No patients enrolled as study did not move to enrollment stage

Patients with an allergy to iodine based intravenous contrast cannot undergo a CT scan with contrast, unless they are premedicated. Premedication can result in a delay in diagnosis, that can be significant with acute conditions such as Pulmonary embolism.

The investigators are evaluating the use of Gadolinium as an alternative contrast agent in CT. Gadolinium has been used extensively in MRI exams but the dose that is used is not sufficient to provide good enhancement on CT. Dual Energy CT is a new technology that has better sensitivity to lower doses of intravenous contrast and the investigators aim to study its effectiveness in improving the enhancement with Gadolinium. The investigators will compare enhancement of Iodine with enhancement of Gadolinium during Dual Energy CT, to assess the possibility that Gadolinium could be used as an alternative for patients who are suspected of having Pulmonary embolism. They will recruit up to 50 patients who are having a routine Chest CT during treatment for a malignancy and administer Gadolinium contrast and Iodine contrast during a chest CT exam. The enhancement with the two agents can be compared.

Although safe in most patients, iodine may be associated with lifethreatening adverse events, particularly in high-risk patient. This application is of particular clinical significance in patients with conditions like pulmonary embolism which could be fatal. CT pulmonary angiogram (CTPA) with sensitivity of 83-100% and specificity of 89-97% is currently the gold standard for diagnosis of pulmonary embolism. However, in patients who have allergy to iodinated contrast agent, CTPA cannot be performed. Gadolinium based contrast agent can serve as a viable alternative in such patients and can be used for Gadolinium enhanced MR pulmonary angiography (MRPA) and CTPA. Gadolinium enhanced MRPA however has inherent limitations in evaluating pulmonary embolism as evident from multicentre prospective study (PIOPED III) which reported technically inadequate exams in as many as 25% of patients (range 11% to 52%). The MRPA sensitivity was also low in the range of 78%. Moreover, unlike CT, low spatial and temporal resolution of MR hampers precise evaluation of lung parenchyma. Previously investigators have evaluated gadolinium enhanced CTPA on a single energy CT (SECT) at different tube potential (80 - 120 kV) and reported encouraging results. However, these patients received higher volume of gadolinium contrast (mean 53 ml, range 30-64 ml) and in spite, the scans performed at 120 kV demonstrated overall suboptimal pulmonary artery enhancement. These limitations can be potentially over come with dual-energy CT which simultaneously acquires low and high energy data. From this data, monochromatic images can be reconstructed at any desired energy from 40 keV to 140 keV.

The investigators aim to recruit up to 50 patients who will guide in determining gadolinium contrast volume, injection rate and image delay required to maximize pulmonary arterial enhancement.

Inclusion Criteria:

• patients required to undergo a clinically indicated CT examination

Exclusion Criteria:

• subject less than 56 yrs of age
• pregnant women
• Non English speaking
• Abnormal Renal function
• Allergy to Gadolinium