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

Clinical contraindication

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 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.

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 is considered present at sonography when the lumen of the fluid-filled small bowel loops were dilated more than 3 cm and the length of a segment of the dilated small bowel was over 10 cm; peristalsis of the dilated segment was increased. This is shown by rapid progression or whirling movement of bowel content (Ko et al., 1993).

Ultrasound can be used for the diagnosis of Crohn’s disease (an inflammatory bowel disease that may involve various segments of the GIT, although ileal and/or colonic involvement is most frequent); however, the specificity and sensitivity of any test depends on the prevalence of the disease in the population (i.e. the pre-test probability) (Fraquelli et al., 2005).

Endoluminal ultrasound is performed using an axial endoscopic type probe with a 7.5–10 MHz transducer. It is a mechanically rotated probe that produces a 360 degrees cross-sectional image.

Endoluminal ultrasound is excellent for local staging of esophageal cancer. It can define tumor infiltration, as well as local nodal staging. This has major prognostic and management implications, and associated morbidity of open surgery if lesions are detected early. Puncture endosonographic scopes have also been developed to enable fine needle aspiration of lymph nodes and more accurately stage nodal burden (Binmoeller, 1999).

Endoanal ultrasound (Figure 17.2) essentially follows the same principle and has several specific indications. It is used to assess the anal–rectal region, in particular the sphincters. The main indication for endoanal ultrasound is in the investigation of fecal incontinence as well as anal pain (Rottenberg and Wiiliams, 2002).

Figure 17.2 Endoanal ultrasound depicting the different sonographic layers of the mid rectum.

These are specialized techniques requiring specific training and are preformed in tertiary referral centers.

Applied terminology

Attenuation: the energy loss of the x-ray beam as it passes through a material. High attenuation – white; low attenuation – dark, gray/black; isoattenuation – gray/light gray.

Density can be used as an alternative to attenuation.

Hounsfield unit: a unit of measurement for CT densities, named after Geoffrey Hounsfield, the inventor of CT.

Windowing: manipulation of the CT image to display differences in the gray scale

Clinical indications for CT assessment

Figure 17.7 Axial CT of the abdomen shows ill-defined low density lesions (proved to be liver metastases) in the liver.

Clinical contraindication for CT assessment

Future developments

CT examination was initially obtained as single slice images, it was prolonged, cumbersome and only 2D views were possible. The advent of multi-slice CT with improved high performance software and powerful computers has added a new dimension to all radiological specialities. Examinations are obtained as spiral acquisitions with thin slices. This equates to faster scanning and thus fewer motion artefacts, especially useful for severely sick patients, patients with respiratory compromise (shorter breath hold) and the pediatric population. Overlapping data are obtained which can be mathematically remodeled to produce 3D multiplanar reconstructed images. The narrow slices obtained with multislice CT result in improved spatial resolution. Currently, there are 4, 16, 32 and 64 slice multidetector CT (MDCT) readily available on the market. Future developments include 256 MDCTs which will allow greater temporal and spatial resolution with a non-significant increase in radiation doses. Temporal resolution is essential for cardiac imaging when capturing small coronary vessels at a rapid heart rate.

Appropriate windowing with review on work stations with the original data set is crucial. This is a potential pitfall if only thick axial slices are used to interpret from. Also altering the window levels is useful, as certain pathologies will be made more apparent to the human eye by changing the relative contrast. Reconstructing the original thin axial dataset with the 3D reconstruction function in the three orthogonal planes is invaluable both for reporting and demonstrating the abnormalities to the clinicians.

Applied terminology

The accepted MRI terminology is signal characteristics.

The MR signal characteristics in physics terms are dependent on the T1 and T2 properties of a given tissue. Every tissue in the human body has its own T1 and T2 value.

T1 weighting: This is the spin/lattice interaction, where energy is given off to surrounding tissue. It is used to indicate an image where most of the contrast between tissues is due to differences in the T1 value. A T1-weighted magnetic resonance image (Figure 17.9) is created typically by using short TE (time to echo) and TR (time to relaxation) times.

Figure 17.9 MR axial T1-weighted image of the liver.

Lesions with short T1 are (bright in T1-weighted sequences):

T2 weighting: This is spin/spin interaction which is the dephasing of magnetic moment. It is created typically by using longer TE and TR times.

Lesions with short T2 are (dark in T2-weighted sequences) (Figure 17.10):

Figure 17.10 MR axial T2-weighted image of the liver.

Clinical indications for magnetic resonance imaging

Figure 17.11 Rectal MR: coronal T1W image.

Image courtesy of Dr E. Loveday.

Figure 17.12 Rectal MR: sagittal T2W image.

Image courtesy of Dr E. Loveday.

Clinical contraindication for magnetic resonance imaging

Magnetic resonance cholangiopancreaticography

The image is heavily T2-weighted so the signal from solid parenchyma is suppressed and fluid-filled structures are highlighted (Figure 17.13).

Figure 17.13 Magnetic resonance cholangiopancreaticography.

Image courtesy of Dr E. Loveday.

CT, MR colonography and enteroclysis

Each year in the UK, around 16 000 people die from colorectal cancer. At disease presentation, around 55% of people have advanced cancer that has spread to lymph nodes, metastasized to other organs or is so locally advanced that surgery is unlikely to be curative (Dukes’ stage C or D) (Drug Therapy Bulletin, 2006). Overall 5-year survival for colorectal cancer in the UK is around 47–51% (compared to 64% in the USA), but only 7% at most in those presenting with metastatic disease (Drug Therapy Bulletin, 2006). It is now known that colorectal carcinoma can start as an adenomatous (precancerous) polyp then develop to carcinoma in-situ and finally to frankly invasive. The risk of malignant progression increases with the increasing size of the polyp: generally adenomatous polyps 5 mm and below have 0.5% risk, greater than 10 mm have 3–10% risk and greater than 20 mm have 10–50% risk (Dahnert, 2003). Thus there is a significant advantage in detecting adenomatous polyps. Direct colonoscopy is the reference standard as simultaneous biopsy and polyp removal can be achieved. However, it is painful, requires analgesia ± sedatives, may not reach the right side of the colon and has a risk of perforation (Lee et al., 1994). (Please refer to Chapter 20 for a more detailed discussion.)

Developments such as MR and CT colonography (CTC) allow examination of the colon and rectum with the possibilities of detecting polyps and/or carcinomas (Pickhardt et al., 2003). It can be used in both symptomatic and asymptomatic patients who have a high risk of developing cancer or even those of average risks.

CTC (Figures 17.14, 17.15, see color insert and 17.16) is less invasive with no clinically significant complications (Yee et al., 2001; Pickhardt et al., 2003) unlike direct colonoscopy; though full bowel preparation is required so fecal residue is not misinterpreted. Antispasmodic prior to the scan is given and a distended colon to patient’s toleration, with full distention being the optimum situation. Distention is achieved by insufflation with air or carbon dioxide and spiral acquisition with breath hold in both supine and prone positions (National Institute for Clinical Excellence). The data obtained are used to produce a 2- and 3-dimensional image of the entire colon and rectum, the software enabling the operator to manipulate the data set in order to define lesions. CTC has high average sensitivity and specificity for large and medium polyps in the symptomatic population, but the findings in the asymptomatic population have not been established due to heterogeneity and small numbers in the cohort groups for the various studies (Halligan et al., 2005). CTC techniques are explored further in Chapter 18.

Figure 17.14 Original dataset of CT colonography showing an intraluminal colonic polyp arrowed.

Figure 17.16 Reconstructed CT colonographic image of the colonic polyp as seen in the original dataset of Figure 17.14.

MR colonography on the other hand has no radiation exposure but still requires antispasmodics to minimize bowel peristalsis and spasm. The colon is distended with several litres of tap water with the patient in the prone position. Once complete filling is achieved with adequate colonic distention, intravenous gadolinium is given. The examination is only performed in the prone position and generally it is well tolerated (Lauenstein et al., 2002).

MR enteroclysis imaging is a technique for evaluation of the small bowel, in particular patients with Crohn’s disease. The technique involves administration of 1.5–2 litres of isosmotic water solution through a nasojejunal catheter, which ensures distention of the bowel and facilitates identification of wall abnormalities. This is similar to the traditional enteroclysis technique, but an antiperistaltic agent is essential to acquire images free of motion artefacts. The patient is then imaged with special MRI sequences: FISP (fast imaging with steady state precession) and HASTE (half Fourier acquisition single shot turbo spin echo) (Prassopoulos et al., 2001). The major advantage is the lack of ionizing radiation especially in patients who are diagnosed at a young age with inflammatory bowel disease and will have to have repeated examinations for follow up, disease status or its complications. It provides excellent soft tissue contrast with 3-dimensional images and is able to provide extramural information. However, the spatial resolution is not yet sufficient to delineate superficial abnormalities. It is emerging, but it is not the initial investigation of choice as its clinical utility has not been fully established.

Applied terminology

Hot: an area of increased radionuclide activity.

Photopenic/cold: an area of decreased radionuclide activity.

Nuclear medicine in abdominal imaging

The role of nuclear medicine in relation to the detection of GI bleeding has specific indications. GI bleeding is typically intermittent and requires active bleeding to be localized. When other investigations have been unable to detect the exact site, it is imperative to ‘stop the tap’ in patients where the bleeding is massive or recurrent. This will determine who may benefit from aggressive treatment and who would be appropriate for medical management. The full scintigraphy protocol has been set by the Society of Nuclear Medicine, which can be downloaded from the website (www.snm.org). It must be emphasized that only hemodynamically stable patients should undergo nuclear scintigraphic scan and the unstable patients should immediately undergo angiography (Hastings, 2000). A high level of certainty in localizing the site of bleeding is possible when located in the stomach or the colon. Localizing the bleeding site is less sensitive in the small bowel, especially from the jejunum and ileum (Holder, 2000).

There are several scintigraphic factors which affect the differences in detecting bleeding sites: they include dynamic imaging, delayed imaging and the extra large field of view needed to visualize the upper and lower gastrointestinal tract simultaneously.

Physiological factors affecting detection are: the bleeding flow rate (rapid, moderate, slow, intermittent, minimal, antegrade and retrograde bleeding); the specific bleeding site; and any previous bowel surgery. An indication of the clinical application for scintigraphic assessment in abdominal imaging is given in Table 17.1.

Table 17.1 Clinical application for scintigraphic assessment in abdominal imaging

Positron emission tomography CT (PET-CT)

PET-CT is a relatively new imaging technique. PET requires radionuclides that decay by positron emission. A cyclotron produces positron-emitting isotopes. The radionuclide is attached to a biological compound to form a radiopharmaceutical. This is injected into the patient and a PET camera detects the emission. PET is similar to the other scintigraphic examinations: it provides functional information, but similar to other nuclear medicine investigations, it is not specific anatomically (Figures 17.17, 17.18, see color insert).

Then, after the acquisition of the PET examination, a CT examination is performed for anatomical localization. The images from the examinations are fused. There are now specific single PET-CT machines where the patient is on a long gantry, the PET is acquired then the CT in a single session. The alternative is two separate machines, whereby the examinations are acquired separately. The disadvantage of this is that the fusion of the examinations is less than ideal.

The fusion of modalities provides functional and specific anatomical information. Its main role is in oncology imaging, whereby primary, secondary sites and recurrences can be identified. Identification can occur prior to the development of a significant mass by which time the carcinoma may be only amenable for palliative therapy.