COMMON IMAGING PROCEDURES

• Ultrasound
• Gastrointestinal Contrast Examinations
• Computed Tomography
• High Resolution Computed Tomography
• Magnetic Resonance Imaging
• Nuclear Medicine
• Positron Emission Tomography

Ultrasound

A pulse of ultrasound is transmitted into the body via a small probe (transducer). Depending on the tissue characteristics and the interfaces present, the beam will be partially reflected, absorbed or transmitted. An image is generated based on the reflected ultrasound beam. Doppler ultrasound depends on the alteration of the frequency of the ultrasound beam when reflected by moving blood cells. The blood flow direction and velocity can be calculated and can provide an assessment of the severity of vessel stenosis. Colour Doppler (CD) also depends on frequency changes induced by blood cell movement; information is colour coded for frequency and direction, allowing a more graphic and immediate assessment. Colour flow imaging is based on echo signal amplitude, rather than frequency (ie. velocity of blood flow). This provides higher sensitivity to the presence of blood flow.

Advantages of Ultrasound

Ultrasound is non-invasive, relatively inexpensive, widely available, does not employ ionising radiation, and provides anatomical information in almost any plane. It has widespread application (see below).

Disadvantages of Ultrasound

In the obese patient ultrasound penetration may be limited so that deep structures may not be well seen. There is a trade-off in ultrasound between using the highest frequency probe possible to achieve high resolution and a lower frequency to achieve beam penetration. The ultrasound beam is also arrested by gas in the abdomen and is unable to penetrate bone. Ultrasound is much more operator-dependent than other imaging modalities.

Indications

For specific indications, please refer to the imaging pathways. Use of ultrasound is widespread for imaging the abdomen, pelvis, small parts (scrotum, thyroid, etc), breast, musculoskeletal applications, and transthoracic echocardiography. It is the main modality used in obstetric scanning. Endoluminal ultrasound applications include transrectal prostate US, transvaginal US, endoscopic US, and transoesophageal echocardiography. Doppler techniques are used to image vascular structures to assess flow characteristics and detect stenoses. Ultrasound is extremely useful as a real-time guiding procedure for fine needle aspiration biopsy or core biopsy and drainage of fluid collections.

Preparation for US

Abdomen: The patient is normally fasted prior to the examination to allow adequate examination of the gallbladder, biliary tract, pancreas and other retroperitoneal structures. The patient should be discouraged from emptying the bladder to faciltate pelvic examination.

Pelvis: A full bladder is necessary, since this acts as an acoustic window for the pelvic organs. In females a transvaginal ultrasound may be an alternative to examine the uterus and adnexa.

Consumer Information

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Ultrasound for Consumers (May 2009)

Last reviewed in December 2011.

Gastrointestinal Contrast Examinations

A barium meal examination includes a study of the oesophagus, stomach and duodenum. A routine examination is a “double contrast” study (a positive contrast (barium) is used in addition to gas-producing agents to distend the viscus). The patient is fasted for 6-8 hours prior to the examination. Essential drugs can usually be taken, but advice should be sought from the Radiologist. Sometimes a single contrast examination (dilute barium only) is used in patients with suspected gastric or duodenal obstruction or in very immobile patients. Barium is contra-indicated if there is a known or suspected perforation. In such cases, either a water-soluble iodinated contrast such as Gastrograffin, or a non-ionic agent is used. Gastrografin is very hyperosmolar and may cause pulmonary oedema if aspirated into the airways. If there is a perceived risk of aspiration, non-ionic contrast is used.

A barium swallow is an examination that concentrates on the oesophagus. This tends, nowadays, to be a multiphasic examination with double and single contrast views. The study is usually tailored to the patients symptoms and a good history is essential. Attention is paid during the examination to morphological abnormalities of the hypopharynx and oesophagus, evidence of gastro-oesophageal reflux and swallowing function - the hypopharyngeal phase and oesophageal motility. Solid boluses (e.g. bread, marshmallows) can be given to assess motility. The examination can be recorded on conventional film, rapid-sequence camera film or video (“video-fluoroscopy”).

A videofluoroscopic study of swallowing can be performed on patients with neuromuscular problems of deglutition and hypopharyngeal function. This examination is often performed in the company of a speech pathologist. Various consistencies of bolus can be tested using thin liquids, purees and solids. The study aids in both diagnosis and management of patients with these problems.

The small bowel is examined by a “dedicated” small bowel barium study rather than a “follow-through” after a barium meal. Radiologists will vary as to whether this is performed by giving the patient a large volume of dilute barium to drink or by a small bowel enema (intubated small bowel study or enteroclysis). Although more accurate for some pathology, the latter examination requires intubation of the jejunum. Unless contraindicated, some form of purgation is given prior to small bowel studies to clear the right side of the colon.

Most examinations of the large bowel are “double-contrast” barium enemas (DCBE). Single contrast examinations may be performed on very elderly, immobile patients or on patients with unprepared bowel to determine the presence and site of obstruction. In the presence of a known or suspected perforation a water-soluble contrast agent is used instead of barium. For an accurate DCBE, thorough colon cleansing is imperative. A variety of cleansing regimes are in use. A barium enema should not be performed within 6 days of a prior rigid sigmoidoscopic biopsy or a polypectomy. However, following a superficial biopsy (ie through a flexible sigmoidoscope or colonoscope) a barium enema can be undertaken immediately. CT colonography is likely to gradually replace DCBE in the investigation of colorectal neoplasms.

Last reviewed in December 2011.

Computed Tomography (CT)

Computed Tomography (CT) involves using an X-ray tube coupled to a detector system, encased within a doughnut shaped gantry. There is a separate table, on which the patient lies, that can move smoothly in and out of the gantry. The X-ray tube emits a finely collimated X-ray beam as it rotates within the gantry around the patient. The detectors receive a series of data profiles depicting the degree of absorption encountered by the X-ray beam as it passes through the body at the different angles of rotation. This information is then transformed into a cross-sectional image after the application of complex mathematical algorithms.

Most studies are obtained in the transaxial plane. The contrast resolution is superior to conventional radiography. Intravenous contrast may be given to outline vascular structures and assess the enhancement characteristics of pathological processes.

Helical CT has almost completely replaced non-helical CT. Helical refers to the ability of the X-ray tube to rotate continuously as the body moves through the gantry. The detectors thus record a continuous volume of data rather than single slices. This information can be processed to provide images in the sagittal and coronal planes. The data can also be manipulated to enhance thresholds of attenuation. This enables structures such as bones to be viewed with the soft tissues removed or provide 3D angiographic display of vascular structures. This facility has led to expansion of CT scanning to such applications as CT angiography, CT cholangiography, 3D reformation for reconstructive plastic surgery, "virtual colonoscopy" etc.

Helical CT also allows for faster scanning, often resulting in the imaging of a structure or organ in a single breath-hold. This facility overcomes previous problems of misregistration - that is of small lesions falling between slices, when single slice breath-hold scans were obtained. This is of importance in situations such as the demonstration of small lesions in the liver, lung & adrenals. In addition, rapid helical scanning allows timing of imaging during maximal intravenous contrast enhancement, which is of particular importance in CT angiography and arterial-phase scanning of the liver. Indeed, with helical CT scanning it is possible to obtain biphasic images of the liver and other regions, that is in the arterial and portal venous phases of contrast enhancement in rapid succession.

Multidetector CT (also known as multislice, multidetector-row, multisection or multichannel CT) is the latest development in computed tomography technology. Single slice helical CT has a single detector that only allows one channel of image information to be recorded for every one rotation of the gantry. In multidetector CT, the single detector is replaced by multiple rows of detectors allowing for registration of more than one channel of variable width per gantry rotation. Depending on the machine, the number of detectors can range from two to sixty four. In addition, these scanners have faster gantry rotation speeds allowing for large volumes to be imaged in a single breath-hold. Multidetector CT enables rapid scanning, with less motion artifact, and a reduction in IV contrast media doses.

The higher resolution of multidetector CT makes it superior to single slice helical CT for 3D-volume reconstructions. Multidetector CT has impacted mainly on applications that require high spatial and temporal resolution, such as CT angiography and trauma imaging where rapid scanning is ideal.

CT has traditionally used an algorithm known as filtered back projection to reconstruct the acquired CT data information into cross-sectional images. Although this was not the first algorithm used in commercial CT machines, it gained popularity due to ease of implementation and faster image reconstruction. The first algorithm used is called iterative reconstruction and is computationally intensive. In recent years, advancements in computer processing power and growing interest in reducing medical ionising radiation has prompted renewed interest in iterative reconstruction for CT imaging. Several vendors have already introduced their own software suites, which all use simplified iterative methods to shorten image reconstruction times.

The general principle of iterative reconstruction is repeated processing cycles to reduce image noise, while maintaining contrast. The benefits are:

• Improved noise reduction (of around 30%). This will theoretically allow for radiation dose reduction of around 40%. This effect will work synergistically with other dose reduction methods.
• Less image artifacts.
• Better imaging of obese patients.
• Enables imaging at higher spatial resolutions.
• Enables re-interpretation of previous poor quality scans.

Preparation for CT

No special preparation is normally required for CT scanning other than for abdominal-pelvic scans. In addition to the frequent use of IV contrast agents, abdomino-pelvic scans usually require contrast within the bowel. Very dilute barium or iodinated contrast (Gastrografin) is usually ingested at the instruction of the Radiologist. In addition, rectally administered contrast may be given. Water or air are alternative endoluminal contrast agents in some circumstances.

Consumer Information

Computerised Tomography (CT) Scan for Consumers

Last updated December 2011.

High Resolution Computed Tomography (HRCT)

High-Resolution Computed Tomography (HRCT) is a widely used technique to image various lung pathology. Compared to helical CT, HRCT uses a narrow beam collimation to take thin slice images of the lung parenchyma. This protocol produces extremely high definition images of lung alveoli, airways, interstitium, and pulmonary vasculature. Expiration images may assist in identifying air-trapping in patients with lung disease.

Click here for HRCT protocol used to evaluate suspected diffuse lung or airways disease

Main Indications

In patients with suspected chronic diffuse lung disease, HRCT is indicated in the following situations:

• for detection in patients with normal or equivocal plain CXR appearances who have symptoms or pulmonary function tests suggestive of diffuse lung disease.
• where the symptoms and/or plain CXR findings are non-specific, to attempt a specific diagnosis.
• to assess activity of disease.
• to select an optimal biopsy site

In patients with suspected acute diffuse lung disease, HRCT is indicated in the following situations:

• for detection in patients with normal or equivocal plain CXR appearances who have symptoms or pulmonary function tests suggestive of acute lung disease (especially in immunosuppressed patients).
• for investigation of haemoptysis in selected patients
• to select an optimal biopsy site.

Other indications

HRCT may also be useful in the following conditions:

• Industrial lung disease
• Idiopathic pulmonary fibrosis
• Connective tissue disease (rheumatoid lung, scleroderma etc)
• Radiation-induced lung disease
• Diffuse metastatic disease (haematogenous, lymphangitis)
• Sarcoidosis (appearances may be diagnostic)
• Bronchio-alveolar carcinoma
• Mycobacterial infection (tuberculosis, non-tuberculous atypical)
• Infections in immunosuppressed patients
• Hypersensitivity pneumonitides (appearances may be diagnostic)
• Interstitial pneumonitis (appearances may be diagnostic in Usual Interstitial Pneumonitis)
• Histiocytosis X (appearances may be diagnostic)
• Lymphangiomyomatosis (appearances may be diagnostic)
• Cryptogenic organising pneumonia (COP)
• Bronchiolitis obliterans organising pneumonia (BOOP)
• Emphysema (appearances may be diagnostic)
• Bronchiectasis (appearances may be diagnostic)

Last reviewed in December 2011.

Magnetic Resonance Imaging (MRI)

MRI Principles

MRI is based on the interaction between nuclei of hydrogen atoms occurring abundantly in all biological tissues and the magnetic fields generated and controlled by the MRI system's instrumentation. Hydrogen nuclei have a non-zero magnetic moment. When a body tissue is placed in the magnetic field of the MR scanner, the magnetic moments of the protons tend to align themselves with the main magnetic field of the scanner. Having aligned the hydrogen protons in a known direction, a pulsed radiofrequency field is applied to the body tissues, causing a number of hydrogen protons to flip or absorb energy. When the RF field is turned off, the protons gradually return to their previous positions , and in the process release the energy they absorbed in the form of a RF signal. It is this signal that is used to develop the MR images by the computer.

Main Indications

The current main indications for a MRI examination in Australia as determined by the Medicare rebate schedule are for the neurological and musculo-skeletal systems. However, as supported by the current literature, MRI has a proven role in assessing liver and breast lesions, in the assessment of breast implants ,in the staging of prostatic carcinoma, in the staging of gynaecological malignancies and in the assessment of the biliary tree and these examinations are being routinely undertaken despite the absence of a rebate.

Main Contra-indications

The main contra-indications to MRI examinations include cardiac pacemakers, cochlear implants, epidural electrodes, ferromagnetic aneurysm clips. Relative contra-indications include first trimester pregnancy and claustrophobia.

Use of Contrast Media

Gadolinium-DTPA is the most commonly used contrast agent in MRI. Its main use is to increase lesion conspicuity in situations where contrast uptake may be expected as in the case of break down in the blood brain barrier or in neovascularity in tumours. IV Gadolinium can also be used to increase vessel conspicuity in MR Angiography but should be used with caution in patients with renal failure due to its association with Nephrogenic Systemic Fibrosis (NSF) pending further study.

Rebatable versus Non-Rebatable Examinations

At present the Medicare rebatable examinations are mainly for neurological and musculo-skeletal indications and these too have a quota imposed on the number of examinations that can be performed per patient per year. There is no rebate available for most MRI examinations of the chest and abdomen (ie. body MRI). Referrals for MRI examinations have to be requested by medical specialists as defined by the Health Insurance Commission.

Consumer Information

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Magnetic Resonance Imaging (MRI) Scans and Procedures (May 2009)

Last reviewed December 2011.

Nuclear Medicine

Nuclear medicine uses short-lived isotopes to image specific body systems. It is a very sensitive modality as most pathological processes will affect cell function before structural changes are evident. The dose received for most studies is similar to many CT procedures. A technique is available to image every organ system in the body (please refer to the appropriate imaging pathways). Most investigations require the injection of the radioisotope either in its free form (eg. pertechnetate for thyroid, salivary or Meckel's scans, Thallium for cardiac or tumour imaging, Gallium for infection, inflammation or tumour imaging) or bound to cells (eg red and white blood cells) or to a chemical ligand that determines the organ of uptake.

The majority of studies are performed using Technetium as the radioactive source. This agent has physical properties that are optimal for detection by a gamma camera, a short half life (6 hours) which reduces the dose received by the patient compared with longer lived agents and can be chemically bound to a wide range of substances. A functional image is created by the concentration and distribution of the radioactive tracer within the target organ. Tomographic images (SPECT) can assist localization of activity within an organ. The distribution of the activity passing through an organ with time can be analysed mathematically to derive functional data such as half-clearance times, ejection fractions, and relative perfusion.

Nuclear medicine demonstrates the in-situ biochemical or physiological disturbances of disease. Although some patterns of tracer distribution are characteristic for certain disease processes most studies have low specificity and spatial resolution and may require other anatomical imaging techniques for further evaluation.

Last reviewed in December 2011.

Positron Emission Tomography (PET)

Positron Emission Tomography utilises short acting, positron emitting isotopes to investigate a number of disease processes. The most commonly used agent is fluorodeoxyglucose (FDG). The positron emitting isotope fluorine-18 is substituted for a hydroxyl group in the glucose molecule. FDG uptake and subsequent retention is a marker of cellular glycolytic activity. The higher metabolic activity of tumours and their preference for glucose as an energy substrate leads to the high sensitivity and specificity of this agent in oncology, which is the main area of use. Combined PET CT scanners are gradually replacing the dedicated PET scanner, and combined imaging will soon be the standard in PET imaging. Many other PET radiopharmaceuticals are available which can be used to look at various biological processes.

Indications

The current reimbursable indications in Australia are:

Consumer Information

Click on icon below to download consumer information in PDF format.

Positron Emission Tomography (PET) Scan (May 2009)

Last reviewed in December 2011.