SURFACE INTEGRAL EXPOSURE (SIE).

Since exposure expressed in roentgens or coulombs per kilogram is a concentration, it does not express the total amount of radiation delivered to a body. The total radiation delivered, or surface integral exposure (SIE), is determined by the exposure and the dimensions of the exposed area. It is also referred to as the exposure-area product.

The surface integral exposure is expressed in the conventional units of roentgens-square centimeters (R-cm²). If the radiation exposure is uniform over the entire area, the SIE is the product of the exposure in roentgens and the exposure area in square centimeters. If the exposure is not the same at all points in the exposed area, the SIE can be found by adding the exposure values for each square centimeter of exposed surface. Mathematically, this is the process of integrating the exposure over the surface area. The SIE can be measured during x-ray examinations by placing a special type of ionization chamber in the x-ray beam. The significance of SIE is that it describes total radiation imparted to a patient, whereas exposure indicates only the concentration of radiation at a specified point.

Comparison of SIE and Exposure during a Fluoroscopic Examination is as shown

Another important example is illustrated below. Here the same exposure (100 mR) is delivered to both patients. However, there is a difference in the exposed area: the patient on the right received 10 times as much radiation as the patient on the left.

Comparison of SIE Values for a Radiographic Examination is as shown

DOSE AREA PRODUCT (DAP)

Dose Area Product (DAP) is similar in concept to surface integral exposure and exposure area product in that they all express total radiation delivered to a patient.  The principle difference is in the units used.  DAP is in dose units, such as Gy-cm².  For a uniformly exposed area, the DAP is just the product of the air kerma, in Gy or mGy, and the exposed area in cm². DAP provides a good estimation of the total radiation energy delivered to a patient during a procedure.

Both radiographic and fluoroscopic machines can be equipped with devices (DAP meters) or computer programs that measure or calculate the DAP for each procedure. It is the most practical quantity for monitoring the radiation delivered to patients.

The important point to remember is that exposure (roentgens) alone does not express the total radiation delivered to a body. The total exposed area must also be considered.

ENERGY

Energy can be defined as the ability or capicity to do work. An x-ray beam and other forms of radiation deliver energy to the body. In principle, the amount of radiation delivered could be expressed in units of energy (joules, ergs, kiloelectron volts, etc.). The energy content of an x-ray beam is rather difficult to measure and for that reason is not widely used in the clinical setting. However, considering the energy delivered by an x-ray beam is helpful in understanding other radiation quantities.

ENERGY FLUENCE

Energy fluence (concentration) is the amount of radiation energy delivered to a unit area. The units for expressing radiation energy concentration are either the millijoule (mJ) per square centimeter or erg per square centimeter. For a specific photon energy, fluence is proportional to exposure. In the relationship between energy fluence and exposure, the relationship changes with photon energy because of the change in photon interaction rates. However, assuming a photon energy of 60 keV, the energy fluence for a 1-R exposure is approximately 0.3 mJ/ cm².

The energy delivered by an x-ray beam can be put into perspective by comparing it to the energy delivered by sunlight, as shown below. For the x-ray exposure, using the fluoroscopic factors of 5 minutes at the rate of 3 R/min. This 15-R exposure delivers x-ray energy to the patient with a concentration (fluence) of 4.5 mJ/cm² assuming effective photon energy of 60 keV.

Comparison of Energy Delivered by an X-Ray Beam and Sunlight

The energy delivered by the sun depends on many factors including geographic location, season, time of day, and atmospheric conditions For example, a typical midday summer exposure on a clear day in Atlanta produces approximately 100 mJ/sec/cm². In 5 minutes a person would be exposed to an energy fluence of 30,000 MJ/cm². This shows that the energy content of an x-ray beam is relatively small in comparison to sunlight. However, x-ray and gamma radiation will generally produce a greater biological effect per unit of energy than sunlight because of two significant differences: x- and gamma radiation penetrate and deposit energy within the internal tissue, and the high energy content of the individual photons produces a greater concentration of energy at the points where they are absorbed within individual atoms.

TOTAL ENERGY

The total energy imparted to a body by an x-ray beam is determined by the energy fluence (concentration) and the size of the exposed area. If the radiation is uniformly distributed over the area, the total energy delivered is the product of the fluence and the surface area.

ABSORBED DOSE

A human body absorbs most of the radiation energy delivered to it. The portion of an x-ray beam that is absorbed depends on the penetrating ability of the radiation and the size and density of the body section exposed. In most clinical situations more than 90% is absorbed. In nuclear imaging procedures, a large percentage of the energy emitted by radionuclides is absorbed in the body. The two aspects of the absorbed radiation energy considered include (i) the amount (concentration) absorbed at various locations throughout the body and (ii) the total amount absorbed.

Absorbed dose is the quantity that expresses the concentration of radiation energy absorbed at a specific point within the body tissue. Absorbed dose is defined as the quantity of radiation energy absorbed per unit mass of tissue. Since an x-ray beam is attenuated by absorption as it passes through the body, all tissues within the beam will not absorb the same dose. The absorbed dose will be much greater for the tissues near the entrance surface than for those deeper within the body.

The conventional unit for absorbed dose is the rad, which is equivalent to 100 ergs of absorbed energy per g of tissue. The SI unit is the gray (Gy), which is equivalent to the absorption of 1 J of radiation energy per kg of tissue. The relationship between the two units is

1 rad = 100 erg/g = 0.01 J/kg = 0.01 Gy

1 Gy = 100 rad.

For a specific type of tissue and photon energy spectrum, the absorbed dose is proportional to the exposure delivered to the tissue.

COMPUTED TOMOGRAPHY DOSE INDEX (CTDI)

The Computed Tomography Dose Index, CTDI, is the special dose quantity that is used extensively to express absorbed dose in CT.

The Concept of Computed Tomography Dose Index (CTDI)

MEAN GLANDULAR DOSE (MGD)

The Mean Glandular Dose (MGD) is the special dose quantity used in mammography.  It is defined as the mean, or average, dose to the glandular tissue within the breast.  The assumption is that the glandular tissue, and not the fat, is the tissue at risk from radiation exposure.  Obviously, it is just about impossible to determine the actual dose to the glandular tissue during a specific mammographic procedure because of variations in breast size and distribution of glandular tissue within the breast.  The MGD is based on some standard breast parameters.

MGD values are determined by following a standard two-step protocol:

1. The first step is to determine the entrance surface exposure, or air kerma, to the breast.  This can be measured directly with small dosimeters placed on the breast or calculated from the known calibration factors for the mammography equipment.
2. Then, the MGD is determined by multiplying the surface exposure value by published dose factors.  The dose factor values are tabulated according to breast size and composition and the penetrating characteristics of the x-ray beam as determined by the anode material, filtration, and KV.

For comparison of imaging techniques, evaluation of equipment performance, general dose management, and regulatory and accreditation purposes, the MGD to a “standard” breast is used.  The standard is a 4.2cm thick compressed breast consisting of 50% glandular tissue and 50% fat.  This corresponds to the standard phantom that is used for image quality evaluation and comparative dose determinations.

Mean Glandular Dose (MGD) in the Breast

INTEGRAL DOSE

Integral dose is the total amount of energy absorbed in the body. It is determined not only by the absorbed dose values but also by the total mass of tissue exposed.

The conventional unit for integral dose is the gram-rad, which is equivalent to 100 ergs of absorbed energy. Since integral dose is a quantity of energy, the SI unit used is the joule.

IJ = 1000 gram rad

Integral dose (total absorbed radiation energy) is probably the radiation quantity that most closely correlates with potential radiation damage during a diagnostic procedure. This is because it reflects not only the concentration of the radiation absorbed in the tissue but also the amount of tissue affected by the radiation.

There is no practical method for measuring integral dose in the human body. However, since most of the radiation energy delivered to a body is absorbed, the integral dose can be estimated to within a few percent from the total energy delivered to the body.

DOSE LENGTH PRODUCT (DLP)

The associated quantity for specifying the total radiation to a patient is the dose length product (DLP) as shown below. The DLP is just the product of the CTDI value and the length of the body area scanned.  It has the units of either rad-cm or Gy-cm.  It is a useful and practical quantity for comparing the total radiation to patients for various CT procedures.  It is, however, not a precise measure of the total radiation or integral dose; that is more difficult to determine.

The Concept of Dose Length Product

DOSE EQUIVALENT

Dose equivalent (H) is the quantity commonly used to express the biological impact of radiation on persons receiving occupational or environmental exposures. Personnel exposure in a clinical facility is often determined and recorded as a dose equivalent.

Dose equivalent is proportional to the absorbed dose (D), the quality factor (Q), and other modifying factors (N) of the specific type of radiation. Most radiations encountered in diagnostic procedures (x-ray, gamma, and beta) have quality and modifying factor values of 1. Therefore, the dose equivalent is numerically equal to the absorbed dose. Some radiation types consisting of large (relative to electrons) particles have quality factor values greater than 1. For example, alpha particles have a quality factor value of approximately 20.

The conventional unit for dose equivalent is the rem, and the Sl unit is the sievert (Sv). When the quality factor is 1, the different relationships between dose equivalent (H) and absorbed dose (D) are

H (rem) = D (rad)

H (Sv) = D (Gy)

Dose equivalent values can be converted from one system of units to the other by:

1 Sv = 100 rem

A summary of the general relationship among the three quantities, exposure, absorbed dose and dose equivalent is as shown:

Relationship of Exposure, Absorbed Dose, and Dose Equivalent

Although each expresses a different aspect of radiation, they all express radiation concentration. For the types of radiation used in diagnostic procedures, the factors that relate the three quantities have values of approximately 1 in soft tissue. Therefore, an exposure of 1 R produces an absorbed dose of approximately 1 rad, which, in turn, produces a dose equivalent of 1 rem.