Fig. 1: Determining Dose Parameters

When an X-ray tube is in operation, so-called X-ray beams, a type of radiation, are released. Using these beams, the technician can create images of whatever is being examined. This radiation penetrates objects and human bodies, passes through them, and is weakened in the process.

In simple terms, this weakening is equivalent to a reduction in the number of individual radioactive particles. A statement concerning the amount of radiation, which is measured at a site, produces the concept of "dose".

Because not all the radiation particles generated during an X-ray are used to produce the resulting images, and because radiation can cause damage to the human body, we try to achieve the greatest possible effect, that is the best possible image with the smallest possible dose of radiation.





In general, the concept of "dose" can mean different things according to the circumstance, for example according to the site where the dose is measured. For this reason, the dose concepts most commonly used in radiology will be explained here.



Dose Parameters
Incident dose

The incident dose is the dose measured in the middle of a radiation field on the surface of a body or a phantom. However, it is only measured at this point if there is no body in the path of the x-ray beam. Thus, there is no scatter radiation from the body during this measurement. When radiation strikes a substance, there is always a certain scattering of radioactive particles. This is comparable to light striking a glass surface; a certain portion of the light is always reflected.

The unit used to measure the incident dose is joules per kilogram, and is known as "Gray" where 1 Gray (Gy) = 1 J/kg. The former unit used to measure the incident dose was the "Rad," and using this unit, 1 Rad (rd) = 0.01 Gy, or 1 Gy = 100 rd. But because today's doses are generally very small, they are usually described using the unit "uGy", that is, 0.000001 Gy.

Incident dose = the dose measured on the intended surface of the patient, but without the presence of the patient

The System International unit (SI unit) used to measure the incident dose is the Gray, where 1 Gy = 1 J/kg



Surface dose

The surface dose is measured with the body in the path of the beam. Because of the scattered radiation that results on the surface and in the depths of the body, the surface dose differs from the incident dose by including the amount of scattered radiation.

Thus we can say:

Surface dose = incident dose + scattered radiation from the body
The SI unit used to measure the surface dose is the Gray (Gy)



Exit dose

The exit dose serves in the evaluation of the X-ray image. It is measured in the radiation field in immediate proximity to the surface of the body where the beams exit from the body. On the basis of the exit dose and the surface dose, we can calculate how much radiation must have remained in the patient's body.

Radiation in the body = surface dose - exit dose
The SI unit used to measure the exit dose is the Gray (Gy)


Image receptor dose

The image Receptor dose is measured at the film cassette, X-ray system's image intensifier assembly or Digital Detector. The image receptor dose is generally smaller than the exit dose, because the radiation weakens before it reaches the image receptor, for example by encountering objects behind the patient's body such as the radiation protection grid, anti-scatter grid or the table.

Image receptor dose <= exit dose
The SI unit used to measure the image receptor dose is the Gray (Gy)



Dose rate at image receptor

In order to measure a dose, the beam must operate for a certain period of time. The dose rate therefore represents the measured dose for the amount of time required to complete the dose measurement. If the image receptor dose is measured in the process, then the dose rate is the image receptor dose rate. If the dose is measured at a different site, then the dose rate is determined using one of the previously mentioned dose parameters.

                    measured dose
Dose rate = -------------------------
                   required time

The SI unit used to measure the dose rate is Gray per second: (Gy/s) or (mGy/s)

Dose-area product

The dose-area product is a measurement of the amount of radiation that the patient absorbs. It is usually measured behind the multi-leaf collimator, that is, on the side of the patient where the radiation enters the body, by attaching a measuring device in front of the X-ray tube and passing a beam through it. The dose-area product is independent of the distance between the X-ray tube and the measuring device because the further away from the X-ray tube this measurement is taken, the more the size of the device increases, and the dose itself decreases (see diagram). The dose to the patient can be calculated from the dose-area product, the size of the measuring device, and the distance to the X-ray tube and the patient.

Dose-area product = dose * surface area of the measuring device
The SI unit used to measure the dose-area product is the Gray * centimeter2 (Gy*cm2)
 

Fig. 2: Dose-area product
The dose-area product at 50 cm from X-ray tube is just as great as dose-area for 100 cm or 200 cm, because the size of the measuring device increases with greater distance to the X-ray tube. But the dose itself decreases with greater distance to the tube. Thus the dose-area product is the same at each position if the size of the measuring device enables it to detect all of the radiation.


Body dose and effective dose

The body dose is the comprehensive concept for the organ or partial-body dose equivalent and the effective dose. In the practical application of radiation protection, however, local and individual doses are monitored, because body doses cannot be measured directly. The Radiation Protection Regulations therefore use the concept of effective dose, in which all the individual doses to the irradiated organs or parts of the body are multiplied by a factor and then added together. The resulting value may not exceed the dose limit for the effective dose that a patient is allowed to receive.

Body dose = sum of all organ or partial-body doses

Effective dose <= patient dose limit

The SI unit used to measure the body dose and the effective dose is the sievert, where 1 sievert = 1 Sv = 1 Joule/kilogram = 1 Gray


Legal provisions

In many countries or states, by means of rules, guidelines or regulations, lawmakers have contributed to improving radiation protection for patients and medical personnel; after all, medical exposure to radiation is the single largest source of radiation exposure among the general population. On an international level, guidelines are laid down by the International Commission on Radiological Protection (ICRP). Many of the rules, guidelines or regulations are governed by the ALARA concept (As Low As Reasonably Achievable), meaning the production of a diagnostically relevant image at minimum possible dose.

Special cases


Pediatrics

Because children have a greater sensitivity to radiation than adults, special conditions apply to pediatric radiology. In particular, an attempt should be made to avoid X-ray examinations altogether and to use alternative procedures instead, such as nuclear spin tomography or sonography. Erroneous X-ray exams should be avoided and the dose measurement should be taken with special pediatric measuring devices. Special X-ray intensifying screens should be used as well to reduce additional dose. And because children have a lower body thickness, the operator generally does not use an anti-scatter grid, which is used when adults are X-rayed. Of additional benefit is a more precise collimation of the area through which the beam will pass; this also reduces the dose. Beam filtration is performed with a pediatric filter consisting of 0.1 mm copper and 1 mm aluminum. It is especially important to use a gonad shield and to time a child's inhalations exactly when making thoracic X-ray images.

Cardiology

Cardiology is another special case. The generator output must reach at least 100 kW, and there must be an additional filter of 0.1 mm copper available for fluoroscopy as well as an appropriate system of collimators for the radiation area. This should include an iris diaphragm, and rectangular and semi-transparent collimators. Calculation of the dose-area product during application has been prescribed by some laws.


General dose reduction measures

Inverse square law

A bundle of X-rays corresponds to the shape of a cone, with the tube at its tip. The intensity or dose of the radiation emitted from the source of the X-ray beam diminishes with the square of its distance from the source. If you double the distance x, the dose changes by a factor of 1/(2²), and if you triple it, the dose changes by a factor of 1/(3²).


 

Fig. 3: Inverse square law

In general, the dose amounts to 1/x². Therefore, if you double the film-to-target distance, you will need four times as much radiation to achieve the same image blackening. If you did not change the patient's position, this would lead to radiation stress in the patient; thus, increasing the distance between X-ray tube and patient helps to reduce the dose.

Collimation at film

Collimation at the site of the film cassette does not result in any dose reduction, because the radiation is not collimated to the appropriate film format until it has passed through the patient. It merely serves to improve image quality by reducing scattered radiation and thereby improving contrast.

Collimation at target

Collimation at the target brings about a genuine dose reduction and also produces better image quality. Collimation is performed using cones and collimators (multi-leaf collimators or iris diaphragms) that are attached directly in front of the X-ray tube. Collimation at the target is the most effective radiation protection for the patient and personnel, because it narrows the area that the radiation can strike.

Compression

Because radiation scatters in a body exposed to X-rays, compression of the body is another way to reduce the radiation dose. Scattered radiation also produces an undesirable reduction in contrast in the X-ray image. With compression, the thickness of the body is reduced, and so a lower dose is absorbed by the body. Additionally, compression ensures that less scattered radiation occurs.

Anti scatter grid

Fig. 4: Cross section of an anti scatter grid

The anti-scatter grid is located between the patient and the image intensifier, or Cassette or Digital Detector. It is the most effective method of reducing scattered radiation. The grid absorbs a portion of the scattered radiation in its lead plates.

This absorbed dose therefore does not reach the image intensifier, Cassette or Digital Detector, even though it has already passed through the patient. Thus, the use of an anti-scatter grid leads to an increase in the dose, because the amount of radiation that reaches the image intensifier, cassette or Digital Detector, is not reduced until it has passed through the patient: if the anti-scatter grid is used, the patient must be exposed to a higher dose of radiation in order for the minimum dose to reach the Image intensifier, Cassette or Digital Detector.

We can differentiate between individual anti-scatter grids using their grid ratios. This is the relationship between the height of the plates to their distance from each other.

The greater the grid ratio, the greater the grid's effect. Thus, the required dose increases with the grid ratio. The typical grid ratio is 8:1 for Radiography and 5:1 for Mammography.

Kilovolt adjustment

The adjustment of the kilovolt values at the operating console also has an important effect on the dose, because if a high kilovolt setting is chosen, the radiation is "harder," that is, richer in energy and more able to pass through the body. High kilovoltage and strong filtration are therefore similar in their dose reduction effects, except that image contrast decreases with high kilovoltage.


 

Fig. 5: Rhodium and Molybdenum energy spectrums for GE Mammography X-ray tube.

For Mammography, the traditional X-ray tube target material is molybdenum, but some equipment feature an additional tube target material, Rhodium or Tungsten, in order to slightly harden the X-ray beam to better penetrate dense breast without compromising image quality or contrast.




Radiation filtration / hardening

The quality of the X-rays also plays a great role in the size of the administered dose. X-ray radiation normally has so-called "hard" and "soft" particles, that is, particles with a lot of energy and particles with little energy. Hard particles are better for the patient, because they pass through the body. Soft particles, by contrast, get caught inside the body because they are too weak to pass through and out of it. Therefore, it is primarily soft radiation that creates unnecessary exposure to the patient. For this reason, copper and aluminum (Molybdenum and Rhodium in the case of Mammography) are used as filters in front of the X-ray tube. The soft radiation is caught in the filter plates, and the remaining radiation emerging from the filter is "harder." This additional filtration can also reduce the dose to the patient without diminishing image quality, because in any case only the "hard" rays reach the image intensifier, film cassette, or Digital Detector.


 

Fig. 6: Target/filtration materials impact on dose

Because the GE Senographe DMR+ and 2000D (Mammography Systems) feature a double Molybdenum / Rhodium X-ray tube tracks as well as two different filters, they provide a good example of the impact of different X-ray target/filtration materials on dose (fig. 6).




Film/screen combinations

Choosing the right film/screen combination can greatly influence the required dose. In general, the dose is a function of the sensitivity of the combination. This sensitivity is the quotient of 1000 uGy and the required dose in uGy.

                          1000 uGy
Sensitivity = -------------------------
                   Required dose in uGy

or:

                                              1000 uGy
Required dose in uGy = -------------------
                                            Sensitivity

For example, a combination with a sensitivity of 400 requires 2.5 uGy in dose. The sensitivity is greatly dependent on the intensifying screen that is used, because the screen is the principal component in image blackening. We differentiate mainly between screens using traditional fluorescent materials such as calcium tungstate, and screens made of so-called rare earths. These rare earths intensify better, which means they transform more X-ray beams into light. They can therefore reduce the dose by up to 50%, because the operator can select a lower dose and still get the same image quality that would be attainable using traditional screens. These screens are stipulated in pediatrics, for example.

Image intensifier input screen

At the input screen of the image intensifier there is a situation similar to that of the screen-film combinations. The input fluorescent screen substantially determines the intensification, that is, the transformation of the X-rays into light. In conjunction with the X-ray image intensifier, the age of the X-ray system plays a role, because the properties of intensification decrease considerably with age. In addition, the radiation field adapts automatically to the format of the image intensifier, which also lowers the dose, because only small portions of the patient are irradiated instead of the patient's entire body. The choice of a small input screen causes collimation, and this too leads to a reduction in the dose.

Automatic exposure timing

The automatic exposure timer or Automatic Exposure Control (AEC) measures the dose of radiation that strikes the X-ray film behind the patient, and turns the X-ray system off when the predetermined dose for that screen-film combination has been reached. This assures that only the smallest required dose is administered. The resulting images all show a uniform blackening, and the danger is reduced that the X-ray examination might have to be repeated owing to an error in the image. In this way, automatic exposure timing also indirectly reduces the dose.

Automatic dose rate adjustment

The dose rate is the dose over the total time in which the patient is exposed to radiation. If the radiation exposure time to the body can be reduced, this leads to a decrease in the total dose to the patient. By automatic dose rate adjustment, the operator tries to reduce the time during which the dose rate is measured at the input of the image intensifier and the kilovolt and milliamp values are, in turn, adjusted at the generator. In the process, the dose rate should be kept as low as possible. Automatic dose rate adjustment is comparable to automatic exposure timing for images made using a screen-film combination.

Tabletop

The material from which the tabletop is constructed is also significant for the required dose, because the tabletop is penetrated by the radiation and weakens it before it reaches the image intensifier. Therefore, if at all possible the tabletop should not contain any material that strongly weakens the radiation or absorbs it well, such as lead or metals in general. Carbon fiber has proven to be the best material for X-ray system tabletops because its radiation absorption is minimal and the tabletop can take a great amount of stress; today a tabletop is expected to be able to support a patient weighing 120 - 150 kg (up to 330 lbs).



Special dose reduction measures
Low-dose fluoroscopy

Fluoroscopy using a reduced dose has become possible primarily through digital technology. Principally, parts of the body with low levels of spontaneous movement are well suited to this method of examination. A few digital fluoroscopy procedures will be described in the following paragraphs.

Pulsed fluoroscopy

In pulsed fluoroscopy, X-rays are no longer delivered continuously; they are delivered in pulses that follow in rapid succession. This reduces the amount of time during which radiation is released. The resulting radiation-free gaps in the imaging process are filled with the last stored digital image until a new and more current image is available. The short X-ray pulses mean that the dose is significantly reduced; additionally, image definition is increased. Pulsing can take place either by using a pulse control at the X-ray generator, or with a grid-controlled X-ray tube; however, the grid control leads to a lower level of radiation exposure.
 

Fig. 7a: Pulse Fluoroscopy The illustration shows the advantages of grid control of the X-ray tube (top) compared to control at the generator (bottom).

Fig. 7.a Top, we can clearly see the exact pulses through the grid control; moreover, they allow rapid switching times. Fig. 7a Bottom in contrast, we can see that during pulsing controlled at the generator, the kilovolt values move more slowly toward the correct value and away from it again, resulting in the patient receiving an unnecessary dose of pulses with a low kilovolt value. Low kilovolt values contribute to radiation exposure but do not result in usable images.
 

Fig. 7b: Pulse Fluoroscopy The Diagram shows the constant pulse width of the selected frequency during pulse frequency control (top), and the constant frequency with variable pulse width using pulse width control (bottom).

Grid control can itself be subdivided into pulse frequency control and pulse width control (Fig 7b). The frequency of pulse frequency control can be varied for example, 12 b/s or 3 b/s, and it controls the X-ray tube continuously. Pulse width control, on the other hand, changes the duration of the individual pulses while at a constant frequency, for example, 25 b/s.

Image integration

Image integration means adding together two or more individual images to create a single image. The dose rate can either be kept the same, resulting in images that are clearer because there is less noise in the image, or the dose can be reduced. This is accomplished by reducing the dose rate and adding individual images to each other until the same image quality is achieved without image integration, but with a reduction in total dose. Combining individual images does result, however, in fewer finished images for viewing at the monitor. The gaps that occur with this method are filled by outputting the same image twice in a row, similar to the method used for pulsed fluoroscopy. The reduction in dose with this method is approximately 50%. A disadvantage of the method is the stroboscopic impression that can arise with fast moving objects.

Digital filtering / SMART Fluoro

With digital filtering, also known as recursive filtering, a fluoroscopy image is mixed or overlaid with one or more previously stored images. The proportion of the previous images is smaller the farther back or longer ago that they were acquired. On the whole, there is flexibility in choosing the proportional mixture of images, and the process represents a compromise between dose reduction and the lag effects that result when mixing images. However, the dose rate can be significantly lowered since the images that are produced are only new, or newly made, to a certain degree; that is, combining individual images means that less overall dose is required. The image mixture proportion can be monitored using a motion detector that lowers the image mixture proportion in the event of a strong shift in the gray scale values in the image, for example when there is movement, so that the output predominantly reflects the current gray scale value.

Last image hold

Last image hold means that the last image obtained during a fluoroscopy is stored until a new image is produced. The physician can then study the image without further radiation exposure. This can lead to a reduction in radiation exposure since the total fluoroscopy time is reduced, and with it, the total dose.

Frame grabbing

Frame grabbing means that the physician can "grab" or extract and view a chosen image from a fluoroscopic series without the necessity of additional radiation. Additionally, spot film radiography can be reduced because the physician can use the "frozen" images from the fluoroscopic series. This dose reduction is particularly well suited to pediatrics.

Roadmapping

Roadmapping is the overlaying of two images. A stored image is superimposed upon a current fluoroscopic image, or a current image can be copied for storage and later used in roadmapping. This is useful primarily in viewing blood vessels, because an existing image of a blood vessel filled with contrast medium can be superimposed on a catheter image made during fluoroscopy. This can save time and contrast medium, and reduce the radiation dose.

Digital Fluoro imaging techniques

Due to digital correction functions and the superior quality of modern X-ray image intensifiers, it is possible to produce spot films from previously captured fluoroscopy images instead of making new images with screen-film systems. Thus, the additional dose that would be needed for new spot film radiography can be completely eliminated, which means a significant reduction in dose. The somewhat lower quality of the fluoroscopy-generated images, which stems from the lower dose used for fluoroscopy, is usually accepted as a reasonable compromise.

Dose levels

The subdivision of the dose into individual levels permits finer gradation. With this practice, the minimum dose for optimum image quality can be selected for every kind of examination. In addition, the standard examination protocols can be individually adapted to the patient.

Virtual collimation

Virtual collimation is a term used to describe the possibility of positioning the collimators via a display or at the monitor. Pre-setting the collimators this way does not require an X-ray beam, and thus reduces the time that the patient is exposed to radiation and consequently, the entire administered dose.

Solid State detector

Another possibility for dose reduction is provided by the use of an electronic flat bed detector, also known as a solid state detector. These silicon-base detectors have a higher degree of effectiveness than traditional detectors, which is expressed in Detective Quantum Efficiency (DQE). The patient dose is in direct proportion to this quantum efficiency:

                                                                    Image Quality
Patient Dose       proportional to       -------------------------
                                                                 Detective Quantum
                                                                 Efficiency (DQE)



On the basis of this equation, we can see that the greater the Detective Quantum Efficiency, the smaller the dose for the patient, yet with the same image quality. An electronic flat bed detector therefore means that a larger amount of the released radiation is actually used, so that from the outset a smaller dose of radiation is needed to produce a comparable image.

Also, because Solid State detectors feature a high dynamic range, they can accommodate for less X-ray and compensate for the lack of film blackening through appropriate Brightness and Contrast adjustment techniques. For some Solid State detectors, the Image Quality does not suffer from a higher energy X-ray beam, thus contributing to a decrease of the overall patient dose.


GE X-ray equipment and dose reduction measures

Revolution XQ/i and XR/d Digital X-ray Systems

The digital radiography systems from GE are designed to meet your clinical requirements today and to address the trends that will make digital x-ray a logical imperative over the next decade.
 

The Revolution XR/d includes a four-way float-top elevating table.

The Revolution XQ/i and XR/d feature a high DQE that can contribute to reduction of dose.

Senographe 2000D and DMR+

Senographe 2000D is the new Digital Mammography System from GE Medical Systems. It is a complete, modular system that eliminates the need for film cassettes and takes full advantage of digital technology from on-screen image display to Networking, Filming and Archiving.

The Senographe DMR+ employs a unique, patented bi-metal mammography tube with a Rhodium track for superior imaging of the most challenging breast tissues.

The Senographe 2000D feature a Solid State Detector with High DQE in addition to the dose reduction measures already incorporated into the Senographe DMR+: