Cross-sectional investigations, nuclear medicine and ultrasound of the small and large bowel

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CHAPTER 17 Cross-sectional investigations, nuclear medicine and ultrasound of the small and large bowel

Jessie Aw, S. Gandhi

Contents

Introduction
Ultrasound

image Doppler ultrasound
image Applied terminology
image Clinical indications
image Clinical contraindication
image Patient preparation
Computed tomography (CT)

image Applied terminology
image Clinical indications
image Clinical contraindication
image Future developments
Magnetic resonance imaging (MRI)

image Applied terminology
image Clinical indications
image Clinical contraindication
image Magnetic resonance cholangiopancreaticography
image CT, MR colonography and enteroclysis
Nuclear medicine

image Applied terminology
image Nuclear medicine in abdominal imaging
image Positron emission tomography CT (PET-CT)
Acknowledgment
References
Further reading

Introduction

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:

1. Formulate an answerable question
2. Analyze the best current available evidence
3. Appraise critically
4. Apply findings to practice
5. Evaluate performance.

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

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

image Safe (no radiation)
image Low cost
image Portable
image Quick
image Non-invasive
image Real-time imaging
image Real-time ultrasound guided interventional procedures
image Excellent local detail using endoluminal ultrasound (applications: endoanal, endoluminal local staging for esophageal carcinoma)

BOX 17.2 Limitations of ultrasound

image Operator dependent
image High body mass index can attenuate the sound waves and deep penetration cannot be achieved
image Limited spatial resolution
image Attenuated by bone and large amount of air/gas

Doppler 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.

The combination of two-dimensional ultrasound imaging with Doppler ultrasound is known as duplex ultrasound. Duplex is an important technique for the examination of arteries and veins.

Applied terminology

The tissues within the human body produce various degrees of sound wave reflection; this is termed echogenicity. The echoes are the ultrasound waves.

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.

Hypoechoic: dark gray, nearly black, e.g. fat, muscularis propria of the bowel wall.

Isoechoic: gray lesion, e.g. liver substance or gut wall.

Anechoic: a very dark lesion, black, e.g. pure fluid; it does not reflect any sound waves back, i.e. all the sound passes through.

Acoustic enhancement: occurs when sound passes through an anechoic structure. The echoes all pass through the anechoic lesion and more sound is available.

Acoustic shadowing: this is produced when a sound wave encounters a very echo dense structure and nearly all the sound is reflected back, resulting in an acoustic shadow.

Clinical indications for ultrasound assessment

• Abdominal pain – ascites
• Query intussusception
• Esophageal local staging – endoluminal ultrasound
• Anal pain – endoanal ultrasound
• Weight loss – liver query metastases
• Enlarged abdominal organs – hepatomegaly in cirrhosis, splenomegaly in hematological disorders
• Pancreas – pseudocyst post pancreatitis, pancreas mass
• Gallbladder – stones and their complications
• Assess intrahepatic and extrahepatic ducts
• Appendicitis – appendix is identified and has thickened wall; however, if the appendix is not identified this does not exclude the diagnosis.

Clinical contraindication

image There is no absolute contraindication for an ultrasound examination, as ionizing radiation is not involved. However, you must be aware of the limitations of an ultrasound examination (see Box 17.2).

Patient preparation

Ultrasound of abdomen

Nil by mouth for 8–12 hours pre-test and a fat-free meal the evening before the test. This is to keep the gallbladder distended. Smoking should also be avoided as this can contract the gallbladder.

Ultrasound of the kidneys

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

Normal sonographic bowel wall anatomy

The sonographic appearance of the normal bowel wall consists of five concentric layers of alternating hypoechoic and echogenic interfaces. We will describe from inside out (Figure 17.1).

Layer 1 This represents the superficial mucosal interface – a thin echogenic layer
Layer 2 The deep mucosa including the muscularis mucosa – hyperechoic layer
Layer 3 This represents the submucosa and muscularis propria interface – hyperechoic layer
Layer 4 This represents the muscularis propria – hypoechoic layer
Layer 5 The marginal interface to the serosa – a thin hypoechoic layer.
image

Figure 17.1 Normal bowel wall anatomical layers.

Normal gut wall thickness is 2–4 mm on average (Ledermann et al., 2000).

Ultrasound of small and large bowel

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.

Ultrasound of the bowel can be used for bowel obstruction and in Crohn’s disease. Bowel obstruction

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