General X-ray
Fluoroscopic Screening
OPG
Mammography
Bone Densitometry
Angiography
Ultrasound
Doppler Ultrasound
Echocardiography
CT Scanning
MRI Scanning
Nuclear Medicine

General X-ray

The familiar 'X-ray' was the earliest of the currently utilized diagnostic imaging techniques to be applied to clinical practice. X-rays are still widely used in general medicine particularly for imaging bones and the chest. A radiograph is a visible photographic record produced on a special type of film by X-rays passing through a body part. A specially trained radiographer positions the patient on the X-ray table and aligns the X-ray beam. X-rays have enough energy to pass through body tissue, however some X-rays are absorbed or scattered. Dense tissue such as bone absorbs more radiation than soft tissue. The remaining X-rays pass into a film cassette where they are absorbed by a chemical layer in the cassette known as a phosphor screen. The phosphor emits light proportional to the intensity of the X-ray beam striking it. In turn, this light exposes the X-ray film contained in the cassette. The exposed film is then processed in an automatic developer.

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Fluoroscopic Screening

Screening or 'fluoroscopic screening' enables radiologists to visualize X-ray images in real time on a television monitor. This technique is used to perform a variety of X-ray procedures. In most instances this would involve the administration of some form of 'contrast' agent to outline the region of interest. Barium contrast is used to coat the gastrointestinal tract and may be swallowed as in a barium meal or introduced via the rectum in a barium enema. Intravenous contrast is used to define the veins in a venogram or to outline an abscess cavity in a sinogram. In all these procedures, the radiologist uses the X-ray images displayed on the television monitor to guide the examination and to position the region of interest to obtain 'spot' films. The spot films provide more detail than is available on the monitor and act as a permanent record of the examination. Spot films may be either conventional X-ray films or alternatively digital images that are temporarily stored on a computer system to be subsequently printed on laser film.

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OPG

OPG is an abbreviation of OrthoPanTomoGram. 'Ortho' as in orthodontics refers to the teeth. 'Pan' refers to the panoramic display of the teeth produced by the technique. A Tomogram is an X-ray image that is focussed in a single plane of the patient. The OPG machine is specifically designed to produce panoramic tomographic X-rays of the teeth, jaws and temperomandibular joints. The physical principles are similar to conventional tomograms however with an OPG the plane of focus is curved to match the curve of the jaws. The images provide an overview of the state of the dentition as well as information regarding the bones of the jaw (the mandible and maxilla), the sinuses in the upper jaw and the joints between the jaw and the skull (the temperomandibular joints). The OPG machines can also perform a 'lateral cephalogram' which is a standard lateral view of the skull. The information assists dentists, dental and medical specialists in diagnosing abnormalities and in planning treatment for known problems.

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Mammography

Mammography is an X-ray examination of the breasts. A dedicated X-ray machine is required for mammography. Compared with conventional X-ray techniques, mammograms are obtained with much lower energy X-rays of around 20,000 volts. The X-rays are generated using a special fixed anode X-ray tube which has a molybdenum target rather than a tungsten target. The softer radiation produced helps to enhance the contrast between different types of breast soft tissue and makes abnormalities such as tumours and cysts more readily visible. Fine calcification, a feature of breast tumours, is also rendered more visible. The tube has a small 'focal spot' which gives excellent spatial resolution i.e. fine detail. Special X-ray film cassettes are also required to help maximize detail and contrast while minimize radiation exposure. Every step in the production and interpretation of a mammogram requires a fastidious approach and quality control is of major importance as  breast cancer is a major cause of death and morbidity. Mammography is the best available technique to detect breast cancers at an early, curable stage.

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Bone Densitometry
Bone densitometry is a technique used to determine the average bone mineral density (BMD) within a particular region. Dual energy X-ray absorptiometry or 'DEXA" is the most accurate method available to perform this function. DEXA machines have an X-ray source consisting of a stationary anode X-ray tube which after filtration produces a pencil beam of X-rays of a specific energy. After passing through the region of interest, the X-rays which have not been absorbed in the body strike a detector consisting of sodium iodide. The X-rays cause a 'scintillation' of light in the detector. This light is measured and the reading digitized for analysis by a computer. By obtaining two sets of data using two different energies of X-ray it is possible to estimate the amount of calcium in the sample. In the clinical setting the regions most commonly scanned are the hip, lumbar spine and less often the forearm. The results obtained from a particular patient are compared with results from age matched and from young adult control groups. From the results, an estimate of fracture risk is obtained. Serial studies are used to monitor bone density especially in patients receiving treatment for known osteoporosis.

Osteoporosis is defined as low bone mass leading to increased bone fragility and greater susceptibility to fractures. It is a condition which is more common with increasing age and is a major contributor to the incidence of fractures in people over 60 years of age. Common sites of osteoporotic fractures are the thoracic and lumbar spines, the hip, the upper arm (humerus) and the wrist (radius).

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Angiography
Digital Angiography is a diagnostic procedure which produces X-ray pictures of blood vessels. It is performed by the insertion of a small flexible catheter into an artery (Angiogram) or vein (Venogram). The usual point of entry is via the groin vessels. Catheters are flexible, small bore hollow tubes about the diameter of coat hanger wire. The radiologist first inserts a small needle into the blood vessel through which a guide wire is inserted. The catheter is threaded over the guide wire and into the lumen of the blood vessel. By monitoring the catheter on a T.V. screen, the radiologist can carefully guide the catheter tip to the region of interest. Once in place, X-ray contrast is injected through the catheter often via a pressure injector. The contrast fills the lumen of the blood vessel which then becomes visible on X-ray images. The pictures enable the diagnosis of narrowings, occlusions, abnormal dilatations or abnormal communications of blood vessels. Before the examination, patients are interviewed by the radiologist and the nurse. Information about the procedure is conveyed and any questions answered. A consent form must be signed prior to the procedure.

Originally angiograms were obtained using conventional cut film. Rapid filming sequences were possible due to a device known as a film changer. In the 1980s, with the development of more powerful computers and suitable hardware, the images could be acquired digitally. The image the radiologist sees on the T.V. screen during the procedure is derived from a device called an Image Intensifier. This consists of a vacuum tube device that is placed near the patient. The X-rays form an image on the face of the device. This image is amplified many fold by a photomultiplier tube. Subsequently the image is viewed by a video camera. The output of the video camera is digitized and fed into a computer system. The advantage over the conventional technique is faster image acquisition rates, instantaneous availability of the images and a lower radiation dose to the patient. The digital images can be manipulated in various ways. A common technique is to acquire an image of the patient prior to the injection of contrast. This image, known as the subtraction mask, is digitally subtracted from the later images in the sequence which contain contrast. The result is an image where the underlying body structures virtually disappear leaving only a picture of the injected contrast.

The Procedure
After positioning the patient on the table, the puncture site in the groin is shaved, cleansed with an antiseptic solution and sterile drapes applied. Local anaesthetic is injected into the tissues at the puncture site prior to introduction of the catheter. Once the catheter is manipulated into position, contrast is injected usually by a pressure injector which automatically regulates the volume and rate of the injection. When the contrast is injected, some patients notice a temporary hot flushed feeling or a metallic smell or taste. A series of digital images are acquired with each injection of contrast. These images are available for review immediately after acquisition. Several 'runs' are often required. At the completion of the procedure, the catheter is withdrawn and firm pressure applied to the puncture site for several minutes to allow sealing of the artery. After an angiogram rest in bed is required for four hours usually in a recovery area or occasionally in a hospital ward. Movement is restricted during this time to allow the seal at the puncture site to strengthen. Vital signs such as pulse rate and blood pressure are monitored and the puncture site and pulses in the feet checked regularly. A responsible person should be available to drive the patient home. Following an angiogram patients are required to rest for a few days and avoid strenuous activities.

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Ultrasound
Ultrasound is a technique for visualising internal structures in the body using sound waves. The sound waves are of a much higher frequency than audible sound. The sound waves are produced by a small hand held device known as a transducer or ultrasound probe. The probe consists of a piezoelectric crystal that vibrates when a voltage is applied. These vibrations are transmitted from the probe into the body and travel through the soft tissues until they hit an interface between two different structures. At a tissue interface some echoes are reflected back to the transducer and some are transmitted deeper into the body. The reflected sound waves travel back to the probe and set up small vibrations in the transducer crystal that produced the sound in the first place. The vibrations produce a small voltage across the crystal. The voltage is read and digitised for subsequent computer analysis. By measuring the time taken for the sound to return to the transducer it is possible to calculate the exact depth the echo came from. A bright dot is placed on a monitor at an appropriate position corresponding to the depth of the echo and of a brightness determined by the strength of the echo. By analysing thousands of echoes returning from multiple positions in the body, a composite picture is built up. The result is a two dimensional image of the region of interest.

The Procedure
Ultrasound has a wide range of clinical applications including imaging of the abdomen, pelvis, breast, small parts such as the thyroid gland, salivary glands, scrotum, musculoskeletal system including muscles and tendons, lumps and bumps etc. Specialised fields of ultrasound imaging include echocardiography and vascular ultrasound. The ultrasound examination is performed by a specially trained radiographer known as an 'ultrasonographer'. The radiologist may also perform part or all of the examination. A gown may be required depending on the region being examined. A gel is applied to the skin surface. This gel allows the probe to glide freely across the skin and also acts as a coupling medium which greatly improves transmission of the sound waves into the body. The procedure is painless and harmless and takes from ten minutes up to half an hour or more to perform. Interested observers can be allowed in the room particularly with obstetric scanning. Usually, however, a preliminary scan is performed prior to admission of interested parties to ensure a thorough examination without distraction. A series of representative pictures is obtained and recorded on hard copy laser film. The images are reviewed by a radiologist who issues a report for the referring doctor. The gel is wiped off the skin at the end of the procedure.

Transvaginal probes are designed to be inserted into the vagina to enable more detailed images of the pelvic structures. Transvaginal imaging is only performed after full explanation of the procedure and with consent of the patient. A hysterosonogram is a transvaginal scan obtained after the introduction of saline into the uterine cavity via a small catheter. This technique enables definition of small lesions growing into the uterine cavity that cannot be adequately delineated with conventional ultrasound imaging.

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Doppler Ultrasound
Doppler or Vascular ultrasound is a technique for visualising arteries and veins in the body using sound waves. The basic principles of ultrasound are described at the ultrasound webpage. There are several different ultrasound modes or techniques that are used in examining blood vessels. Two dimensional ultrasound images directly visualise the vessel walls and lumen and can demonstrate plaques causing narrowings in arteries, dilatations of arteries known as aneurysms, clots within the blood vessels etc. Duplex scans use the Doppler principle to study the velocity, direction and character of flowing blood through the vessels. Echoes returning from a moving target experience a shift in frequency that is proportional to the velocity of blood flow. The frequency shift can be analysed by the systems computer to estimate the velocity of blood flow. Narrowings in arteries are associated with increased velocities and therefore an estimate of the severity of a narrowing can be obtained from the velocity of blood flow recorded at the site of narrowing. Colour Doppler mode superimposes Doppler information on the two dimensional images by colour coding flowing blood according to the direction and velocity of flow. This provides an instantaneous graphic display of the character of blood flowing through a vessel. A new technique known as 'B' flow imaging directly visualises flowing blood in a vessel lumen by amplifying signals from moving targets. Using a combination of the above techniques it is possible to document the presence and severity of narrowings and complete occlusions, the presence of blood clot in the vessels, the extent and size of aneurysms or varicosities, the presence of abnormal connections between arteries and veins and any anatomic variations in vascular anatomy. The examination is performed by a specifically trained ultrasonographer. A report is issued to the referring doctor by the radiologist or physician on site.

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Echocardiography
Echocardiography is simply an ultrasound examination of the heart. The instrumentation and techniques for echocardiography have evolved rapidly over the last 20 years. During the examination, various different ultrasound modes or techniques are employed. 'M' mode produces a graphic tracing of the movement of a cardiac structure such as a valve leaflet over time. Two dimensional echocardiography allows real time cross sectional imaging of the heart from multiple different projections. This technique provides most of the information regarding the anatomy of the heart. Most measurements are obtained using this technique. Doppler echocardiography uses ultrasound to study the velocity, direction and character of flowing blood through the structures of the heart. Echoes returning from a moving target experience a shift in frequency that is proportional to the velocity of blood flow. The frequency shift can be analysed by the systems computer to estimate the velocity of blood flow. Colour Doppler mode superimposes Doppler information on the two dimensional images by colour coding flowing blood according to the direction and velocity of flow. Using a combination of the above techniques, information about the structure and function of the heart is obtained and any abnormalities documented. The examination is performed by a specifically trained ultrasonographer with subspecialty skills in echocardiography. A report is issued to the referring doctor by the radiologist or physician on site.

The Procedure
Echocardiography is performed in a standard ultrasound room. The patient lies on the examination couch and is rolled slightly to one side or the other to improve visualization of cardiac structures. Ultrasound gel is applied to the skin surface and the probe is placed in contact with the skin. Various views are obtained from either side of the sternum, from the left lower chest and from beneath the rib cage near the midline. The procedure is recorded on videotape and on hard copy laser film. An examination takes between 30 minutes to an hour depending on the ease of obtaining good images and on the type of pathology being investigated. The technique is completely harmless and does not cause any discomfort.

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CT Scanning
The technique of CT scanning was developed in 1973 by Hounsfield. A thin fan beam of X-rays generated by a conventional X-ray tube passes through a single 'slice' of a patient through to a bank of X-ray detectors. The detectors can be manufactured as a 'solid state' component or may be composed of the inert gas xenon. The detectors produce a voltage proportional to the intensity of incident X-rays. The voltage is read and digitized for subsequent processing in a computer. By taking thousands of readings from multiple angles around the patient, a two dimensional image can be reconstructed. 'CT' stands for Computerized Tomography, tomography referring to a cross sectional image in a single plane. Modern CT scanners acquire images rapidly with each slice taking as little as a fraction of a second to obtain. Multiple slices are obtained in sequence. By stacking the data from multiple slices together, a three dimensional image can be reconstructed. The processed images are transferred onto special laser film with between 6 to 24 images per film.
The radiographers responsible for the examination undertake special training in CT scanning. The images are interpreted by a Radiologist who issues a diagnostic report. CT scanning is now commonly performed particularly for imaging the head, sinuses, spine, chest, abdomen, pelvis and limbs. The radiographers and radiologist are happy to answer any questions you may have.

An injection of intravenous contrast may be given. Contrast increases the density of blood vessels and vascular structures, improving their visibility. A pressure injector may be used to inject the contrast automatically. Patient movement creates severe artifacts on the images and therefore it is important to keep still. Examinations take between 10 minutes to 30 minutes to complete.

Spiral CT
In the 1990's spiral CT scanners were developed. A Spiral CT has a slip ring which enables the tube to spin continuously at one revolution or more per second. Data is obtained in a continuous spiral or helix rather than in individual slices. The advantages of spiral CT include faster scanning with reduced examination time, greater accuracy especially for small lesions in the liver or lungs, better intravenous contrast usage and the ability to obtain seamless 3D images. Reduced examination times are important for seriously ill patients, children, restlessness or claustrophobia.

Multi-slice CT
A major development has been the introduction of multi-slice CT. The latest multi-slice scanners have 16 detector rows allowing the acquisition of 16 slices of information each tube rotation. The advantage is improved volume coverage and longitudinal spatial resolution. New applications have evolved including cardiac scanning, virtual endoscopy and CT angiography.

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Advantages of MRI
By using magnetism instead of X-Rays, MRI avoids exposing patients to ionising radiation. The information in MRI can be obtained in any plane and is 3D. Magnetic Resonance Imaging provides more detailed information of the soft tissue structures of the body than has previously been possible. The images are not degraded by adjacent bone as with CT. It is the modality of choice for demonstrating many of the disease processes which involve the brain and spinal cord. MRI is able to demonstrate the anatomy of fine joint structures including muscles, ligaments and cartilage.

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Nuclear Medicine
Nuclear Medicine examinations involve the injection of a small quantity of a radioactive material which is designed to specifically target a region of interest in the body. After a variable period of time, the radioactive substance accumulates in the target organ. By placing the region of interest under a gamma camera, the radioactivity emitted from the patient can be detected and a composite picture built up over time. Unlike conventional X-ray images, the spatial detail of nuclear medicine images is not great. Conversely, nuclear medicine is very sensitive in detecting abnormalities, sometimes long before X-rays can. Stress fractures, tumours or infections in bones can all be demonstrated earlier than with X-rays. Nuclear medicine provides information on the function of an organ where as conventional imaging techniques are generally more concerned with the structure of an organ. The radioactive materials used (or radiopharmaceuticals) have short half lives. This means that the radioactivity only persists for a short period of time. The radiation dose from most nuclear medicine procedures are small and are comparable to the dose from other X-ray imaging procedures.

Nuclear Medicine Procedures are tailored to the region of interest. The radioactive substance is attached to other compounds which are designed to target specific organs in the body. In this way many different kinds of examinations can be performed using the same basic equipment. A specially trained nuclear medicine technologist interviews patients prior to the examination to obtain relevant medical history.

Adrenal Scans
Tumours of the adrenal glands often produce specific hormones. Technetium can be attached to substances that mimic precursors of these hormones. Adrenal tumours take up these radiopharmaceuticals and can be diagnosed by demonstrating a 'hotspot' over the involved adrenal gland. This technique can be used to diagnose 'phaeochromocytomas', an adrenaline secreting tumour which can cause severe hypertension. Different radiopharmaceuticals are used to diagnose tumours that produce steroids as in 'Conn's Syndrome' and 'Cushing's Syndrome'. No preparation is required for adrenal scans.

Bone Scans
A bone scan involves an injection in the arm which may be performed either in the injection room or under the gamma camera if the pattern of blood flow to the region of interest is required. The radiopharmaceutical used is distributed via the bloodstream and is taken up by actively metabolizing bone. After three hours, the images are obtained. The scans take up to one hour to complete and usually the entire skeleton is imaged by positioning the gamma camera over multiple regions. Occasionally a special tomographic study (SPECT) may be required in which the gamma camera rotates around the region of interest. This takes additional time. Bone scans can detect most diseases of bone usually earlier than other techniques and are more sensitive than x-rays in depicting stress fractures, infections of bones and joints, infections of artificial joints and most tumours. No special preparation is required for a bone scan. Fluids are required to be drunk between the injection and scan to help clear the radiopharmaceutical from the soft tissues which enhances the quality of the subsequent bone scan. The bladder must be emptied before the scan.

Gallium Scans
Gallium is a radioactive substance which is taken up in certain types of tumours and in areas of active infection. Gallium scans can therefore be used to follow the progress of these tumours or to document the presence of active infection such as in an artificial joint. Occasionally a scan is also performed to study inflammation in areas such as the lungs, heart etc. The gallium is administered by a vein injection. Several scans are obtained between two and five days after the injection. The first scan is the most detailed and may take more than an hour in total. No preparation is required for a gallium scan.

Lung Scans
There are two parts to a lung scan. The 'ventilation' scan is first obtained in which the patient breathes a gas labelled with technetium. A series of images is obtained depicting the pattern of ventilation in the lungs. Next an injection of technetium is administered, attached to minute particles that are temporarily trapped in the capillaries (tiny blood vessels) of the lungs. Another series of images is obtained depicting the pattern of perfusion in the lungs. By comparing the two scans, regions where there is a ventilation / perfusion mismatch can be defined. This enables the diagnosis of pulmonary emboli where blood clots from the legs travel into the lungs and obstruct the pulmonary arteries. No preparation is required for a lung scan.

Parathyroid Scans
Tumours of the parathyroid take up thallium but not technetium, whereas the thyroid gland takes up both. By performing scans of the neck with both radiopharmaceuticals, two sets of images are obtained. Using the system's computer, the technetium scan can be digitally subtracted from the thallium scan leaving an image of the parathyroid tumour alone. This is a useful technique because parathyroid tumours are often small and difficult to image on CT or ultrasound. No preparation is required.

Renal Scans
There are two basic types of renal scan both of which are designed to evaluate the kidneys. A 'DMSA' scan involves an injection of a radiopharmaceutical that binds to the renal substance. The images obtained give an indication of the function of each portion of the kidney and can define areas of kidney scarring related to previous infection. A 'DTPA" scan involves an injection of a radiopharmaceutical that is excreted by the kidney into the renal collecting systems and ureters and into to the bladder. By tracking the radioactivity from the substance of the kidney and from the collecting systems over a period of time, graphs of kidney function can be obtained. Delay in function of a kidney can be defined by comparing the graphs obtained from each side. This technique is used to assess overall renal function where a kidney has been damaged in the past. Scans can also be used to assess obstruction of the kidneys, during which an injection of a diuretic (Lasix) is administered late in the scan. DTPA scans are also used to diagnose narrowing of the renal arteries in patients with hypertension. When assessing for hypertension, a drug known as Captopril may be administered after the first scan and the scans subsequently repeated.

Thallium Scans
Thallium scans are used to evaluate coronary artery disease. Coronary artery disease is a major cause of death and morbidity in Western cultures. Narrowing of the blood vessels supplying the heart leads to a lack of oxygen. The heart muscle cannot function properly without an abundant supply of oxygen. Under periods of increased cardiac workload during exercise, the lack of blood supply can lead to chest pain known as angina. If there is a severe or sudden reduction in blood supply a myocardial infarct may result where the heart muscle dies. Thallium is injected into the bloodstream about one minute prior to the actual scan and enters the heart muscle via the coronary arteries. The gamma camera is placed over the heart and tomographic (SPECT) pictures are obtained depicting the level of radioactivity in each portion of the heart muscle. The level of radioactivity is directly related to the perfusion or blood flow in that portion of the heart. Two separate scans are obtained, each requiring an injection of thallium. The scans are performed at rest and after exercise on a treadmill or bicycle to define areas of reversible 'ischaemia' i.e. areas where the muscle is still viable but the blood supply compromised. During the exercise an ECG trace will be monitored and blood pressure and pulse rate checked. In preparation for the test, products containing caffeine should be avoided for twelve hours beforehand. A four hour fast is required immediately before the scan to ensure an empty stomach. Smoking should also be ceased four hours prior to the scan. Usual medication should be continued.

Thyroid Scans
An injection of technetium. Is given via an arm vein. After half an hour, the thyroid gland is placed under the gamma camera and images obtained. Thyroid scans demonstrate the structure, location and size of the gland. The pattern of function in the gland is also depicted and any 'hot' or 'cold' areas defined. The pattern of activity is usually correlated with other imaging studies, particularly ultrasound. Any cold areas that correspond to a nodule seen on ultrasound must be biopsied to rule out a thyroid cancer. An estimate of overall thyroid function (thyroid uptake) is also obtained during the examination.

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