The biologic effects of ionizing radiation are of two types: dose dependent (deterministic) or non-dose dependent (stochastic).

Deterministic “effects” are dose dependent and begin when a certain threshold dose is reached and increases with increasing dose. These effects are caused by cell death and include burns, hair loss, cataracts, radiation sickness, sterility, and fetal damage [11].

Stochastic “effects” are non-dose dependent and can occur with any dose (i.e., not threshold dependent), and the severity of the “effect” (i.e., DNA mutation) is independent of dose. The “risk” of these events occurring is however dependent on dose. These effects result in permanent DNA damage, and this includes cancer induction and genetic mutations. In all cases the radiation causes damage to DNA either directly by breaking DNA links with ionization or indirectly by ionizing a water molecule into a free hydroxyl radical which then damages DNA. DNA is double stranded, thus breaks can be single or double with single being more easily repaired, but double breaks open the potential for lethal DNA connections and cell death. Alternatively DNA breaks may rejoin as a “symmetrical translocation.” In this situation two different chromosomes suffer a double-strand breakage, and the broken fragments are exchanged, and the sticky ends rejoin. This may result in an oncogene fusion protein which allows potential expression of an oncogene during cell division and the development of a subsequent malignancy. It can also cause abnormal division in the reproductive cells in the ovaries and testicles, giving rise to hereditary disorders (also a stochastic effect).

As radiation damage occurs through DNA breakage, any actively dividing cell with more mitosis or a less differentiated cell is more radiosensitive. Thus tissues more sensitive to radiation include the gonads, basal epidermis, mucosa, bone marrow, and thymus and lens cells. The muscle, bones, and nervous system tissues are relatively radio resistant [11, 17].

It is this predilection for damaging DNA that accounts for the increased radiation risk in children. Children are growing with more mitotic figures in their rapidly dividing cells in general, thus their tissues are more radiosensitive. There is also a longer lifetime to manifest the late effects of radiation-induced injury such as cancer and cataracts. Each radiation exposure is cumulative over the life of the child [12].

Now that we know what radiation is and how it causes injury, we must be able to measure the dose or amount of exposure.

Before we learn to measure dose, where does most radiation exposure come from? Since 2006 the greatest radiation exposure of an individual in the United States no longer is derived from background radiation but is received directly from medical imaging at 49 %. 24 % is from computed tomography, 12 % nuclear medicine, 7 % interventional fluoroscopy, 5 % conventional radiography, and 2 % fluoroscopy (Fig. 20.3). The next greatest exposure is the natural radon and thoron background radiation at 37 %. Note the largest contribution from medical imaging is computed tomography or CT [13]. The modern X-ray tube (Fig. 20.4), where electrons produced at the heated negative cathode filament are accelerated across the vacuum striking the positive tungsten anode to produce X-rays, is still the principle component of CT scanners. CT began as a single X-ray tube with an opposing single detector translating around the head and evolved into a complete ring of stationary detectors (Fig. 20.5) [14]. The latest generation of CT scanners has solid rings of X-ray tubes and detectors, which rotate in opposite directions at the same time [15]. These configurations can now permit scanning of the heart in a second without a breath hold. Another development has been dual energy system where two X-ray tubes and two sets of detectors are set at 90° to one another (Fig. 20.6). Each new generation of CT has produced faster images with less radiation than previous generations [16].

The dose of all X-ray beams is related to the energy and number of the electrons within it and the time over which it is applied. The first factor affecting radiation dose is the kilovolt peak (kVp) which is the kinetic energy in kV (1 V × 1000) of the most energetic electrons arriving at the anode. If you recall your high school physics, a volt is a measure of electric potential. The kVp is the maximum voltage applied across the X-ray tube. This determines the kinetic energy of the electrons accelerated in the X-ray tube and the peak energy of the X-ray emission spectrum . This is essentially the energy of the electrons in the beam and would be analogous to the water pressure in your garden hose.

The contrast of an X-ray image which is the amount of difference between the black and whites is primarily controlled by the kVp. Each body part contains a certain type of cellular composition which requires an X-ray beam with a certain energy or kVp to penetrate it. Increasing the kVp increases the energy of the electrons producing a better quality of X-ray beam and greater penetrability which also increases the quantity of X-rays reaching the detector. Thus kVp affects both the quality and quantity of the X-ray beam [17].

The next factor controlling radiation dose is the milliampere/second (mAs) or one thousandth of an ampere per second. The ampere is a measure of the amount of electric charge passing a point per unit time, 6.241 × 1018 electrons passing a given point each second constitutes 1 A, which means this is the number of electrons per X-rays. This could be considered analogous to the size of the garden hose. Radiographic density, i.e., how many photons get through to produce the image, is primarily controlled by the mAs. Increasing mAs causes more photons (radiation) of the particular kVp energy to be produced. The tube current (mAs) affects the number of electrons and the quantity of X-rays produced. The quality of the X-ray beam produced is not affected by the mAs, only the quantity [17].

Now that we know the electrical power factors that produce the emitted radiation, how do we measure it?

The first measurement of radiation exposure was called the Roentgen . It measured ionization in air independent of area and has been redefined multiple times over the years. Currently the Roentgen is defined as 2.58 × 10⁻⁴ C of charge produced by X or gamma rays per kilogram of air. This does not indicate what dose actually penetrated the tissue concerned so is not very practical for medical imaging. An absorbed dose would be more useful as different tissues are more radiosensitive. The absorbed dose in Roentgen absorbed dose or Rad = 100 ergs of energy per gram (erg = 10⁻⁷ J) and measures the energy absorbed in a unit mass of tissue. The old units of Rad were replaced in 1985 by the new System International (SI) by Gray (Gy) which measures Joules absorbed per kilogram [J/kg] (see Table 20.1). Measurements of “absorbed dose” are used for deterministic (dose dependent) effects of radiation, but an “effective dose” accounting for biological damage per unit dose is required to measure stochastic (dose independent) effects. The old units of effective dose were Rem (Roentgen effective man) and were replaced in the new SI system by the Sievert where 1 Sv = 100 Rems. Effective dose must take into account a quality factor (QF) for the type of radiation but fortunately for us mathematical cripples, the QF for diagnostic X-rays is 1. The QF is different for other forms of more powerful radiation like gamma rays (Table 20.2) [17].

Table 20.3 illustrates the comparative effective dose of various imaging procedures as compared to the number of equivalent chest X-rays or days of background radiation. These metrics are very helpful in explaining dose to patients and/or parents. For example, a head CT of an adult produces an effective radiation dose equivalent to 100 chest X-rays or 8 months’ worth of background radiation. Table 20.4 demonstrates the comparative effective radiation dose for imaging procedures in a 5-year-old child. Note how the effective dose for a head CT of a 5-year-old child is 150 chest X-rays.

As we previously mentioned, the largest effective dose of radiation that people receive in this country is from diagnostic CT. CT utilization continues to increase: 85 million CT scans were performed in the United States in 2011 compared to 62 million in 2006. Over four million of these scans were performed on children. Fortunately, pediatric CT utilization began to decrease in 2003, and recent studies note the decreasing trend has persisted [18, 19], led by academic institutions [19]. This likely is due to the professional and public alarm raised by Brenner’s 2001 paper “Estimated risks of radiation-induced fatal cancer from pediatric CT” [20] and the subsequent implementation of the Image Gently campaign . It also helped that the FDA issued an alert on October 8, 2009 informing the public that patients had received extremely high radiation doses during perfusion CT imaging due to operator error. Patients were expected to receive a dose of 0.5 Gy (max) to their head but instead received 3–4 Gy which resulted in hair loss and skin erythema with the possibility of long-term effects. The FDA began an investigation which resulted in their working with manufacturers of CT scanners to improve their instructions and training programs for this complex equipment and to provide software safety checks that would prevent unreasonably high radiation doses from being delivered unintentionally [21].

There are many different dose measures in medical imaging, but due to the prevalence of CT, the Computed Tomography Dose Index (CTDI ) is something we see every day on the scanner console under the Dose Report header (Fig. 20.7). CTDI is the usual metric to estimate dose generated by a CT scanner. The CTDI volume is a standardized measure of the radiation output of a CT system, measured in a cylindrical acrylic phantom that enables users to gauge the amount of emitted radiation and compare the radiation output between different scan protocols or scanners. To measure the CTDI, the total radiation dose in mGy from a single CT scan is collected by a 100 mm long ionization chamber for a single axial slice divided by the nominal slice thickness. Thus CTDI is an average dose over a volume. Although CTDI is measured by using a single scan, it can be used to estimate the average dose from multiple scans where the table is incremented between successive scans [22]. It is an estimate of effective dose to either a 16 or 32 cm diameter phantom (depending on the patient’s size) based on the CT parameters (kVp, mAs, length scanned) selected. It does not indicate dose to the child in the CT scanner [23]! It is just the radiation dose this scanner will emit with the particular scanning protocol programmed. The actual dose to any given patient is directly dependent on the size and shape of the patient and calculations with conversion factors (k factors) which are required to convert the CTDI to that particular patient’s effective dose. Factors must include the patient’s size, the body part, organs irradiated, age, and scan length [23].

To calculate an estimate of the effective radiation dose (“cancer risk”) [23, 24] from the child’s CT scan the dose length product (DLP ) incorporating the length of the area scanned must be known. It also is found on the scanner console on the Dose Report header. DLP = CTDI x total length in millimeters (mm) scanned. The published “k factors” used to convert the DLP to effective dose previously assumed a standard patient model with organs of both sexes which of course does not exist. No adult is a perfectly cylindrical 70 kg hermaphrodite which was the “standard” patient used for adult k factors. For children the standard k factors are at 5 age intervals: newborns, 1, 5, 10, and 15 years of age (Table 20.5) [25, 26]. This also refers to a generic hermaphrodite child of each age. It is well known that the size assignments for age do not correlate well with the actual size of the individual patient [27]. Conversions of CTDI to effective dose are thus only rough estimates for children. As these conversion factors are not ideal, the International Commission on Radiological Protection (ICRP ) publication 103 in 2007 [28] produced new recommendations for conversion factors. They were implemented by Deake et al. who concluded that separate conversion factors that take the tube voltage into account should now be used to determine the effective dose from DLP in pediatrics and the conversion factors should be specific for sex and age [29]. Lately more specific estimates of organ doses and effective whole body doses from CT are being published by age and sex [30, 31].