Imagine of the Abdomen

• The abdomen can be imaged using:

  • ultrasound
  • computerised tomography (CT)
  • magnetic resonance imaging (MRI)
  • optical imaging
  • nuclear medicine imagine
• Planar X-rays have poor tissue contrast since soft tissue does not have the highest attenuation.

  The order of attenuation of tissues from highest attenuation to lowest attenuation is: metal, bone, soft tissue, fat, air.
• Planar X-rays, coupled with injecting an iodine-based contrast agent, can be used for looking at blood vessels. An example would be endovascular repair (EVAR) of an abdominal aortic aneurysm (AAA) using X-ray fluoroscopy for guidance. In that procedure, an iodine-based contrast agent is injected into the abdominal aorta to show the AAA before a stent is delivered to the site to open up the lumen. Interventional devices such as stents are purposefully made to be attenuating to X-rays so they can be seen easily. Visualisation can be increased is plastics are doped with barium sulphate.
• Planar X-rays can be used intravenous pyelograms (IVPs), which are used to track the flow of an iodine-based contrast agent through the urinary system. An image is taken before the agent is administered, and then serial images are taken to see the agent flow through the urinary system. Urinary stones can be picked up via this method. IVPs using X-rays have largely been replaced by CT scans.
• A voiding cystourethrogram (VCUG) is an X-ray taken while a patient urinates. An iodine-based contrast agent is injected into the bladder via the urethra, and fluoroscopy helps to diagnose vesicoureteric reflux. VCUGs are also useful when looking for issues in micturition.
• X-rays are used alongside barium sulphate compounds to image the abdomen. The patient can be administered barium sulphate either alone, or with air. Barium sulphate administered by itself is known as a single contrast, and will show the lumen of the GI tract. Barium sulphate administered with air will show the walls of the GI tract (since the barium sulphate is pushed to the walls).

  Barium sulphate can be swallows to show the upper GI tract, or be administered via the rectum to show the lower GI tract.
• CT scans will give a 3D imagine when rendered. Normally, the scan will give axial slices of the image, but these scans can be viewed in many anatomical planes.

  CT imaging has a high spatial resolution, high soft tissue contrast, very ionising, and uses the contrast agents iodine and barium. It has little artefacts, and can give some dynamic imaging.
• Planar X-rays give 2D images, have high spatial resolution, but poor soft tissue contrast. They're intermediately ionising, and use the contrast agents iodine and barium. Planar X-rays have no artefacts. Radioscopy will provide static images, but fluoroscopy will provide very good dynamic images.
• Ultrasound imaging can give either 2D or 3D images. It has a very high spatial resolution, an intermediate soft tissue contrast, and is not ionising. Ultrasound can use a contrast agent of microbubbles to aid imagine, because the acoustic properties of microbubbles are different to that of surrounding visceral structures. Ultrasound has a lot of artefacts, but can give very good dynamic imaging.

  Doppler ultrasound is used to image blood flow. The colours used indicate direction of blood flow, and speed is measured on a scale.
• Needles, which are very reflective in an ultrasound image, create artefacts in ultrasound images so they can be seen easily. Interventional devices (such as needles) can be modified to make them more visible to ultrasound.
• MRI provides 3D images. They have an intermediate spatial resolution and have a high soft tissue contrast. They are no ionising, and can use the contrast agent gadolinium. MRI images contain some artefacts, and MRI gives poor dynamic images.
• Optical imaging is imaging using light; an image is built based on tissue properties to light (reflection, scattering, adsorption). Optical imaging can provide both 2D and 3D images and have a high spatial resolution. They have no soft tissue contrast and aren't ionising. Contrast agent is fluorescent dyes, optical images have no artefacts, and optical imaging can give very good dynamic images.
• Optical imaging is usually done via endoscopy, with is low risk and gives enough clinical information to make a decision. Endoscopy is often used to guide procedures, such as key-hole surgeries in the abdomen.

  Flexible endoscopes are used to look at the GI tract. The upper GI tract is viewed when the endoscope is inserted through the oesophagus, and the lower GI tract is viewed when the endoscope is inserted through the rectum.
• Nuclear medical imaging can provide 2D or 3D images. They aren't very ionising, have poor spatial resolution, and no soft tissue contrast. Contrast agent used are radiotracers. Nuclear medical imaging has minimal artefacts and gives poor dynamic images. Nuclear medical imaging is mainly used in oncology to image tumours.
• Nuclear medical imaging involves injecting a radioisotope into the GI tract to be detected and imaged. The radioisotope doesn't distribute itself everywhere however, so imaging the anatomy of the abdomen is difficult. As a result, this imaging technique is usually combined with either CT or MRI images as part of a hybrid scan, where the nuclear medical imaging will give an isotopic image, and the CT/MRI will provide an anatomical scan. The isotope will be colour-coded with a gradient and then superimposed onto the anatomical scan.
• Nuclear medical imaging is very sensitive because it involves introducing a foreign, radioactive substance into the body. Hence, detecting the substance is easy, even when the radioisotope is only present in the body in very small amounts.