Reducing the effective radiation dose and the amount of contrast medium in CTA of the abdominal aorta

Since the introduction of MDCT technology, CT angiography (CTA) has become a
standard tool for the evaluation of disease of the aorta and its major
branches. CTA permits the selective visualization of vascular structures after
the IV injection of contrast material and the reconstruction of 3D images. As
the use of MDCT has become routine in clinical practice, concerns have been
raised regarding radiation exposure.
The current literature assumes a small but not negligible risk for radiation
induced cancer from CT examinations. Therefore, reducing the radiation dose
from CT has become an important issue. Various techniques and patient-based
strategies have focused on reducing the radiation dose delivered during CT
studies.
Numerous methods have been evaluated for radiation dose reduction during CTA
including the use of a lower tube voltage. Lowering the tube voltage represents
an important radiation reduction approach because the radiation dose varies
with the square of the tube voltage. Sigal-Cinqualbre et al were the first to
hypothesize that low-kilovoltage scanning may facilitate the reduction in
iodine load by increasing the vasculature enhancement due to the lower
effective energy more closely approximating the k edge of iodine (33 kV). By
approximating the k edge of iodine, a low-tube voltage computed tomographic
technique increases the x-ray absorption of iodine by facilitating
photoelectric interactions compared with Compton scattering effects. This
technique can substantially increase the contrast enhancement of vascular and
parenchymal structures while simultaneously reducing radiation dose to the
patient.
Low tube voltage CTA of the body with 100 keV or 80 keV represents the most
commonly applied technique for radiation dose reduction, with savings ranging
from 20% to 50% when compared to the conventional protocol employing 120 keV.
With this approach, the increased contrast between the arterial system and the
surrounding tissue at the lower tube voltage offsets the greater image noise.
The amount of contrast medium injected in a patient is of concern because of
the risk of complications, especially contrast induced nephropathy (CIN). CIN
is defined as acute kidney injury caused by exposure to intravascular iodinated
contrast medium, resulting in a decrease in the glomerular filtration rate. CIN
is the third leading cause of all hospital-acquired renal insufficiency and is
associated with increased long-term mortality.
The amount of contrast medium can be redused, using a test bolus prior to the
scan bolus. The test bolus is used to synchronize the data acquisition with the
arrival of contrast material in the abdominal aorta.

Primary Objective: Is it possible to reduce the effective radiation dose and
reduce the amount of contrast medium used in CTAs for patients, without losing
image quality, using a reduced tube voltage protocol (80 kV) or CARE kV
scanning protocol compared to patients, who are scanned with the conventional
scanning protocol of 120 kV and 100 ml contrast medium?

Intervention study, Technical Efficacy study.

Randomly, 15 Patients are assigned to undergo the CARE kV protocol using the
Combined Applications to Reduce Exposure (CARE kV) tool of Siemens,
CARE kV automatically suggest kV and effective mAs to optimize the
contrast-to-noise-ratio (CNR) of the image while limiting the applied effective
radiation dose. The system*s proposal is based on the attenuation as measured
in the topogram and the user defined acquisition type (non-contrast, bone, soft
tissue, vascular).

The remaining 15 patients are assigned to undergo the low voltage protocol
which scans the patients with the dual-source CT technique at a voltage of 80
kV and 140 kV instead of the single source CT at a voltage of 120 kV. When the
image quality of the 80 kV scans would be insufficient the 140 kV scans can be
used for diagnostic purpose.
The scans of the 30 patients mentioned above will be compared with 15 patients
scanned in a previous study using conventional settings; a tube current of 120
kV and 100 ml contrast medium [9].
In our previous study we used a test bolus reduction protocol with a test bolus
of 10 mL contrast medium (ioversol 350, 350 mg iodine per milliliter; Optiray
350, Tyco Health care, Mansfield, Mass) to synchronize the data acquisition
with the arrival of contrast material in the abdominal aorta. The test bolus
was followed by a bolus injection of 40 mL of undiluted contrast medium
(ioversol 350) and the a saline chasing bolus of 20 mL at 4 mL/sec. With this
study we will use the same test bolus reduction protocol only the injected
volume is a mixture of 1:1 contrast medium and saline. This results in a total
contrast medium injection of 25 mL, which means a 50% reduction in iodine load
compared to our previous study and even a 75% reduction in iodine load when
compared to the conventional CTA protocol which uses 100 mL of contrast medium.

There is no risk that the scans of patients undergoing the low tube voltage or
CARE kV protocol will be of less quality. The low tube voltage is associated by
a dual energy scan of 140 kV, which is performed with a second x-ray tube. The
total amount of radiation dose of the 80 kV and 140 kV scan is comparable to a
normal 120 kV scan.
There is however a risk in the reduction of contrast material. When the
reduction is too large, the image quality will not be sufficient. If this
occurs a new scan will be made immediately after the first scan, so the patient
doesn*t have to come back. This scan will be made with the multiphasic
injection method and with the use of a test bolus. When this is done the total
amount of contrast used for this patient will be 75 ml, which is still less
than 100 ml used in clinical practice. However this will be 3 times the amount
of contrast material used in patients were the image will be sufficient the
first time. Also the second scan will increase the radiation exposure about
5-11 mSv.