Diagnostic imaging Imaging techniques
Conventional radiographs depend on the differential absorption by soft tissue, bone, gas and fat of X-rays passing through the body. The unabsorbed rays blacken a photographic film, contained within light-sensitive screens, which is then processed to produce the hard copy. Modern radiology involves the use of many technical modifications to reduce the dose of X-rays to the patient. Plain X-rays remain the primary diagnostic tool in the chest and abdomen, and in trauma and orthopaedics. With careful interpretation, accurate diagnosis can be achieved and it is vital that the plain film is not jettisoned in favour of more complex and expensive imaging techniques.
When X-rays strike a fluorescent screen, light is emitted which, by means of an imaging intensifier, can be projected on a television screen. This is the basis of fluoroscopy (screening) which allows continuous monitoring of a moving process. It also provides guidance for many interventional and angiographic procedures and for barium investigations of the gastrointestinal tract. Barium studies remain a standard technique for evaluating disorders of swallowing and oesophageal function and for the small bowel. The role of the barium meal and enema is challenged by the expansion of endoscopy. However, there is little evidence to indicate that in the diagnosis of significant disease, e.g. ulcer/cancer, endoscopy is superior (Fig. 2.4). Choice of examination depends on local expertise and availability. Endoscopy is preferable where there is gastrointestinal bleeding (upper or lower) or inflammatory bowel disease.
Intravenous contrast contains iodine which absorbs X-rays by virtue of its high atomic number. It provides arterial or venous opacification depending on the route and timing of injection. Contrast injected intravenously is excreted rapidly by the kidneys which forms the basis of the intravenous urogram(IVU) where the nephrographic (renal parenchymal) and pelvicalyceal (collecting system) phases, ureters and bladder are successively demonstrated and recorded over approximately 30 minutes following contrast injection. The IVU remains the best method for investigating renal stones and haematuria. No other technique can equally visualise the pelvicalyceal systems and ureters (Fig. 2.5).
Tab 2.3 Imaging in the acute abdomen.
CXR (erect) Gas under diaphragm
Abdominal X-ray (AXR) (supine) Dilated bowel/gas pattern
Gas inside/outside bowel
Bowel wall oedema?
IVU Renal colic
Ureteric obstruction by stone?
Ultrasound (US) Ascites
Obstruction?— dilated fluid-filled
Focused high-resolution US Diverticulitis
Bowel wall thickening/abscess
CT Severe pancreatitis
Small bowel obstruction (high
Focused CT Appendicitis
Ureteric colic (if contrast allergy)
Ultrasound is inexpensive, quick, reliable and noninvasive and is an excellent initial investigation for a wide range of clinical problems. It is technically demanding and requires an experienced operator to maximise the potential of the examination. Despite the advances in technology, there are still problems with gas (which reflects sound completely) and obese patients, who are often unsuitable for ultrasound. As ultrasound is so accessible there is a tendency to overload departments with requests which may be on the margins of appropriateness. As with all investigations, clinicians should consider whether the request for ultrasound is justified as to its likely yield and its subsequent effect on patient management.
Ultrasound depends on the generation of high-frequency sound waves, usually of between 3 and 7 MHz, by a transducer placed on the skin. Sound is reflected by tissue interfaces in the body and the echoes generated are picked up by the same transducer and converted into an image which is then displayed in real time on a monitor. The scope of ultrasound has increased vastly over the last decade with higher frequency probes of diminishing size producing high-resolution images. The current range of ultrasound includes probes measuring only millimetres and operating at 20 MHz, which can be introduced via a catheter into a blood vessel to image the vessel wall; probes combined with fibre-optic endoscopes to visualise the gut wall at echo endoscopy (EUS) (7.5—20MHz) (Fig. 2.6); endoluminal probes for transvaginal and trans-rectal scanning (7.5 MHz); dedicated very-high-frequency probes of up to 15 MHz for scanning the breast, other superficial structures and musculoskeletal work; and an increasing array of specialised probes for abdominal scanning. Ultrasound is the first-line investigation in hepatobiliary disease, suspected pancreatic, aortic and many other intra-abdominal disorders (Fig. 2.7).
There is an increasing recognition of the value of intraoperative ultrasound scanning, acknowledging the fact that visualisation at surgery is frequently incomplete, the surgeon seeing only the exposed surfaces. These limitations are accentuated by the restrictions imposed by minimally invasive and laparoscopic surgery.
Doppler ultrasound measures the shift in frequency between transmitted and received sound and can therefore measure blood flow. The spectral Doppler wave form and ultrasound image are combined in duplex scanning. Colour Doppler imaging displays flowing blood as red or blue, depending on its direction, towards or away from the transducer (Fig. 2.8). Power Doppler is not dependent on frequency or direction of flow but is exquisitely sensitive to low flow and has the potential to demonstrate tissue perfusion (Fig. 2.9). Contrast agents have been developed based on micro-bubbles to enhance the Doppler effect. These techniques have revolutionised the diagnosis of both arterial and venous vascular disease.
To create a CT scan, a thinly collimated beam of X-rays passes through an axial ‘slice’ of tissue and strikes an array of very sensitive detectors which can distinguish very subtle differences in tissue density. By analysis of the collected data, the digital information is translated to a greyscale image where the attenuation value of tissues is related to water, which is given a CT number of zero Hounsfield units (HU). Tissue densities range from + 1000 (bone) down to —1000 (air). An observer working at a viewing console can, by varying the range and centering of densities represented (window width/level), display an image appropriate to the tissue being examined (Fig. 2.10).
In conventional CT, a series of individual scans is acquired during suspended respiration. Helical or spiral CT involves continuous rotation of the X-ray tube with the beam tracing a spiral path around the patient such that a volume of tissue is scanned. In this way, during a single breath-hold of up to 30 seconds, 30 cm or more of tissue can be covered in one acquisition. Further developments have allowed thinner collimation and stretching of the spiral to cover greater distances. The volumetric data can then be processed to produce conventional transaxial images or multiplanar (sagittal and coronal) and three-dimensional images (Fig. 2.11). The development of spiral scanning has greatly enhanced the diagnostic potential of CT (Fig. 2.12). It is now possible to exploit the enhancement characteristics of tissues in both the arterial and venous phases of imaging and this modification has opened up the fields of CT angiography, three dimensional imaging and ‘virtual endoscopy’ of the bronchial tree and colon (Fig. 2.13).
CT scanning is usually performed after simpler investigations such as plain films or ultrasound. In many centres, however, CT is often used as a first line examination in the evaluation of abdominal trauma and severe pancreatitis. It
• Reduced scan time: advantages in critically ill and children
• Imaging at peak levels of contrast: arterial and venous phase
• Overcomes the problem of ‘mis-registration’ — lesion ‘missed’
because of different depth of respiration
• Ability to review and reconstruct data retrospectively — improved lesion detection
• Multiplanar and three-dimensional analysis
— CT angiography
— Complex joints
— Facial bones
— ‘Virtual endoscopy
— Spiral pneumocolon
has a major role in cancer staging and an increasing role in ‘problem solving’ in the chest and abdomen. Some centres advocate early CT in assessment of the acute abdomen (vide in Ira). The development of the technique of CT spiral pneumocolon is challenging the barium enema and colonoscopy in the investigation of large bowel disorders (Fig. 2.14).
Magnetic resonance imaging
The basic principle of magnetic resonance imaging (MRI) centres on the concept that the nuclei of hydrogen, most prevalent in water molecules, behave like small spinning bar magnets and align with a strong external magnetic field. When knocked out of alignment by a radio frequency pulse, a proportion of these protons rotates in phase with each other and gradually returns to their original position, releasing small amounts of energy which can be detected by sensitive coils placed around the patient. The strength of the signal depends not only on the proton density but on the relaxation times, T1 and T2. T1 reflects the time taken to return to the axis of the original field and T2 on the time the protons take to dephase. T1 images usually demonstrate exquisite anatomical detail because of the high soft tissue discrimination. Most pathological processes increase T2 relaxation times, producing a higher signal than the surrounding normal tissue on T2-weighted scans.
The complexity of the imaging process is compounded by the variety of pulse sequences available. In general, image acquisition time is longer than CT. Respiratory and cardiac motion degrade the image but this can be largely overcome with cardiac and respiratory gating. Technological developments are fast and scanning times are shortening. Intravenous gadolinium acts as a contrast agent by reducing Ti relaxation and enhancing lesions which then appear as areas of high signal intensity (Fig. 2.15). Specific sequences have been developed to demonstrate flowing blood and produce images resembling conventional angiography. This technique of magnetic resonance angiography (MBA) can be achieved without the risks of intravascular injection of contrast and may ultimately replace conventional studies (Fig. 2.16). Heavily T2-weighted sequences which demonstrate fluid-filled structures as areas of very high signal intensity have been developed to show the biliary and pancreatic ducts in magnetic resonance cholangiopancreatography (MRCP). It seems likely that this technique will take over from diagnostic endoscopic retrograde cholangiopancreatography (ERCP) (Fig. 2.17).
The major strength of MRI is in intracranial, spinal and musculoskeletal imaging, where it is superior to any other imaging technique because of its high contrast resolution and multiplanar imaging capability. Cardiac MRI is firmly established and the value of breast MRI, particularly in multifocal and recurrent cancer, is increasingly recognised. It is currently the best investigation for staging cervical cancer and for anorectal sepsis (Fig. 2.18).
Open access magnets have been developed which allow interventional procedures to be performed with MRI guidance and there is no doubt that this will revolutionise the operating room of the future (Fig. 2.19). There is a vast potential for MRI in the assessment of disease in the abdomen and pelvis and undoubtedly the role of MRI will continue to expand. However, because of the expense of the equipment and its installation, the provision of scanners cannot keep up with the demands for scanning time and most hospitals have to impose strict guidelines for access.
Radionuclides can be tagged to substances which concentrate selectively in certain tissues of the body. These radiopharmaceuticals are injected intravenously and, in general, emit gamma radiation detected by a gamma camera. The emitted radiation strikes a sodium iodide crystal which generates a small flash of light which is then enhanced by photo-multiplier tubes to produce the image. Many studies employ technetium-99m (99mTc) which has a short half-life and imparts a radiation dose to the patient which is lower than many other imaging investigations.
In general, spatial resolution is poor as the technique demonstrates physiological and functional changes rather than anatomy (Fig. 2.20). A standard gamma camera provides only a two-dimensional display of activity. Single photon emission computed tomography (SPECT) creates a three-dimensional image by means of an array of photomultiplier tubes that surround the patient in the same way as with CT and MRI. This technique uses conventional radionuclides. Positron emission tomography (PET) scanning is more sensitive, depending on the coincidence detection of annihilation protons resulting from radionuclides that decay by positron emission. However, these studies require specially designed, dedicated and currently expensive cameras and an in-hospital cyclotron to generate the radionuclide. These scans are therefore not widely available.
Cross-sectional imaging techniques have replaced many radionuclide studies (liver colloid scans, brain scans). Bone scanning remains a sensitive tool for detection of bone metastases and occult fractures. Ventilation—perfusion scans are widely used to detect pulmonary embolic disease, although contrast-enhanced spiral CT of the pulmonary vessels is challenging this role. New exciting techniques of radiopharmaceutical labelling of monoclonal antibodies are opening up possibilities of targeted cancer therapy and early detection of micrometastases.
Imaging in the acute abdomen
The term ‘acute abdomen’ encompasses many diverse entities. Imaging tests are selected based on the likely diagnosis (Fig. 2.21). The erect CXR and supine abdomen remain the investigation of choice where perforation or intestinal obstruction is suspected (Fig.2.22 and Fig.2.23). In many patients this will provide sufficient information to determine further management. When the diagnosis is less clear, new imaging techniques are challenging the traditional approach. Both ultrasound and CT may contribute valuable information in inflammatory disease within the abdomen —notably in diverticulitis, appendicitis and in inflammatory bowel disease. In some centres — particularly in the USA — the use of spiral CT as a first-line investigation is being promoted as a cost-effective alternative to increase the specificity of primary diagnosis (Fig. 2.24).
Imaging in oncology
Modern surgical treatment of tumour requires an understanding of tumour staging systems, as in many instances this will define appropriate management. The development of stage-dependent treatment protocols involving neoadjuvantchemotherapy and preoperative radiotherapy relies on the ability to define tumour stage accurately by imaging before surgical and pathological staging. Once a diagnosis of tumour has been established, often by percutaneous or endoscopic biopsy, new imaging techniques have considerably improved the ability to define the extent of tumour, although the pathological specimen remains the gold standard. Many staging systems are based on the TNM classification (tumour/node/metastasis).
In most published studies, cross-sectional imaging techniques (CT, ultrasound, MRI) are more accurate in staging advanced (T3, T4) than early (T1, T2) diseases and the staging of early disease remains a challenge. In gut tumours, endoscopic ultrasound is more accurate than CT or MRI in staging early disease (T1 and T2) by virtue of its ability to demonstrate the layered structure of the bowel wall and the depth of tumour penetration (Fig. 2.25). Developments in MRI may also improve staging accuracy of early disease.
Accurate assessment of nodal involvement remains a challenge for imaging. Most imaging techniques rely purely on size criteria to demonstrate lymph node involvement with no possibility of identifying micrometastases in normal sized nodes. A size criterion of 8—10 mm is taken but it is not usually possible to distinguish benign reactive nodes from infiltrated nodes. This is a particular problem with intrathoracic neoplasms where enlarged benign reactive mediastinal nodes are common. The echo characteristics of nodes at endoscopic ultrasound have been used in many centres to increase the accuracy of nodal staging and nodal sampling, via either mediastinoscopy or transmural biopsy under EUS control. New radioisotope techniques are being developed using radiolabelled monoclonal antibodies against tumour antigens which may increase detection of nodal involvement by demonstrating micrometastases in nonenlarged nodes.
The demonstration of metastatic disease will usually significantly affect surgical management. Modern cross-sectional imaging has greatly improved the detection of metastases but occult lesions will be missed in between 10 and 30 per cent of patients. CT is the most sensitive technique for detection of lung deposits, although the decision to perform CT will depend on the site of the primary tumour, its likelihood of intrapulmonary spread and the effect on staging and subsequent therapy of the demonstration of intrapulmonary deposits.
Ultrasound and CT are most frequently used to detect liver metastases. Contrast-enhanced CT can detect most lesions of greater than 1 cm, although accuracy rates of CT vary with the technique used and range from 70 to 90 per cent. Recent studies suggest that MRI may be more accurate than CT in demonstrating metastatic disease. While enhanced CT is used in most centres for screening for liver deposits, CT AP (CT with arterial portography), which requires contrast injection via the superior mesenteric artery, is used in many centres as the most accurate technique for staging liver metastases if surgical resection is being considered. Preoperative identification of the segment of the liver involved can be determined by translation of the segmental surgical anatomy as defined by Couinaud to the cross-sectional CT images (Fig. 2.26).
Intraoperative ultrasound is an alternative method of staging that provides superb high-resolution imaging of sub-centimetre liver nodules that may not be palpable at surgery.
Imaging in trauma
The response of the skeleton to trauma changes both with the nature and force of the injury and with the maturity and strength of the skeleton. In children the ‘physis’ or growth plate provides the weakest link and therefore epiphyseal injuries or apophyseal displacements are common. The skeleton is less brittle, resulting in buckling of the cortex or incomplete ‘green-stick’ fractures. In the mature adult skeleton the soft tissues — ligaments and muscular insertions are the weakest link, and sprains and strains occur more commonly than fractures. The elderly osteopenic skeleton is brittle and susceptible to fracture often with minimal force.
Fracture radiographs should be performed in two planes and where possible should include the adjacent joint. Most fractures are easily diagnosed but some may be subtle and occult. Where a fracture is strongly suspected but not demonstrated, a repeat X-ray 5—10 days after the injury may identify the fracture line when bone absorption has begun. Stress fractures, either ‘fatigue fractures’ (normal bone) or ‘insufficiency fractures’ (abnormal bone), can be difficult to diagnose. Radionuclide bone scanning and more recently MRI are useful additional investigations if stress fractures are strongly suspected. The ability of CT to scan in the axial plane, together with excellent resolution of bony detail and the ability for multiplanar reconstruction, makes CT valuable in assessment of fractures of the spine, foot and pelvis (Fig. 2.27).
In patients who survive the immediate injury, imaging is considered after clinical evaluation and acute resuscitation. Acute spinal trauma is initially assessed by plain films. In approximately 10 per cent of patients there are multiple levels of injury. If spinal instability or spinal canal disruption is suspected, thin-section CT scanning with reconstruction of the images is required. Suspected cord damage may require an urgent MRI scan (Fig. 2.28). Evaluation of severe head, chest and abdominal trauma usually necessitates CT scanning after initial plain films (Fig. 2.29).
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