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IMAGING METHODS AND SPECIAL PROCEDURES
A) CONVENTIONAL RADIOGRAPHY – X-RAYS
1. Plain films:
X-rays are a form of electromagnetic radiation, their frequency and energy being much greater than visible light. X-rays are produced in an X-ray tube by focusing a beam of high energy electrons onto a tungsten target. On hitting the target X-rays are produced which are directed out of the tube through a “window”. They pass through the patient onto X-ray film. This produces an image on processing, similar to the process in photography.
While passing through the patient the X-ray beam is decreased in energy according to the density & atomic number of the various tissues through which the beam passes. This process is known as attenuation.
X-rays turn film black, therefore the less dense parts of the body which allow more X-rays to pass through, will appear darker on the film e.g. air. The film is enclosed between 2 fluorescent screens within a metal cassette. These screens emit light when exposed to X-rays. The film records the visible light emitted by the intensifying screens in response to irradiation by X-rays.
A high voltage generator supplies the required power to the X-ray tube. A collimator is placed at the tube exit port to limit the extent of the X-ray field. An electronic timer is used to keep the X-ray exposure to a precise, finite duration.
There are 5 principle densities recognised on plain X-rays:
- air/gas = black (e.g. lung, bowel, stomach)
- fat = dark grey (e.g. subcutaneous tissue layer, retroperitoneal fat)
- soft tissues/water = light grey (e.g. solid organs, heart ,muscle, bladder)
- bone = off white
- contrast material/metal = bright white
An object will only be seen on conventional radiography if its borders lie against tissue of different density, e.g. right heart border is only seen because it lies against aerated lung which is less dense. If that part of the lung collapses & loses its’ air the R heart border is no longer seen.
There are several factors contributing to the quality of the film produced
-obesity. The more tissues the X-ray beam has to pass through the more the beam will be attenuated. A larger dose will be needed to produce the same film blackening. Increasing the kilovoltage (KV) decreases the contrast on the film. Compression may be used to advantage when X-raying certain parts of the body with a low inherent contrast e.g. breast
-movement. The slightest movement will result in unsharpness of the image. This is especially important in the lungs when slight blurring may mask small miliary opacities. It is important that immobilisation devices & accessories are available to the technicians in order to produce satisfactory films. In an obese patient a longer exposure time will be necessary which is more likely to result in movement unsharpness
-magnification. As the X-rays fan out obliquely from source some magnification is inevitable. The amount of magnification will depend on the distance the X-ray tube is from the film & also the distance the object is from the film. If the X-ray tube is placed nearer to the object & film then magnification will be more marked. If an object is placed further away from the film then again the magnification will be greater
-scatter. As X-rays pass through the patient some are scattered & do not pass through in a forward direction, their exit from the patient being at random. This results in blackening of the film at the expense of detail. This means that for thicker parts of the body it is necessary to use a grid to absorb the scattered rays before they reach the film. The use of a grid inevitably leads to a higher patient dose
Plain films are particularly useful for:
- Chest
- Abdomen
- Trauma
- Skeletal diseases
2. Tomograms: are performed using conventional X-rays. They are useful when an object is obscured by overlying structures e.g. during an intravenous pyelogram when the kidneys are obscured by bowel gas.
The X-ray tube & film move in opposite directions blurring out structures not at the level of interest while structures in the plane of interest are seen in sharper outline. The pivot height is set according to the depth of the lesion within the patient.
In modern practice this technique is being replaced by newer cross sectional imaging techniques although it is still useful in intravenous pyelography and some orthopaedic cases. It used to be commonly performed for chest opacities before the advent of computed tomography.
3. Fluoroscopy: is real time radiography. Fluoroscopy allows continuous viewing of an X-ray image & permits live visualisation of dynamic events. A continuous low power X-ray beam is passed through the patient An X-ray image intensifier coupled to a TV camera converts the X-ray energy to visible light, the fluoroscopic image is then viewed on a monitor, located in the fluoroscopy room. Fluoroscopy is used for many radiological procedures. A few of these are barium studies, arteriograms, myelograms and interventional techniques.
4. Digital radiography: this uses the same basic principles but the X-ray film is replaced by a digital screen. The information received by the screen is then manipulated using computers and the resulting image viewed on a monitor. Computed tomography, magnetic resonance imaging and ultrasound already use a digital method of imaging. Digital imaging applied to plain film radiography is a relatively new technique leading to a film-less department as X-ray films are no longer needed. The system is called PACS (picture archival & communicating system). It is very expensive to install but has several advantages over use of X-ray film:
-reduction in exposure
-digital enhancement means that all images are of adequate quality & do not need to be repeated
-images can be transferred out of the department to other locations
-no problem finding film storage space
-rapid finding of previous images for comparison
The system however is expensive to install and maintain. Many countries lack suitable engineers to install and maintain the system
B) COMPUTED TOMOGRAPHY – CT SCAN
Computed tomography uses X-rays to produce cross sectional images. The patient lies on a movable table within a round gantry. The gantry contains an X-ray tube opposite a set of detectors. These rotate around the patient at the level of interest while a collimated X-ray beam passes through the patient. X-rays passing through the patient are detected by photomultiplier tubes and the information stored in a digital format. Computer analysis of the digital readings gives information about the absorption patterns of each tissue in the body. This is displayed as an image. Owing to the use of computer analysis a much greater array of densities can be displayed than on conventional X-ray films. This allows differentiation of solid organs from each other & from pathological processes such as tumour or fluid collections. It also makes CT extremely sensitive to the presence of minute amounts of calcium or contrast material.
As with plain radiography, high-density objects cause more attenuation of the beam and are displayed as lighter grey than objects of lower density. White and light objects are said to be of “high attenuation” while dark grey and black objects are said to be of “low attenuation”. Absorption values are expressed on a scale of +1000 units for bone, the maximum absorption of the X-ray beam, to –1000 units for air, the least absorbent. These units are called Hounsfield units. The value for water is zero.
The image information can be manipulated by the computer to display the various tissues of the body in different degrees of lightness and darkness. This is called “altering the window settings”. For example in the chest, the mediastinum is imaged at a different window setting to that of the lungs. Window width is used to select a range of absorption values then displayed as a grey scale. Attenuation values below this range will appear black, whereas attenuation values above this range will appear white. Window level is used to select the centre point of the window. Different windowing is needed for different parts of the body and this technique can be used to accentuate a subtle difference in tissue density.
When looking at a CT scan it is important to look at the images with different window settings rather than just relying on the images printed out by the technician, which may not always be the optimum.
Each picture represents a section through the body. The thickness of the section (or slice) can be varied between 2-10mm. Tissues lying outside this setting are not imaged. A series of slices are taken to cover the region. Thinner slices give greater detail. Imaging of the chest and abdomen can only be done in the transverse plane.
The latest generation of CT scanners are spiral scanners. With spiral CT the patient is moved continuously through the gantry tunnel while the X-ray tube and detector system rotate continuously. Previously only one section of the body could be imaged at any one time and there was delay moving the table into the next position. With spiral scanning the scanning time is greatly reduced and it is possible to scan a whole chest in one breath hold.
By computer manipulation it is possible to obtain accurate 3D reconstruction due to the ability with spiral CT to process data as overlapping sections. These overlapping sections can be reformatted to give high resolution images in coronal, sagittal, transaxial and oblique anatomical planes.
The principal advantages of spiral computed tomography are much faster scanning times and the ability to reconstruct images enabling blood vessels to be imaged (CT angiography).
Image quality depends on many factors such as slice width, scanning speed, matrix size but it also depends on the attenuation coefficients of adjacent structures. In abdominal scanning good visualisation of structures is dependent on the presence of abdominal fat. The absence of fat makes it difficult to differentiate between organs & structures. This is the opposite of ultrasound when imaging is much better in thin people.
Some scans are performed without any contrast enhancement but when imaging the abdomen oral contrast medium is given to outline the stomach and bowel which enables it to be differentiated from other structures. This is normally very dilute barium or dilute gastrografin. Intravenous contrast is also commonly used for a number of reasons:
-differentiation of normal blood vessels from abnormal masses (hilar vessels versus lymph nodes)
-to make an abnormality more apparent (liver metastases)
-to demonstrate the vascular nature of a mass
Main applications for ComputedTomography
Head
-trauma
-tumours
-stroke
-complications of HIV
-brain abscess
Chest
-mediastinal disease
-tumour staging prior to surgery
-diffuse lung disease using high resolution techniques
-pleural disease
-detection of early metastatic disease
-aortic aneurysm
Abdomen:
-liver lesions
-pancreas, tumour, pancreatitis
-trauma
-tumour staging
-retroperitoneum
-undiagnosed abdominal masses
-to identify an infective focus in PUO
Spine:
- vertebral fractures
-disc prolapse
-infection
-tumour
Musculo-skeletal
-calcaneal fractures
-recurrent dislocation shoulder – CT arthrography
-fractures pelvis/acetabulum
-fractures tibial plateau
-soft tissue tumours
We must remember that the dose rate is high with CT scanning compared to conventional X-rays.
I head CT scan gives a dose equivalent to 115 CXRs
I chest CT scan gives a dose equivalent to 400 CXRs
I abdominal CT gives a dose equivalent to 500CXRs
X-RAY HAZARDS
Is the radiation used in diagnostic radiology harmful? Potentially yes.
Electromagnetic radiation consists of several types of rays. Radio waves have the longest wave length and do not harm the body in any way. As the wavelengths decrease in size radiation becomes potentially harmful.
Radio waves
Micro-waves
Infra red rays
Light rays
Ultraviolet rays------
X-rays
Gamma rays
X-rays and gamma rays have the ability to change molecules into ions within the body. They are called ionising radiation. This type of radiation may harm the body .
In actual practice, adverse health effects from ionising radiation in diagnostic radiology is rare. There is still disagreement over the risk of biologic injury from low levels of radiation delivered over an extended period of time. The available data do not provide a definitive answer as radiation induced cancers are indistinguishable from non radiation induced cancers. However, it is believed that any amount of radiation is potentially harmful.
Radiation hazards occur as a result of damage to cells caused by radiation. This damage takes many forms:
-cell death
-mitotic inhibition
-chromosome damage/genetic damage leading to mutation
Actively dividing cells are particularly sensitive, these are the gonads and bone marrow
The nature and degree of cell damage vary according to:
-radiation dose
-dose rate
-irradiated volume
-type of radiation
In general 2 types of effects are seen as a result of radiation damage
- Stochastic effects: these are effects in which the probability of occurrence increases with radiation exposure. Examples of these are carcinogenesis and genetic effects. The probability but not severity is related to the quantity of radiation
- Deterministic effects: these are associated with a threshold radiation dose, below which the effect is not observed & above which the probability that the effect will occur is virtually 100% & the severity increases with the dose. Examples are
-skin erythema
-loss of hair
-skin desquamation
-cataracts
-fibrosis
-depression of the bone marrow- anaemia
Dose is now measured in dose equivalent which takes into account the fact that some kinds of radiation can produce more damage in tissue than others even though the absorbed dose may be the same. The equivalent dose is the absorbed dose multiplied by a weighting factor.
The number of chest X-rays which would be needed to reach the erythema skin dose threshold (2-3Gy) is approx. 10,000 The number of CT studies to produce the same effect is only 100, but note fluoroscopy time only 30mins.
Background radiation:
All inhabitants of the earth receive a certain amount of radiation exposure each year in the form of “natural background” radiation. This comes from rocks and soil, from outer space, from radon gas produced in the ground and natural isotopes of elements found in living tissues. The total average background radiation including cosmic and terrestrial radiation is 1-2 mSivert per year. Each time we fly in an aeroplane we receive radiation from cosmic rays. Flying 200 air miles has been equated to give the same radiation dose as a mammographic examination. A plain abdominal X-ray is equivalent to 6 months of natural background radiation while an intravenous pyelogram averages 14 months of background radiation.
The radiation risk is really unknown but based on survivors of the atomic bomb explosions in Japan it is calculated that if 100 people are exposed to 1 Sivert of radiation, 5 will theoretically develop a fatal cancer. A dose of 5-6Sv over a short period of time leads to acute radiation sickness. The skin erythema dose threshold is 2-3Sv.
In considering the possible risks caused by radiation exposure factors are considered:
- Probability of developing a cancer
- Probability of severe hereditary effects
- Length of life lost if harm occurs
In diagnostic practice only Stochastic effects need be considered. A Barium Enema examination gives a dose of approximately 7 mSv. There is some legitimate reason to be concerned about these effects since they have no known dose threshold. This implies that even the smallest amount of radiation exposure may increase the probability of the induction of genetic effects or malignancy.
Hospital personnel in the course of normal clinical work receive at most only a small radiation exposure, due primarily to scatter. X-ray workers fall well below the set dose limit. Lead aprons reduce the radiation exposure by 95%. It is important when using fluoroscopy that the exposure time should be kept as short as possible and lead aprons worn by all staff present in the room.
Pregnancy:
Radiation before implantation is thought to have no effect or to prevent implantation so that the embryo is lost at the next menstrual period. Organogenesis commences soon after the time of the first missed menstrual period and continues for the next 3-4 months. Hence, during this time, the foetus is considered to be radiosensitive. Exposure to radiation may result in developmental problems such as a small head size & mental retardation. Examinations of the abdomen or pelvis should be delayed if possible to a time when foetal sensitivity is reduced i.e. after 24 weeks or ideally until after delivery.
Protection in practice:
The aim is :
- To prevent deterministic effects
- To limit the probability of stochastic effects by keeping all justifiable exposures as low as is reasonably achievable. With this in mind the following guidelines are used for radiographic exposures:
-each exposure is justified on a case-by-case basis
-minimise number of X-rays taken
-minimise fluoroscopy screening time
-focus beam accurately to area of interest
-only trained personnel should operate equipment
-minimise use of mobile equipment
-use ultrasound whenever possible
-use restraining devices in children
-gonadal shield protection
-only necessary people should be present in a room where X-ray procedures are being performed
-staff should wear lead aprons
-at no time should anyone other than the patient be irradiated directly by the primary beam
-all X-ray rooms should have lead lining in their walls, ceilings & floors
ULTRASOUND
Ultrasound uses high frequency sound waves to produce cross sectional body images. The basic component of the ultrasound probe (transducer) is the piezoelectric crystal. Excitation of the crystal by a small electric current causes it to emit ultra high frequency sound waves (the piezoelectric effect). Sound waves are reflected back to the crystal by the various interfaces in the body and these are turned into small electric signals which are analysed by a computer and represented as a cross sectional image. A moving image is obtained as the transducer is moved over the body. Sections can be obtained in any plane and viewed on a monitor.