Iodine Mapping using Subtraction in Pulmonary Embolism Computed Tomography versus Dual Energy Computed Tomography.

Pulmonary embolism (PE) is the third most common acute cardiovascular disease
after myocardial infarction and stroke and is fatal in up to 30% of patients.
Prompt diagnosis and treatment is critical and has been presumed to reduce
mortality in up to 10%. Since 2007, multidetector CT pulmonary angiography
(CTPA) has been accepted as the reference standard for the diagnosis of acute
PE. However, conventional pulmonary CTPA provides only morphologic information
and does not allow functional assessment of the effects of thromboembolic clots
on lung blood perfusion. Blood clots associated with pulmonary embolism cause
abnormal blood flow with perfusion defects in a segmental or lobar
distribution. Especially in case of small, peripheral pulmonary embolisms, it
is these perfusion defects that are visible signs of pulmonary embolism. In
conventional CTPA, enhancing pulmonary parenchyma cannot be distinguished from
unenhancing tissue due to almost similar attenuation values. Color-coded maps
of lung-attenuation tackle this problem to a little extent but are not able to
visualize the contrast medium distribution selectively.

Dual energy CT does enable selective blood flow imaging by spectral
defragmentation and generation of iodine maps. These iodine maps are able to
demonstrate abnormalities that correspond to the histopathologic changes in
acute and subacute pulmonary embolism. This increases sensitivity for detection
of embolisms, in particular for small emboli at a subsegmental level or in more
distal vessels. In addition, perfusion imaging might help in determining
prognosis and therapy monitoring. Dual energy based perfusion imaging has
therefore been successfully implemented in clinical practice as a complementary
tool in detection of PE.

To evaluate image quality and accuracy of detection of perfusion defects
associated with pulmonary pathology on iodine maps of the lung that are created
by two different CT techniques: 1. A standard of care CTPA with DECT and 2. A
new technique that subtracts a low radiation dose unenhanced CT from
mono-energetic CTPA (subtraction).

A total of 375 patients will undergo a standard CTPA with DECT according to
local clinical guidelines. For the purposes of this study, patients will
undergo an additional unenhanced, low-radiation dose chest CT. Standard
reconstructions of all scans and DECT iodine maps will be obtained for clinical
reporting and subsequent treatment decisions, according to standard clinical
routine. For research purposes, selected mono-energetic images will be
post-processed using a novel subtraction algorithm to create iodine maps of the
lungs. The iodine maps based on the subtraction algorithm will not be used for
clinical management, only the additional unenhanced scan will be used in
clinical management.

CT imaging is associated with risks related to the use of radiation and
iodinated contrast administration. No additional contrast will be used as
compared to standard clinical practice as patients will only undergo one CTPA
scan. The CT protocol of this study has been carefully designed to have a
radiation dose identical or even lower than standard CT protocols for pulmonary
embolism detection. The estimated dose-length product (DLP) of standard CTPA
with DECT in Meander Medical Centre is 167 mGy-cm (effective dose is 2.4 mSv,
using 0,0146 mSv/mGy-cm as a conversion factor. We will expose patients who
participate in the study to an estimated additional DLP of 72 mGy-cm due to the
unenhanced scan, resulting in an additional estimated effective dose of 1,0
mSv. This implies that the total radiation dose is within the same range as
radiation doses of other scans for PE detection in the Netherlands. The
additional scan is not obligatory in pulmonary embolism diagnosis, but will be
used for clinical evaluation of these patients.