Introduction to imaging technichs Absorption of x-rays Radiographic and CT images depend on the fact that x- rays are absorbed to a variable extent as they pass through the body. The visibility of both normal structures and disease depends on this differential absorption
Radiography With conventional radiography there are four basic densities : gas, fat, all other soft tissues and calcified structures. X-rays that pass through air are least absorbed and therefore cause the most blackening of the radiograph calcium absorbs the most and so the bones and other calcified structures appear virtually white. The soft tissues, with the exception of fat, e.g. the solid viscera, muscle, blood, a variety of fluids, bowel wall, etc., all have similar absorptive capacity and appear the same shade of grey on conventional radiographs. Fat absorbs slightly fewer x-rays and therefore appears a little blacker than the other soft tissues.
Basic Radiographic Opacities four basic densities Air Fat Water/ST Bone Metal/+Contrast WHITE BLACK GRAY- GRAY GRAY -WHITE
Radiography The image on an x-ray film is two-dimensional. All the structures along the path of the beam are projected on to the same portion of the film. Therefore it is often necessary to take at least two views to gain information about the third dimension.
Computed tomography CT also relies on x-rays transmitted through the body. differs from conventional radiography in that : 1. a more sensitive x-ray detection system is used 2. the data is manipulated by a computer. 3. The x-ray tube and detectors rotate around the patient very small differences in x-ray absorption values can be visualized and range of densities recorded is increased approximately 10-fold. Not only can fat be distinguished from other soft tissues, but gradations of density within soft tissues can also be recognized, e.g. brain substance from cerebrospinal fluid, or tumor from surrounding normal tissues.
Spiral CT With modern spiral (also known as helical) scanners, the patient is moved past an array of detectors within the machine so the data at multiple adjacent levels are collected continuously, during which time the x-ray beam traces a spiral path to create a 'volume of data’ within the computer memory. The data obtained from each set of exposures are reconstructed in to an image by computer manipulation
Multislice CT Multislice (multidetector CT) is a recent innovation whereby up to 16 slices (sections) can be acquired during one rotation of the x-ray tube. Multislice CT enables the examination to be performed much faster, thereby allowing many more thinner slices which in turn allows high quality multiplanar and three-dimensional reconstructions, as well as CT angiography.
CT Angiography Rapid intravenous injections of contrast media result in significant opacification of blood vessels, which, with multiplanar or three-dimensional reconstructions, can be exploited to produce angiograms. CT angiography, along with magnetic resonance angiography, is gradually replacing conventional angiography.
Contrast agents in conventional radiography and CT Radiographic contrast agents are used to visualize structures or disease processes that would otherwise be invisible or difficult to see. Barium is widely used to outline the gastrointestinal Tract. all the other radio-opaque media rely on iodine in solution to absorb x-rays. Iodine-containing solutions are used for urography, angiography and IV contrast enhancement at CT.
Ultrasound very high frequency sound is directed into the body from a transducer placed in contact with the skin. to make good acoustic contact, the skin is smeared with a jelly-like substance. As the sound travels through the body, it is reflected by the tissue interfaces to produce echoes which are picked up by the same transducer and converted into an electrical signal. Since air, bone and other heavily calcified materials absorb nearly all the ultrasound beam, ultrasound plays little part in the diagnosis of lung or bone disease. The information from abdominal examinations may be significantly impaired by gas in the bowel that interferes with the transmission of sound.
Ultrasound Fluid is a good conductor of sound, and ultrasound is a particularly good imaging modality for diagnosing cysts, examining fluid-filled structures such as the bladder and biliary system. Ultrasound can also be used to demonstrate solid structures that have a different acoustic impedance from adjacent normal tissues, e.g. metastases. Cysts or other fluid-filled structures produce large echoes from their walls but no echoes from the fluid contained within them. Also more echoes than usual are received from the tissues behind the cyst, an effect known as acoustic enhancement. with a calcified structure, (e.g. a gall stone), there is a great reduction in the sound that will pass through, so a band of reduced echoes, referred to as an acoustic shadow is seen behind the stone.
Ultrasound scan of longitudinal section through the liver and right kidney. A cyst (C) is present in the upper pole of the kidney.
Ultrasound scan of gail bladder showing a large stone in the neck of the gall bladder (white arrow). Note the acoustrc shadow behind the stone (horizontal arrows).
Ultrasound The time taken for each echo to return to the transducer is proportional to the distance travelled. Also by knowing the velocity of sound in tissues it is possible to measure the distance between interfaces. Ultrasound images are capable of providing highly detailed information : for example, very small lesions can be demonstrated
Magnetic Resonance Imaging The basic principles of MRI: nuclei of certain elements align with the magnetic force when placed in a strong magnetic field. Hydrogen nuclei (protons) in water molecules and lipids are responsible for producing anatomical images. If a radiofrequency pulse at the resonant frequency of hydrogen is applied, a proportion of the protons change alignment, flipping through a preset angle, and rotate in phase with one another. Following this radiofrequency pulse, the protons return (realign) to their original positions.
Magnetic Resonance Imaging As the protons realign (relax) they induce a radio signal which, although very weak, can be detected and localized by antenna coils placed around the patient and an image representing the distribution of the hydrogen protons can be built up. The strength of the signal depends not only on proton density but also on two relaxation times, T1 and T2; T1 depends on the time the protons take to return to the axis of the magnetic field, and T2 depends on the time the protons take to dephase
Magnetic Resonance Imaging Advantages of MRI over CT : information can be directly imaged in any plane. MRI gives very different information to CT( brain and spinal cord, spine, pelvic organs, biliary system, urinary tract and heart, bone marrow and the soft tissues inside and surrounding joints). it involves no ionizing radiation, and no adverse biological effect. Disadvantages of MRI: In most instances, MRI requires a longer scan time (often several minutes) compared to CT calcified tissues do not generate any signal at MRI.
Contrast agents for MRI The most widelv used agents are gadolinium compounds which only cross the blood-brain barrier when it is damaged by disease and which concentrate in tissues and disease processes with a high blood supply.
Chest : Imaging techniques The plain chest radiograph: Routine chest radiography consists of a posteroanterior (PA) view also known as a frontal view, with the optional addition of a lateral view, on full inspiration and patient in the upright position.
The abnormal chest radiograph Localizing the lesion: If the shadow is surrounded on all sides by aerated lung it must arise within the lung. many masses will clearly be within the mediastinum. when a lesion is in contact with the pleura or mediastinum : If the shadow has a broad base with smooth convex borders projecting into the lung and a well-defined outline it is likely to be pleural, extrapleural or mediastinal in origin.
(a) Extrapleural mass. The mass has a smooth convex border with a wide base on the chest wall(myeloma lesion arising in a rib). (b) located pulmonary mass such as a primary carcinoma of the lung. (b) (a)
The abnormal chest radiograph The silhouette sign : It is an invaluable sign for localizing disease from plain chest radiographs. An intrathoracic lesion touching a border of the heart, aorta or diaphragm obliterates that border on the chest radiograph. 1. localize a shadow by observing which borders are lost. 2. diagnose disorders such as pulmonary consolidation or collapse even when the presence of an opacity is uncertain; causing loss of their normally sharp outlines.
The abnormal chest radiograph Any abnormal intrapulmonary shadow is placed into one or more of the following broad categories: 1. air-space filling a) pulmonary oedema b) Pulmonary consolidation 2. Pulmonary collapse (atelectasis) 3. spherical shadows 4. line shadows 5. widespread small shadows. The presence of cavitation or calcification should be noted.
The abnormal chest radiograph Air-space filling: Air-space filling(infiltration) means the replacement of air in the alveoli by fluid or, rarely, by other materials(infiltrate). The fluid can be either a transudate (pulmonary oedema) or an exudate. The causes of an alveolar exudate include infection, infarction, pulmonary contusion, haemorrhage and immunological disorders, e.g. collagen vascular diseases and extrinsic allergic alveolitis.
The abnormal chest radiograph The signs of air-space filling are: A shadow with ill-defined borders, except where the opacity is in contact with a fissure. An air bronchogram. normally, it is not possible to identify air in the bronchi within normally aerated lung, because the walls of the bronchi are too thin and air filled bronchi are surrounded by air in the alveoli, but if the alveoli are filled with fluid, air in the bronchi contrasts with the fluid in the lung. This sign is seen to great advantage on CT. The silhouette sign.
ill-defined pulmonary shadowing in both mid- zones
Pneumococcal pneumonia - massive opacity left lung with air bronchogram.
The abnormal chest radiograph pulmonary oedema: The causes of pulmonary oedema are broadly divided into: 1. Cardiogenic pulmonary oedema 2. Non-cardiogenic pulmonary oedema(e.g. ARDS) There are two radiographic patterns of pulmonary oedema: alveolar and interstitial. Since oedema initially collects in the interstitial tissues of the lungs, all patients with alveolar oedema also have interstitial oedema.
The abnormal chest radiograph Pulmonary oedema: Alveolar oedema is always acute. It is almost always bilateral. In the early stages, the shadowing is maximal close to the hila and fades out peripherally, leaving a relatively clear zone around the edges of the lobes and involves all the lobes (butterfly or the bat's wing pattern). Later the shadowing becomes more widespread, but is often most obvious in the lower zones. Interstitial oedema causes thickening of the interstitial tissues of the lungs. The hallmarks of interstitial oedema are septal lines and thickening of the pleural fissures.
The abnormal chest radiograph Pulmonary consolidation (alveolar infiltrates): Lobar consolidation produces an opaque lobe, except for air in the bronchi (air bronchograms). Patchy consolidation(one or more patches of ill- defined shadowing ) is usually due to: Pneumonia, infarction, contusion and immunological disorders.
The abnormal chest radiograph Cavitation Cavitation( abscess formation) within consolidated areas in the lung may occur with many bacterial and fungal infections. Abscess formation is only recognizable once there is communication with the bronchial tree, allowing the liquid centre of the abscess to be coughed up and replaced by air and an air-fluid level may be Present. CT is better and more sensitive than plain films for demonstrating cavitation.
The abnormal chest radiograph Pulmonary collapse (atelectasis): The common causes of collapse (loss of volume of a lobe or lung) are: bronchial obstruction and pneumothorax or pleural effusion. The best sign of lobar collapse is displacement of structures
The abnormal chest radiograph Spherical shadows (lung mass, lung nodule): The usual causes of a solitary pulmonary nodule are: bronchial carcinoma/bronchial carcinoid, benign tumour of the lung, infective granuloma, tuberculoma,fungal granuloma, metastasis, lung abscess, rarely, spherical (round) pneumonia. If the centre of the mass undergoes necrosis and is coughed up, air is seen within the mass. An air-fluid level may be visible on erect chest radiographs (cavitation). Calcification can be difficult to recognize on plain chest radiography. CT is of great value in detecting calcification in a solitary pulmonary nodule.
Plain Abdomen The standard plain films of the abdomen are supine and erect AP views. An alternative to the erect AP view in patients unable to sit or stand is a lateral decubitus view Relatively large amounts of gas are usually present in the stomach(air-flurd level beneath the left hemidiaphragrh) and colon in a normal patient.
Plain Abdomen Radiologic manifestations of Malabsorption: The signs that may occur with any of the causes of malabsorption are: 1. Small bowel dilatation, the jejunum being affected more than the ileum. 2. Thickening of mucosal folds. 3. The barium may be diluted by the excessive fluid in the Small bowel and so appears less dense. 4. Excess fluid in the lumen leads to segmentation of the barium column into separated clumps and flocculation of barium in severe disease.
Normal Intravenous pyelogram An injection of x-ray contrast media is given to a patient via a needle or cannula into the vein. The contrast is excreted or removed from the bloodstream via the kidneys, and the contrast media becomes visible on x-rays almost immediately after injection.