip, vertebrae
The pelvis, hip joint
Shoulder, shoulder joint
Thoracic vertebrae in direct projection
Thoracic vertebrae in the lateral projection
Lungs, heart in direct projection
The larynx, trachea
The skeleton of a chest in the direct projection
The stomach, kidneys, gall bladder
Skull lateral projection
The skull is in direct projection
Lumbar and sacral vertebrae in direct projection
Lumbar and sacral vertebrae in the lateral projection
0,5
0.75
1,8
3,0
5,0
5,0
7,0
12,0
24,0-30,0
6,0-10,0
15,0
20,0
2,5-3,0
3,0
10,0
20,0
15,0
30,0
30,0
100,0

At the average values of hardness expressed in kilovolts, we can assume that the increased rigidity on 10 kV allows to reduce the exposure in 2 times, and an increase of 15 kV - 3 times.
Photos screening bars require increased radiation hardness 25% or increasing exposure in 3 times.
Amendments to the age produced the introduction of correction factors (table. 4).
At the pictures fat or thin people should be on every inch of increase (or decrease) the thickness versus conventional respectively to increase (or decrease) the exposure of 25% or voltage 2.5 square
Example: suppose that x-ray of the wrist joint set the exposure time of 10 mA/sec at almost 50 kV and focal length 70 see Then for picture skull in direct projection (coefficient. 30,0) with screening array (coefficient. 3,0) when the focal length of 100 cm (coefficient. 2,1) will need 10X30X3,0X2,1 = 1890 mA/sec. Increasing the stiffness from 50 to 80 kV, you can reduce the exposure time in 9 times. Thus, exposure to x-rays of the skull in these new conditions will be 1890 : : 9=210 mA/sec. The restraint must be appropriate to the capacity of the x-ray machine. On a powerful machine you can use a shutter speed of 2.1 sec. at 100 mA, for low-power machine is better to reduce the load up to 30 mA, using the shutter speed and 7 seconds.