CHAPTER 17 Cross-sectional investigations, nuclear medicine and ultrasound of the small and large bowel
The role of cross-sectional imaging is an integral part of the hospital service and an essential part of the multidisciplinary team approach to the care of the patient both in hospital and in the community. Imaging can clarify clinical assessment, provide presurgical roadmaps, postsurgical complications and aid management decisions. Cross-sectional imaging is also a prerequisite for preassessment planning and performance of an interventional procedure. It is critical in the multidisciplinary meetings and fundamental in the patient care pathway.
Most abdominal imaging is performed to investigate general abdominal symptoms, such as pain, weight loss, bloating or abdominal distension. Abdominal imaging involves a multitude of complex investigations to the uninitiated. Ultrasound, computed tomography (CT), magnetic resonance imaging (MRI) and radionuclide scanning are all important adjuncts to first line imaging: the abdominal radiograph. They are complementary to each other and the majority of clinical questions can be answered by cross- sectional imaging. Occasionally specialist investigations will be required e.g. indium-111 labelled white cell nuclear scintigraphy when the other imaging modalities have been negative and there are ongoing clinical symptoms or concern. Each investigative method is appropriate for certain clinical questions, when used appropriately it will help make an accurate diagnosis, preventing any treatment delays. To understand the role of each imaging modality you must understand the principle technique, including its value and limitations.
Evidence-based medicine can be defined as the integration of best available research with clinical expertise and patient values (Erturk et al., 2006). You can apply evidence-based practice to any clinical discipline using five basic principles:
Evidence-based radiology (EBR) is an effective tool for radiologists, radiographers, advanced practitioners and clinicians skilled in a particular radiological skill to regularly update their knowledge, deepen their understanding of research methods and (if applicable to clinical setting EBR) can allow effective clinical practice. More importantly for patients this can translate to providing the most up to date clinical practice and care.
Ultrasound (US) imaging uses high-frequency (greater than 20 kHz) sound waves, which are emitted and received by the ultrasound probe and are inaudible to humans. The frequency of the sound humans hear determines the pitch. Frequency is defined as the number of oscillations per second and Hertz is the unit measurement of frequency. Medical imaging normally uses higher frequencies, in the range of 2.5–10 mHz, with specialist imaging such as intravascular studies requiring frequencies of 20 mHz or more mHz.
The basic essential component of an ultrasound probe is the piezoelectric (PZE) crystal in a shape of a rectangle or disk. When an alternating (AC) current is applied to the crystal, it expands and contracts with the same frequency: this is the ‘piezoelectric effect’. This produces sound waves or echoes. The echoes are transmitted undergoing ‘reflection’, returning at different velocities depending on the media the echoes have encountered within the body. Different tissues will have different acoustic impedance. The proportion of energy (or sound) reflected and transmitted depends on the acoustic impedances of the two materials. In physics terms, the acoustic impedance of a material is defined as the product of the density of the material and the velocity of the sound within it. The greater the acoustic impedance mismatches between two materials the greater the fraction of sound which is reflected and thus imaging is limited. The reflected/returning sound waves act on the PZE crystal in the ultrasound probe to produce an electric signal. The probe is connected to a powerful computer processor that converts the electrical signal into a cross-sectional image. The images are captured in real time and it can show the solid organs and the movement and flow through blood vessels using a special technique of Doppler ultrasound. The image is then displayed on a monitor and may be stored or printed.
The properties of ultrasound are unique. Unlike x-rays or light waves, which can travel through a vacuum, sound waves require a material medium to travel through and thus cannot penetrate air gaps. Ultrasound cannot image through bone or large collections of air. There is also a high acoustic impedance mismatch between the patient’s skin and the probe. This is overcome by the use of ultrasound scanning gel. The advantages and limitations of ultrasound as an imaging modality are outlined in Boxes 17.1 and 17.2.
BOX 17.1 Advantages of ultrasound
BOX 17.2 Limitations of ultrasound
The Doppler effect occurs when there is a relative movement between a sound source and a sound receiver. This occurs in everyday life, when you hear a police siren. As the siren is travelling towards you (the receiver), the sound waves are compressed producing a higher frequency, then as it passes and moves further away the sound waves are further apart producing a lower frequency. This principle is used in Doppler imaging: the change in sound wave frequency is used to detect movement and the change in frequency can be calculated in order to quantify the blood flow. Doppler is used clinically for detection and assessment of peripheral arterial disease, deep venous thrombosis, portal vein thrombosis, assessing arterial and venous flow. This is very useful in native and transplanted kidneys and livers to assess any evidence of rejection.
Colour Doppler is an augmentation of the Doppler principle. The colours are superimposed on the cross-sectional imaging to allow instant assessment of the direction of flow. Blood flowing towards the transducer is colored red and away from the transducer is colored blue. In venous imaging, any poor infilling of color with associated turbulence of color is suggestive of a vascular stenosis or narrowing secondary to a mural thrombus.
Hyperechoic: on the gray scale a hyperechoic lesion is bright, white or light gray, e.g. muscularis mucosae of the bowel wall. Hyperechoic lesions reflect more sound than a tissue of low echogenicity.
To evaluate the bladder, it must be distended fully. This is achieved by drinking one litre of water or more, an hour before the examination. If the patient is catheterized it should be clamped off, again to fill the bladder. If the bladder is only partially filled, then only comment can be made on any gross/large abnormalities. Small lesions cannot be confidently detected.
Ultrasound of the bowel is a sub-specialist technique only performed in specialist centers worldwide. The technique, however, has been found to be of value in the diagnosis of the acute abdomen, in particular diagnosis of acute appendicitis. It is one of the commonest causes for an acute abdomen for the surgical on-call team.
Acute appendicitis in experienced hands is usually diagnosed on history, physical examination and laboratory investigations. The aim of imaging is either to confirm or refute the diagnosis; however, if the appendix is not visualized then appendicitis cannot be excluded. In a female, diagnoses which mimic appendicitis include enlarged ovarian cysts or torsion of the ovaries. Thickening of the wall of a normal appendix wall to 6 mm (3 mm or less is normal) is suggestive of appendicitis (Guillerman and Ng, 2005). Other features include non-compressibility and round in cross-section rather than oval.