The clinical importance of diagnostic modalities: X-Rays

2. The clinical significance of diagnostic modalities: X-rays

Author: Katalin Klára Kiss

 

2.1. Aim of the Chapter

• Review the physical principles of generation of X ray.
• Become acquainted with the formation of X ray image and functioning of fluoroscopy.
• Become familiar with the clinical indication (its place in the diagnostic algorithm) and usage (examinable organs) of conventional radiography and fluoroscopy.
• Know the advantages and disadvantages of the technique

2.2. Physical basics of generation of X ray image

The physical basics are valid for all conventional radiographic imaging techniques (analogue, indirect digital and direct digital). The difference is the principles of X-ray detection.
Definition of X-ray:
X-ray radiation is a type of electromagnetic radiation, which can be described by the following equation:
C=μ×λ

μ= frequency
λ =wavelength
C=speed of travel, that has a constant value

Wavelength and frequency are inversely proportional to each other.
X-ray is characterized by the wavelength.
The shorter the wavelength, the harder the radiation and more penetrative it is.

According to the principle of quantum theory, all electromagnetic resonance (X-ray included) is made of energy units, called photons, which show wave-like characteristics, and according to the laws of classical mechanics display collision phenomenon.
X-ray radiation is characterized by its intensity, that is the energy delivered by the radiation. By definition it is the energy density which perpendicularly passes through a unit of surface.

X-ray is generated by the X-ray tube.
Electrons from a high voltage, direct current cathode tube travel in an electric field, then a vacuum-tube accelerates them until they collide with a heavy metal object (anode). The accelerated electrons impact on the anode and in various steps they emit energy.

X-ray tube structure:

Cathode Wolfram
Anode Wolfram, Molibdén -Rénium
Power voltage 10-20 kilovolt
Acceleration voltage 6-600 kilovolt

The generation of X-ray
There are two types of X-ray radiations differentiated by the way of their production
-characteristic X-ray radiation
-bremsstrahlung (“deceleration”) X-ray radiation

• Characteristic radiation:

The electron beam collides with the electron shell and pushes out an electron from an inner level, which is replaced by an outer orbital electron.
The electrons on different electron-shells, have different energy levels, therefore, the replacing energy has a distinct value which means that the produced quantum has a characterictic wavelength.

• Bremsstrahlung

The emitted electron pushes through all the electron shells, and in the proximity of the atomic core it decelerates. The energy released at deceleration will produce a photon of equal energy to the deceleration. The point where the electron completely loses its momentum is called wavelength limit.

The spectrum of X-ray radiation
A continuous curve can describe the spectrum of wavelengths with superimposed characteristic peaks, that are specific to the material used as the anode. Molybdenum (used in mammography) has a characteristic peak at 35kV acceleration voltage.
Wolfram has its peak at 60-70kV. These metals are effective anodes, because their peaks occur at diagnostically applicable values. (Medical X-ray diagnostics)

The energy loss is extreme, 99% of the kinetic energy is emitted as heat and visible light.
The excitation mostly occurs on the outer electron orbit, as only one electron is pushed out.
The beam energy depends on the tube current used. The spectral composition can be altered with increasing the voltage and also by filtering the beam.

Filtering
The produced X-ray beam is made up of photons of various wavelengths. The photons unnecessary for image production or the ones that distort image quality need to be filtered. This is done by using aluminum and copper plates. Filtering also decreases the radiation burden.

Inverse-square law
X-ray beam intensity is inversely propotional to the second power of the distance measured from the source of radiation.
The dose registered at 2 m from the source is distributed on a 2x2 square (4 m2).

Absorption
As X-ray travels through matter it loses its intensity as it interacts with it. Transferring its energy to the matter it changes the state (biologic, chemical and physical) of the material.

The radiation absorption ability of a given material depends on its thickness, density and atomic number. The atomic number influences this ability by the power of 4.
As X-ray beam passes through matter five different phenomena occur. This process is called absorption.

• Transmission: It passes through without energy loss
• Rayleight scatter
• Compton scatter
• Photo effect
• Pair production

Compton scatter is the most important factor in image quality distortion.

Central projection
Distorts and enlarges the image.
X rays originate from a single point source, as they travel along straight lines they diverge from one another, which results in magnification and distortion. Structures closer to film will be magnified and distorted less than those located at a greater distance.

2.3. 2.3. The formation of X-ray image

When a homogenous radiation beam travels through a body, it is scattered into the background and in some amount it penetrates through it. As the beam is absorbed the distribution of the X-ray quantums will change, it will decrease unevenly in the plane of travel, thus causing a variable blackening effect on the film or on the detector (digital). This is how the so called beam image is produced. It is an inhomogeneous relief of the beam and it greatly depends on the quality of the matter. This beam relief has to be detected by some kind of an image transforming system. On analogue technique this happens through a large-format film-foil combination. This is the simplest detector system.

The detector is the plain film and contains silver halogenids.
The amplifier screen and the foil are made up of calcium tungstate and zinc-sulfide (blue foil).
The rare earth metal foils are made up of titanium or gadolinium (green foil).
The latter achieves better quantum utilization, and less X-ray photon is needed for image production. It is an important radiation safety (efficacy, hygiene) issue as well. Faster exposition time in turn decreases motion/blurring effects. The particles in the foil, when hit by the X-ray, fluoresce and emit light photons. Blue foils will emit 2-3 photons per one X-ray photon, while green foils will emit 8-10 light photons. Image quality will be determined by the granularity of the foil. The grainier the foil, the worse the image resolution is going to be.
The quality/resolution of an image transformation system is measured in line pair /mm units.
If image formation would occur directly on plain film the resolution could reach 50 line pairs/ mm, although then the delivered dose would be huge. This resolution ability with the use of foils decreases to 5-10 line pair / mm, at significantly lowers doses.

2.4. Factors influencing image quality

Scattered radiation decreases picture quality. It decreases image sharpness, it makes the image hazy and decreases the contrast.

(Filtering, tube, grids, Bucky, Potter-Akerlund)

Image quality is increased by:

• smaller distance between the object and the imaging plane
• greater the distance between the focus and object
• smaller focal point
• Tele-imaging can achieve the best image quality, but this is hindered by the generator’s capacity (see square law)

The quality of X-ray image considered better as it carries more information; this is influenced the most by the quality of the detector system. Image quality also depends on the size of the patient, i.e. in overweight patients more scattered radiation is produced.

Fluoroscopy
During fluoroscopy generation of X-ray beam is continuous, which is achieved using rotating anode tubes.
The image first appears on a primary zinc-cadmium sulfide or cesium iodine containing screen. An electron-optic chain in turn amplifies this image into a several thousand times stronger one, which will appear on the secondary screen. Finally, a camera delivers the resulting image on the monitor screen.

Indirect digital technique
This technique registers the image on a digital plate (such as phosphorous plate). The phosphorous plate at the end of the exposition will be scanned and the produced image delivered onto a monitor. This image is postprocessable and can be delivered to another medical workstation.

The phosphor storage disks are made up of barium-fluoro-bromine molecules immersed in phosphorous-crystals whose electrons are pushed to higher energy levels proportional to the energy of the impacting x-ray photons.
Scanning the phosphorous plate with a laser light the barium-flouro-bromine electrons show a luminescent phenomenon and by emitting light, they return to their original energy level. These light photons are detected. When the cassette is lit by normal light it loses its excited state and turns reusable again. Read-out should be performed within 15 minutes after exposure, because in 2-3hours of time the data stored in the crystals vanishes.

Direct Digital technique
The exposition takes place directly on the detector plate. The detector is a thin transistor panel sensitive to electric signals, covered by an amorphous selenium layer. X-ray induces voltage differences within the selenium which are directly proportional to the x-ray intensity that hits the layer. This electric signal is detected by the transistor panel. The voltage differences are read out by lines and columns.
Nowadays many types of detector systems exist, which all have different advantages and disadvantages. The detector system should be chosen with the its future task in mind.
The registered data are sent to the clinical informational system and workstations through the Hospital Information System (HIS) or the Radiologycal Information System (RIS). Patient data and the digital image can be combined and fused on the same image.

2.5. The clinical uses of X-ray examinations

Conventional radiography has several advantages even today and in several casees, it still has preserved its priority over other diagnostic methods. In most cases, X-ray is the first choice of examination of the diagnostic algorithm. In fact, X-ray examination is still the most frequently used modality. In case of chest screening, if the result is negative it is considered enough. All X-ray exams need to be documented with an image and a report.

Advantages of X-ray exams:

• cheap
• widely available
• can be specific for some diseases
• can set up a preliminary differential diagnostics and help in choosing diagnostic methods in order to acquire a final diagnosis the fastest and cheapest way. This is especially valuable in emergency cases

such as the differentials of acute abdominal pain, traumatology and the diagnostics of postoperative complications.

Disadvantages of X-ray examinations:

• non-specific in many cases
• certain diseases do not have radiological X-ray sings
• certain lesions are invisible on X-ray (non X-ray absorbing bile stone or renal stone)

 
X-ray diagnostic methods

• chest X-ray
• plain abdominal X-ray
• contrast X-ray examinations
• bone X-rays
• interventional radiologic examinations
• special ENT X-rays

 

2.6. Methods of chest X-ray imaging

- The so called Zeiss and Odelka examination stations use a roll film technique. Images are made from a 2 meter distance; they are seemingly small, only 10 x 10 cm or 6 x 6 cm, but have a very high resolution.
This technique was used for national chest screening exams, but it is not accepted nowadays.
-The 1:1 ration posterior-anterior (PA) chest X-ray image
-Lateral chest X-ray
-Chest fluoroscopy is always an additional examination possibility, when the chest radiograph identifies a suspicious lesion. Chest fluoroscopy is never used alone, because its radiation dose is higher, and its resolution is lower than that of normal radiographs. It also has great inter-observer variability and hard to document properly.
-Lateral decubitus or Frimann-Dahl projection

Contrast material examinations

• gastro-intestinal exams
• biliary examinations
• fistula imaging with contrast materials, fistulography
• feeding tube filling exams
• cannula positioning
• radiologic interventions
• control examinations after surgical interventions

 

Summary

• Knowledge of the physical principles of which X ray technique is based is vital for the proper evaluation of conventional radiography.
• The formation of X ray image and functioning of fluoroscopy,
• Clinical indication (its place in the diagnostic algorithm) and usage (examinable organs) of conventional radiography and fluoroscopy and
• Advantages and disadvantages of the technique were overviewed.

 
Translated by Balázs Futácsi

 

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