Diagnostic imaging Imaging techniques
Conventional
radiology
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.
Imaging
Indications/signs
CXR (erect)
Gas under diaphragm
Abdominal X-ray (AXR) (supine)
Dilated bowel/gas pattern
Gas inside/outside bowel
Obstruction?
Closed loop?
Bowel wall oedema?
IVU
Renal colic
Ureteric obstruction by stone?
Ultrasound (US)
Ascites
Cholecystitis
Abscess
Obstruction?— dilated fluid-filled
bowel
Focused high-resolution US
Diverticulitis
Appendicitis
Bowel wall thickening/abscess
CT
Severe pancreatitis
Diverticulitis
Abscess
Small bowel obstruction (high
grade)
Bowel infarction
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
There
is an increasing recognition of the value of intraoperative ultrasound
scanning, acknowledging the fact that visualisation at surgery is frequently
incomplete, the surgeon
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.
Computerised tomography
In
conventional CT, a series of individual scans is acquired during suspended
respiration. Helical or spiral CT involves
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
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
Radionuclide
imaging
Radionuclides
can be tagged to substances which concentrate selectively in certain tissues of
the body. These radiopharmaceuticals are injected intravenously and, in
general,
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
Cross-sectional
imaging techniques have replaced many radionuclide studies (liver colloid scans,
brain scans). Bone scanning remains a sensitive tool for detection of bone
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
Tumour
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
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
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
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.
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
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).
Severe
trauma
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).