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Medical Diagnostic Imaging - Introduction, X-Ray Imaging, Film-Screen Combinations, Computed Radiography, Digital Radiography, Fluorscopy, Computer Tomography

rays image radiation energy

DONALD MCROBBIE
Charing Cross Hospital

Introduction

Medical imaging and the discipline of diagnostic radiology began just over a century ago with Wilhelm Conrad Roentgen’s dramatic and accidental discovery of “X-rays” in 1895. Visit a local hospital today and one will find a battery of diagnostic imaging modalities: X-ray, nuclear medicine, MRI, and ultrasound. These are often connected to each other and distributed throughout the hospital via sophisticated computer networks.

Medical imaging is used for the diagnosis of diseases, for assessing acute injuries, for assessing the severity of disease or the response to a particular therapy, for guiding surgical interventions, and for health screening, as in mammography. Its practitioners are not primarily photographers but instead medical diagnostic specialists: technologists or radiographers, who acquire images and radiologists who interpret them using invisible energy in the electromagnetic spectrum.

With the exception of ultrasound, all medical imaging modalities utilize portions of the electromagnetic (EM) spectrum (Figure 55). As if to emphasize the modern physics view of the unity of matter and energy, in medical imaging these EM interactions all relate to the behavior of atoms.

The basic wave is equation is

c = f ?                              (1)

where c is the speed of light (3 × 10 8 ms -1 ), f the frequency in hertz (Hz), and ? the wavelength in meters (m). As the speed of light is constant, wavelength and frequency bear a reciprocal relationship to each other.

According to quantum mechanics, light has a dual nature, possessing the properties of both waves and discrete particles. The smallest unit or quantum of EM radiation is the photon, which has an energy E (joules) given by de Broglie’s equation

E = h f                     (2)

where h is Planck’s constant (6.6 × 10 -34 Js). The higher the frequency (i.e., the shorter the wavelength), the greater the photon energy.

X-rays are formed principally by the deceleration (Brehmsstrahlung) of electrons as they collide with a target material (usually tungsten) in an X-ray tube (Figure 56). Additionally, characteristic X-rays are produced when an atomic electron moves between shells to fill the gap caused by the ejected electron. A spread, or spectrum, of different energies (and hence wavelengths) is produced. The small wavelength of the X-rays is comparable with the dimensions of an atom and means they can penetrate biological tissues. The X-rays interact with matter through a combination of photoelectric absorption and Compton scattering. In the former, the X-ray photon is fully absorbed by the atom and an electron is ejected. In the latter, some of the X-ray photon’s energy is transferred to an electron and the photon is deflected (or scattered) with reduced energy. The relative contribution of these interactions depends upon the atomic weight of the element in question. In general, the heavier the atom, or the denser the material, the greater the absorption will be.

X-Ray Imaging

Like photography, a conventional X-ray produces a two-dimensional planar image. Unlike photography, radiographs are shadowgraphs, revealing the internal structures of objects. Conventional X-rays are thus projection images, they have no depth information, and all opaque and semi-opaque structures in the beam are superimposed.

The image contrast is generated by differences in the attenuation of the X-ray beam described by the linear attenuation coefficient µ

I 0 = I i exp(- µx)                        (3)

where I i is the incident beam intensity, I 0 the resultant intensity, and x the distance through the material. The attenuation coefficient itself comprises contributions principally from photoelectric absorption and Compton scattering. The mass attenuation coefficient is

µ m = µ/?                     (4)

where ? is density. The probability of photoelectric absorption has a dependence on atomic weight, Z, to approximately Z 3 , while Compton scattering is proportional to electron density
and is broadly independent of Z. At lower photon energies photoelectric absorption is the dominant mechanism.

Differences in linear attenuation coefficient of tissues are shown in Table 3. As the densest tissue, bone produces a shadow in the image. The X-rays blacken the film, hence bone with the greatest attenuation shows up as lighter. To improve the visualization of soft tissues, contrast media are sometimes used. These contain heavy elements, e.g., barium or iodine, which absorb more X-rays than the surrounding tissues. Scattered radiation results in a loss of image information and contrast and may be reduced by using a grid of lead strips, which only accept rays within a certain angle of incidence.

Film-Screen Combinations

Historically, the first X-ray detector was photographic film. In radiography a two-stage detection process is usually employed that uses film-screen cassettes (Figure 57a). In these cassettes, the X-rays impinge upon a phosphorescent intensifying screen that produces visible light photons, which then expose the photographic emulsion. Intensifying screens typically employ rare earths (such as gadolinium, lanthanum, or yttrium) for their photoelectric absorption properties, with light emissions primarily in the blue-green region. Conventional X-ray film has a double emulsion layer (used with two screens) 5-10 (µm thick consisting of silver iodide and silver bromide grains (1-2 (µm in diameter) suspended in gelatin. Photographic processing is required. As in conventional photography, X-ray film has alimited latitude or dynamic range and a non-linear characteristic curve. Dental X-rays do not use intensifying screens but rely on the direct effect of the incident X-ray photons. These films must be processed in a radiographic film developer.

Computed Radiography

In computed radiography (CR) the X-rays interact with a photo-stimulable phosphor plate that stores the latent image, which can then be read out by exposing the plate to a scanning
laser beam (Figure 57b). The CR plate may be reused and the dynamicrange of CR exceeds that of X-ray film. The resultant image is of a digital nature and is compatible with image processing and medical image computer networks or Picture Archiving and Communication Systems (PACS).

Digital Radiography

In digital radiography (DR), a solid-state semiconductor detector array is used. The conversion process may be two-stage (Figure 56c) where incident X-rays interact with a phosphor screen (often cesium iodide) to product light photons, which then are converted to electrical signals by the semiconductor array, usually amorphous silicon (a-Si). Alternatively an amorphous selenium (a-Se) detector array is used to directly convert the X-rays to electrical signals (Figure 57d). The semiconductor arrays used are much larger than in, e.g., DR, as they have size equivalent to that of an X-ray film.

Fluorscopy

In fluoroscopy, dynamic, real-time X-ray images are produced. These enable surgical guidance, e.g., the positioning of a catheter in the heart for angiography, angioplasty for repairing arteries, or the placement of implants and other medical devices. Fluoroscopy originally involved the radiologist or doctor staring at a fluorescent screen directly in the radiation beam. The development of the image intensifier in the 1950s enabled good quality fluoroscopy to be achieved at the fraction of the radiation dose for both patient and doctor. In the image intensifier, X-rays are converted to light photons that impinge on a photo-cathode which produces electrons. These are then accelerated across a vacuum and hit the output phosphor producing an intensified optical image. This can then be captured by a television or charge-coupled device (CCD) camera and digitized. More recently digital flat panel detectors have been deployed. These utilize large semi-conductor arrays as in digital radiography.

Computer Tomography

X-ray computed tomography (CT or computer-assisted tomography) permits the visualization of the internal anatomy as a series of thin slabs or slices. To do this the CT scanner acquires various views or projections of the body from different viewing angles. A complex mathematical algorithm known as filtered backprojection permits the reconstruction of the image as a thin slice. The term tomography derives from the Greek tomos which means slice. In a CT scanner, the X-ray tube and detector are mounted on a gantry which rotates about the patient in one second or less (Figure 58). The image intensity in CT uses hounsfield units (HU) defined as Formerly only one slice could be acquired at a time. To perform a full examination a slice was acquired then the
patient moved to a new position, the whole eXamination was performed stepwise. In spiral CT a whole volume of data is acquired while the patient is moved continuously through the gantry. This allows for faster scans, better three-dimensional information, and the possible reduction of the radiation dose. In multi-slice scanners the X-ray detector consists of a two-dimensional array and multiple slices are obtained simultaneously. Current commercial models may capture up to 64 slices. Both spiral and multi-slice CT techniques enable multi-planar reformatting where different views of the anatomy are possible (Figure 59).

Other X-Ray Techniques

Mammography

Mammography is a technique optimized for imaging the breast, particularly for detecting microcalcifications that could indicate breast cancer. A lower energy X-ray beam is used to achieve greater image contrast. Mammography film utilizes a single layer emulsion with one intensifying screen and has a higher spatial resolution than conventional radiography. Digital techniques (CR and DR) are also available but have lower resolution.

DEXA

Dual-energy X-ray absorptiometry (DEXA) is a low-dose technique used for bone scanning, particularly for the detection of osteoporosis and for determining body composition. The use of two photon energies enables an estimation of the relative contributions of photoelectric and Compton absorption and an indication of the average Z of the tissue, which can be used to categorize bone versus soft tissue.

Nuclear medicine

Radioactivity was discovered by Becquerel and the Curies in 1896. When unstable atomic nuclei disintegrate, quanta of EM radiation known as gamma rays may be produced. Gamma rays have a higher energy than X-rays and are highly penetrating. In nuclear medicine a gamma ray emitter with a short half-life (commonly technetium 99m) is attached to a larger molecule and injected into the bloodstream. The distribution of this radiopharmaceutical can then be traced through the body. Nuclear medicine is often used to assess the function of the kidneys and the heart.

Gamma camera

The most commonly used nuclear medicine imaging system is the Anger or gamma camera. This consists of a large sodium iodide crystal that converts the incident gamma rays into light which is then detected by an array of photomultiplier tubes. Subsequently a digital image is produced. The image is usually noisier and of less spatial resolution than an X-ray image but has the advantage of containing information about organ function by tracking the distribution or time course of the concentration of radioisotope.

PET and SPECT

As for X-ray imaging, tomographic images or slices may also be produced. The simplest way to achieve this is to rotate the gamma camera around the patient and have the computer reconstruct the slices. This is called single photon emission computed tomography (SPECT). Another way involves a different technology called positron emission tomography (PET).

When a positron encounters an electron, annihilation occurs. This results in destruction of both particles and the production of a pair of high-energy gamma rays traveling 180 degrees in opposing directions. By precisely detecting these pairs of photons, the PET scanner can determine the path of the photons. Fluoro-deoXy-glucose (FDG) is a commonly used positron-emitting radiopharmaceutical incorporating fluorine 18. The positron-emitters are usually produced by a cyclotron. PET is useful in oncology, detecting metastases and also for studying brain metabolism.

Radiation dose

The energy of X-rays and gamma rays is such that they may dislodge an electron from its parent atom, producing a positively charged ion plus a negatively charged electron. X- and gamma rays are types of ionizing radiation. Ionization can lead to damage to biologically significant molecules (e.g., DNA) and can produce free radicals, which chemically cause further damage to cells. The biological consequences of ionization depend upon the amount and type of radiation absorbed. Broadly two types of effects occur. The first are acute tissue reactions or deterministic effects, which include loss of fertility, skin burns, and cataract formation, but these require large amounts of radiation in excess of 1 gray (Gy) and rarely occur in diagnostic radiology or nuclear medicine. The second effect is cancer induction, including leukemia and solid tumors, but with a long latency of ten or more years after the exposure. These are stochastic effects where the probability, rather than the severity, of the effect is related to the radiation dose. Usually a linear relationship between risk and radiation dose is assumed. Typical radiation doses are shown in Table 4. The sievert (Sv) is the unit of effective dose (E) which relates to the health detriment defined as

where w T is the tissue weighting factor (an indication of the tissue’s radiosensitivity), w R is the radiation weighting factor (= 1 for X- and gamma rays), and D T is the mean absorbed dose (Gy) in each organ.

Magnetic resonance

Magnetic resonance imaging (MRI) also uses EM radiation generated by the atom, this time the nucleus, but without causing it any damage. The original name for this technique was nuclear magnetic resonance (NMR) imaging, but it was changed to MRI to avoid the negative connotations of the word nuclear. MRI makes use of the magnetic properties of the nucleus, particularly of the hydrogen nucleus, two of which are found in every water molecule. The hydrogen nucleus consists of a single proton, which possesses both angular momentum (or spin) and electrical charge and by the laws of electromagnetism acts like a tiny magnet. Under quantum mechanics the proton spin may only assume two orientations when subject to an external magnetic field: either parallel to the field or anti-parallel. There is a natural preponderance for protons to adopt the lower energy orientation, parallel to the field. In magnetic resonance, protons are induced to change their orientation by the application of EM fields at the appropriate frequency, known as the Larmor or resonant frequency. This happens to fall in the radiofrequency (RF) portion of the EM spectrum. Once stimulated or eXcited the protons return to their preferred state by processes known as relaXation, and in doing so they emit a photon of EM radiation at the Larmor frequency. This can be detected by an RF coil. Magnets used in MRI are very strong with field strength, typically between 0.5 and 3 tesla (T), often using superconductivity to produce the field. By comparison the earth’s magnetic field is of the order of 0.05 mT The wavelength of the RF photons used in MRI is long, and MRI consequently produces no ionization. It is not believed to have any serious side effects.

Magnetic resonance imaging

To form an image, temporary variations in the applied magnetic field are introduced resulting in a localized variation in the Larmor frequency given by

f = ?B(x,y,x)                     (7)

where ? is the gyromagnetic ratio ( for hydrogen equal to 42 × 10 6 Hz T 1 ) and B is the magnetic field ( Tesla ).Thus if B is a function of position, then so is frequency. A process known as two or three-dimensional Fourier transformation converts the MR signals into their constituent frequencies and hence localizes the origin of the signals, forming the image.

In MRI usually only water and fat are visible. The images relate to the water (or proton) density and also to the freedom of movement of the water. In addition to producing static and angiographic images, MRI can be used to investigate organ function, e.g., heart wall motion, valvular function, the velocity of blood, and many other parameters. In functional MRI of the brain (or fMRI) localized changes in oXygenation can be detected that relate to cortical neuronal activity, thus indicating the part of the brain active in a given task (Figure 59).

MR spectroscopy

Hydrogen nuclei, which are part of larger molecules, may be shielded from the external magnetic field resulting in a slightly reduced Larmor frequency. In MR spectroscopy (MRS) this feature, known as the chemical shift, is exploited to yield
information about the chemical environment of the nuclei. Imaging techniques may also be applied simultaneously to yield maps of metabolites, or the body’s chemistry. MRS is useful in eXamining the response of tumors to therapy. MRS may be applied using other nuclei, most notably phosphorus (P 31 ), giving an indication of energy metabolism.

Ultrasound

Ultrasound was developed in the 1950s following the development of SONAR in World War II and is unique in not involving EM radiation. Instead, acoustic pressure waves are transmitted into the body and the reflections detected by a piezo-electric transducer. The strength of these reflections, or “echoes” are dependent upon the mechanical properties of the tissues. By computing the time between transmission of an ultrasonic pulse and the detection of the echoes from that pulse, the depth within the tissue can be determined, assuming the speed of sound in tissue (1540 ms -1 ). Echoes arising from flowing blood display a velocity-dependent frequency shift, subject to the Doppler effect, similar to the change in pitch heard from a moving siren on an emergency vehicle. This enables ultrasound to image blood flow and measure blood velocity. The ability of ultrasound to image in real time and its sensitivity to flow, through the Doppler effect, have been key factors in its widespread role in obstetrics, cardiology, abdominal and vascular disease, real-time biopsy guidance, and minimally invasive surgery. Ultrasound is also non-ionizing and carries minimal risks.

Image Networks

Modern medical images in the Digital Imaging and Communications in Medicine (DICOM) format are freely exchangeable between a wide variety of devices and different vendor’s equipment. This makes it possible to share images over a computer network in aPACS. In this form they may be viewed in multiple locations, e.g., on the ward or in the operating room or clinic. Medical informatics is a rapidly expanding field.

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almost 7 years ago

its well written....good job