Imaging Strategies

Author:  Charles Allison, MB ChB

Editor: Ashley Davidoff MD

The Common Vein Copyright 2006

IMAGING STRATEGIES

BACKGROUND: 

Epigastric pain is usually attributable to foregut structures (including the esophagus, stomach, duodenum and pancreas). Liver pathologies, though derived from the foregut, typically present as RUQ pain. Possible etiologies of epigastric pain include:

Esophagitis

Gastritis, gastric ulcer, gastric carcinoma

Duodenitis, duodenal ulcer

Pancreatitis, pancreatic carcinoma

Myocardial ischemia

Choice of imaging clearly hinges on the suspected diagnosis. Weight loss and chronicity is more suggestive of malignancy. Exertional pain with cardiac risk factors may suggest angina. Pain related to food is more likely to represent peptic ulcer disease. Acute, severe epigastric pain raises the possibility of a perforated duodenal ulcer.

WHAT STUDY?

Endoscopy, CXR or CT are the modalities most likely to be required.

Upright chest radiographs can be used to exclude free air under the diaphragm if a perforated ulcer is suspected.

EGD (esophagogastroduodenography) is the investigation of choice in suspected peptic ulcer disease. It directly visualizes the esophagus, stomach and duodenum and can thus aid diagnosis of esophagitis, hiatus hernia, gastritis, peptic ulcers and gastric carcinoma. It has the added benefit of allowing biopsies to be taken in suspicious masses or ulcers and can be used therapeutically if there is bleeding.

Barium meal can be used to aid diagnosis of peptic ulcer disease but is now rarely used.

CT abdomen is the investigation of choice for suspected pancreatitis or suspected cancer. Oral contrast (gastrograffin) and intravenous contrast should be used if possible. Mucosal lesions in the upper GI tract such as gastritis or ulcers may be missed. It also involves a relatively large dose of radiation; approximately 500 times that of a plain chest radiograph, the equivalent of about 3.3 years of normal background radiation.

MRI provides excellent cross sectional imaging, providing superior images of the soft tissues and organs. It is non invasive but expensive and may be more geographically limited than CT. It also takes much more scanning time in somewhat claustrophobic conditions hence some patients are not able to tolerate the procedure.

Nuclear myocardial imaging may be used if angina is suspected.

Cardiac catheterization may be used for acute epigastric pain accompanied by ECG changes consistent with ST elevation myocardial infarction.

WHY?

Upright chest radiographs cause any peritoneal free air to collect as an abnormal lucency under the diaphragm; allowing radiological diagnosis in around 70% of cases of perforation. Free air may also be seen on plain abdominal films, but is more easily missed.

EGD is the investigation of choice in suspected peptic ulcer disease. This is because in addition to diagnosis it can be used to obtain tissue samples in gastric ulcers to exclude malignancy and to control active bleeding through cautery or injection.

Barium meal involves swallowing of a barium preparation followed by a series of radiographs taken of the upper abdomen. Mucosal defects representing ulcers in the stomach and duodenal bulb may be seen, as may masses consistent with tumor. Delayed emptying may support a diagnosis of duodenal ulcer in the bulb causing pyloric ‘stenosis’. Barium contrast in the gut makes subsequent CT examinations difficult to interpret and hence several days are often required for the barium to pass before further examinations can be performed. The test has now been supplanted by EGD and CT.

CT scanning is very fast and provides information on a wide spectrum of pancreatic and gastrointestinal pathologies. It provides information of the severity of pancreatitis and helps detecting spread (lymph nodes, distant metastases) and hence staging of tumors. The administration of iv contrast may enhance the rich vascular supply of tumors.

MRI provides equivalent diagnostic accuracy to CT in most pathologies. Though it does not carry the radiation burden of CT, it is more expensive, time consuming and may be more geographically limited and less acceptable to the patient. These factors make it less used than CT as a diagnostic modality in acute pain.

Nuclear myocardial stress imaging allows visualization of tissue perfusion at rest and under stress, supporting the diagnosis of angina if this reduced perfusion related to symptoms. This may be of particular use when the pain is atypical, the patient female, non-ambulatory or if the baseline ECG is abnormal, all of which make regular treadmill stress testing less reliable or unfeasible. At rest, myocardial perfusion is impaired only when coronary stenosis exceeds 90%. During stress, perfusion is impaired with stenosis above 50%. Pharmacological agents may be vasodilators (adenosine / dipyridamole), which rely on stenosed vessels not dilating to allow increased flow, or inotropes (dobutamine), which produce true myocardial ischemia. Dobutamine is favored only when the patient is unable to tolerate the vasodilators (e.g. patients with asthma or COPD in whom adenosine may provoke bronchospasm). Such nuclear tests can also detect increased pulmonary uptake of the isotope during stress, which correlates to left ventricular dysfunction, and transient ischemic dilatation of the left ventricle after exercise, which correlates to a higher cardiovascular risk.

Cardiac catheterization directly images the coronary arterial supply and can be used to dilate or place stents across acute thromboses in order to restore flow (in the setting of acute myocardial infarction).

WHEN?

Imaging should be performed at time of presentation if the patient is in acute pain to expedite diagnosis / management.

HOW TO ORDER?

It is important to reference the symptom or the sign as the clinical indication and not the diagnosis.

ie Patient with “epigastric pain” is acceptable, whereas R/O pancreatic cancer is not acceptable.  

Optimal ordering would list symptoms/signs as well as the putative diagnosis being investigated: “epigastric pain, weight loss, r/o pancreatic cancer”. This allows the radiologist to suggest alternative imaging strategies if a suboptimal approach has been ordered.

PATEINT PREPARATION

Plain radiography for an upright chest involves the patient standing with a plate in front of them and the radiation source behind for a standard PA view. They will be asked to take a deep breath in, and hold it for a second while the x-ray is taken. The entire procedure takes less than a minute.

EGD is typically performed by a gastroenterologist, under waking sedation. Local anesthetic spray is use to numb the oropharynx and benzodiazepine sedation is administered prior to the procedure. The procedure itself usually takes under 30 minutes, though time is required after the procedure for the effects of sedation to wear off. Patients should not drive the same day of the procedure. The most common side effect is sore throat.

Barium meals require ingestion of a barium solution followed by a series of plain radiographs over a period of time.

CT preparation depends on the use of contrast. For renal stones no contrast in required and the patient may be scanned immediately. Administration of oral contrast required drinking or NG administration of 30cc gastrograffin diluted in 900cc water to ensure it adequately coats the length of the bowel. Intravenous contrast in given in the scanner and may be timed to highlight the arterial or venous supply to a specific organ. The patient’s allergies, medications and renal function need to be identified before hand as the contrast is iodine based and carried risk of precipitating renal failure in at risk patients (diabetic, on metformin etc). The study itself takes about 1 minute once the patient is on the table

Patients undergoing MRI should be warned that they will be required to lie still in a confined tunnel-like space for up to 30 minutes. The machine is very noisy and ear protection will be provided. They will have a call button to abort the procedure if a problem should arise.

Nuclear stress testing involves intravenous administration of the radioisotope technetium 99m sestamibi to map myocardial perfusion. This is typically done both at rest and immediately after stress (exercise or pharmacological) to build a three dimensional image of healthy myocardium, ischemia and infarct. The patient will be asked to lie still on a gantry on two occasions (at rest and after intravenous administration of pharmacological stressor) while a detector array maps perfusion. Patients with asthma or pulmonary obstructive disease should not receive adenosine, the standard ‘stressor’ (causes vasodilation but also bronchospasm) but should receive dobutamine instead. The entire procedure may take a half-day in the nuclear medicine department.

Cardiac catheterization involves passage of a catheter through a peripheral artery (typically femoral) under local anesthesia, and subsequent retrograde passage of the catheter to the coronary arteries for injection of iodine based dye. Fluoroscopy is used to view the patency of the coronary arterial supply. Stenoses can be opened with balloon dilatation or placement of expandable stents, all via the catheter. Diabetic patients should receive prophylactic n-acetylcysteine for renal protection; patients with chronic renal disease are at risk of worsening renal function, a fact that should be considered pre-procedure. The procedure length may vary from under 15 minutes to longer depending on the therapy given and patient factors. Patients will remain lying flat for four to six hours post-procedure to prevent bleeding from catheter site. Patients with no co-morbidities catheterized on an elective basis may go home the same day, but in the setting of emergent catheterizations (e.g. for pain) will usually require hospitalization.

CLINICAL RED FLAGS:

Pancreatic cancer is notoriously difficult to diagnose symptomatically. There should be a low threshold for scanning in patients with a history of unintentional weight loss with chronic epigastric pain, particularly with increasing age and a past history of smoking.

REFERENCES:

What are the radiation risks from CT? – US Food and Drugs Administration

http://www.fda.gov/cdrh/ct/risks.html