Gastro Radiology

A. Negative contrast Media - Air, CO2, O2. 

• B. Positive contrast media - 
• 1. Iodine based 
• 2. Barium based

Ionic - Urograffine (Sodium Diatrozoate 76%)

Non ionic - Amnipaque, Omnipaque, Ultravist and Iosvist

Diatrizoate (Gastrografin, Iothalmate, or Urografin) is an iodinated ionic radiocontrast agent with high osmolality

Antecubital vein to abdominal aorta – takes 15 seconds approx.

Early arterial phase -  15-30 seconds after injection of dye

Late arterial phase (pancreatic phase) - 45 seconds after injection of contrast.

Hepatic or late portal venous phase - 90 seconds after injection of contrast.

Nephrogenic phase - 100 seconds after injection of contrast

Delayed phase - After 6-10 seconds


Note – Most of the hepatic pathology is supplied by hepatic artery. Normal liver parenchyma is supplied by portal vein. So, most of the hepatic pathologies are best seen in arterial phase. Any enhancing lesion in arterial phase should be considered pathological.

• - Precontrast phase scan
• - Arterial phase 
• - Portal phase 
• - Delayed phase 

Most of the vascular neoplasms of liver will wash out the contrast media very rapidly.  They take the contrast rapidly and wash out the contrast rapidly. In the below picture, in arterial phase, the lesion takes the contrast rapidly, in portal phase the lesion is masked, and in delayed phase, it is clearly seen because the contrast is rapidly washed away while liver is still accumulating the contrast.

• Hepatic artery - seen only in arterial phase.
• Hepatic vein – seen only at the level where IVC is located in visceral surface of liver.
• Portal veins – rest of the vessels  except hepatic artery and hepatic veins. At porta hepatis or hilum of liver, we can find portal vein

• Hepatic artery
• Portal vein – most commonly seen structure, other structures are rarely seen
• CBD

• - Lies obliquely, tail is above, and head is below.
• - Head – 3cm, body – 2.5cm, tail – 2 cm
• - Pancreatic density is similar to unopacified bowel, almost similar or less dense compared with liver or spleen.
• - Vessel running posterior border of pancreas – Splenic vein
• - Pancreatic head – lower film than that of body and tail. Anatomical landmarks – SMA and SMV. SMV is larger.
• - In normal variants, the vessel coming from anterior aspect of aorta is always the SMA. We cannot see the celiac artery because it is short. IMA is also not seen because it is shield. So, the only vessel seen in regular abdominal scan coming from anterior aspect of aorta is SMA.

Pancreas protocol - 3mm cut

• The 'double contrast' refers to the use of positive and negative contrast agents to increase the sensitivity of the examination.
• The double contrast study is sensitive to visualize mucosal irregularities.
• Positive contrast: barium or barium-like agent, e.g. Gastrograffin
• Negative contrast: air or CO2. 

The purpose is to detail radiographically the mucosal lining of the large intestine.

Procedure - As the barium fills the intestine, x-rays of the abdomen are taken,  the rectal tube is removed and the patient expels as much of the barium as possible. A thin film of barium will remain in the intestine, and air is then slowly  injected to expand the bowel lumen.

• Here vessel (artery) is delineated in a better way by eliminating other tissues through computer system.
• AV fistulas, haemangiomas, lesion in circle of Willis, vascular tumours, other vascular anomalies are well made out.
• Dye is injected either to an artery or vein. Injecting into a vein is technically easier but larger dose of dye is required. Injecting into an artery is technically difficult but small dose of dye is sufficient.

Advantages: Only vascular system is visualised; other systems are eliminated by computer subtraction. Small lesion, its location and details are better observed with greater clarity.

Disadvantages: Cost factor and availability.

Complications: Anaphylaxis, bleeding, thrombosis

The hydrogen proton can be likened to the planet earth, spinning on its axis, with a north-south pole. Under normal circumstances, these hydrogen proton “bar magnets” spin in the body with their axes randomly aligned.

When the body is placed in a strong magnetic field, such as an MRI scanner, the protons' axes all line up. This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner.

When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected. The radio wave frequency (RF) that causes the hydrogen nuclei to resonate is dependent on the element sought (hydrogen in this case) and the strength of the magnetic field. The strength of the magnetic field can be altered electronically from head to toe using a series of gradient electric coils, and, by altering the local magnetic field by these small increments, different slices of the body will resonate as different frequencies are applied.

When the radiofrequency source is switched off the magnetic vector returns to its resting state, and this causes a signal (also a radio wave) to be emitted. It is this signal which is used to create the MR images. Receiver coils are used around the body part in question to act as aerials to improve the detection of the emitted signal. The intensity of the received signal is then plotted on a grey scale and cross sectional images are built up.

• Positron Emission Tomography (PET) is a nuclear imaging technique that produces a 3-D image of functional processes in the body by detecting the radiation emitted by photons. 
• The system detects pairs of gamma rays emitted indirectly by positron emitting radionuclide (tracer), which was previously injected in body on a biologically active molecule.
• 3-D images of tracer concentration within the body are then constructed by computer analysis.

• Principle - 
• - Injection of Short lived Radioactive Isotope in body.most commonly used is FDG (fluoro-2-deoxyglucose).
• - Wait till tracer gets accumulated in tissues of interests.
• - Subject is placed in the imaging scanner
• - Tissue concentration is recorded with time.
• - As isotope decays in body, it releases a positron in body.On interaction with an electron, it produces a pair of photons.
• - PET scanner detect these photons and with the help of a computer creates pictures offering details on both the structure and function of organs and tissue in the body.
• 

• Applications - 
• - Neuroimaging
• - Clinical oncology (medical imaging of tumors).
• - Musculo-skeletal imaging
• - Cardiology
• - Pharmacology
• - Neuropsychology

• Side effects 
• - Contraindications in pregnant woman 
• - Allergic reactions 

• Drawbacks 
• - Expensive 

[Note - Positron is the mirror image of an electron. Positron has the same mass as that of electron but with opposite charge]