Coronary computed tomography angiography with 320-row detector and using the AIDR-3D: initial experience

ABSTRACT Coronary computed tomography angiography (coronary CTA) is a powerful non-invasive imaging method to evaluate coronary artery disease. Nowadays, coronary CTA estimated effective radiation dose can be dramatically reduced using state-of-the-art scanners, such as 320-row detector CT (320-CT), without changing coronary CTA diagnostic accuracy. To optimize and further reduce the radiation dose, new iterative reconstruction algorithms were released recently by several CT manufacturers, and now they are used routinely in coronary CTA. This paper presents our first experience using coronary CTA with 320-CT and the Adaptive Iterative Dose Reduction 3D (AIDR-3D). In addition, we describe the current indications for coronary CTA in our practice as well as the acquisition standard protocols and protocols related to CT application for radiation dose reduction. In conclusion, coronary CTA radiation dose can be dramatically reduced following the “as low as reasonable achievable” principle by combination of exam indication and well-documented technics for radiation dose reduction, such as beta blockers, low-kV, and also the newest iterative dose reduction software as AIDR-3D.


INTRODUCTION
Coronary computed tomography angiography (CTA) examination's role was established on the last American College of Cardiology/American Heart Association (ACC/AHA) guidelines as a non-invasive imaging method to evaluate coronary artery disease and some cardiovascular diseases (1) . Nowadays, coronary CTA estimated effective radiation dose can be dramatically reduced by using state-of-theart scanners, such as dual-source CT (DSCT) and 320-row detector CT (320-CT) (2) , without changing coronary CTA diagnostic accuracy (3) . To optimize and further reduce the radiation dose, new iterative reconstructions algorithms were recently released by several scanners manufacturers (4) , and now they are routinely used in coronary CTA. This paper presents our first experience in using coronary CTA with 320-CT and the Adaptive Iterative Dose Reduction 3D (AIDR-3D). In addition, it describes the coronary CTA indications as well as the acquisition protocols related to this new CT application for radiation dose reduction.

THE CORONARY CTA INDICATIONS
Each coronary CTA indication demands a proper CTA scan protocol, which might increase the radiation dose. For example, protocols designed to evaluate coronary artery bypass graft surgery and "triple-rule-out coronary CTA" often require higher doses than standard coronary CTA (1) .
The majority of patients referred to our institution for coronary CTA have prior equivocal cardiovascular exams, such as the treadmill stress test and the singlephoton emission computed tomography (SPECT), as main exam indication.
Other indications for coronary CTA related to current appropriate criteria score (ACCF/SCCT/ACR/ A H A / A S E / A S N C / N A S C I / S C A I / S C M R 2 0 1 0 Appropriate Use Criteria for Cardiac Computed Tomography) are displayed on chart 1 (1) .

Indication
Clinical features Score*

PATIENT PREPARATION TO CORONARY CTA
Patients referred to coronary CTA can receive oral or intravenous beta blockers to reduce heart rate unless they have contraindications, such as overt heart failure, asthma, or atrioventricular conduction abnormalities (5) . Beta blocker dose protocols of our institution are detailed on chart 2. Sublingual isosorbide dinitrate (3.75mg) is administrated routinely prior to coronary CTA if not contraindicated due to pulmonary hypertension, severe aortic stenosis, the use of phosphodiesterase type 5 inhibitors (such as the use of sildenafil citrate in the last 24 hours, or tadalafil in the last 72 hours) and migraine (5) .

THE SCANNER
In our institution, coronary CTAs are performed in two 320-row CT scanners (Aquilion ONE, Toshiba Medical Systems, Tochigi-ken, Japan). All patients are scanned with prospective electrocardiographic (ECG) gating/triggering, independently of heart rate. This technique uses forward-looking prediction of R-wave timing, step-and-shoot non-spiral acquisition with no table motion during imaging, and unique cone beam reconstruction (6) .
The scanning plan is based on body mass index (BMI), in order to apply the lowest kV and mA for each patient (Chart 3), using the Sure Exposure 3D ® (Tochigi-ken, Japan) with an automatic exposure control system (7) .
The z-axis coverage or the range varies from 10 to 16cm, and the 12 to 14cm range is used in about 75% of our patients. Scanner standard values provided by the manufacturer are displayed on chart 3. There is no table movement, so pitch is zero. Reconstruction algorithm uses the "half" protocol, which increases temporal Chart 2. Institutional lowering heart rate protocols with orally and intravenously beta blockers resolution to 175ms (8) , and Xact + (on), that corrects cone beam angle. Iodinated contrast media (Henetix ® 350mg/mL, Guerbet, Lille, France) is injected using dual-head injection system, volume ranges from 50 to 100mL, according to patient's BMI and coronary CTA indication, followed by a 50mL flush of saline solution (5) .

DOSE REDUCTION STRATEGY: ITERATIVE RECONSTRUCTION
CT scan images are formed from reconstructions of projections of the radiation detected in multiple angles in a tomography scan, such as back-projection (BP) or filtered back-projection (FBP) associated to iterative reconstructions, since 1970 (4) . The term "iterative" refers to a method of successive approximations until satisfactory agreement with an arbitrary starting image. Therefore, iterative reconstructions by definition repeat the reconstruction process several times, and are much slower than analytic methods (4) .
The increase of low-dose CT scanning implies reduction of the number of photons reaching the detector, and results on a decrease in the signal-to-noise ratio and more strike artifacts (8,9) .
The AIDR-3D is a recent iterative reconstruction algorithm composed by lots of operations launched by Toshiba Medical Systems (Tochigi-ken, Japan). The aim of the operations in the projection data space is to reduce streak artifacts caused by photon starvation. Therefore, a 3D-smoothing filter is applied to the photon count values, which performance is fu rther enhanced with statistical models of the noise and the scanner. In the meantime, the AIDR-3D operations occurs in the image reconstruction domain, in order to obtain iterative noise reduction (8) . The final process involves a weighted blending of the iterative and the primary reconstruction to create AIDR-3D image. As a result of this blending, the images retain a more typical CT appearance, as if they were simply acquired with standard CT exposure parameters (10) (Figure 1). Nowadays, the AIDR-3D can be applied to all acquisition modes for routine clinical use and is able to eliminate up to 50% of image noise, resulting in dose reduction of up to 65% (8) (Figure 2).

CONCLUSION
In conclusion, coronary computed tomography angiography radiation dose can be dramatically reduced, following the ALARA ("as low as reasonable achievable") principle, combining the exam indication with well-documented technics in coronary computed tomography angiography, such as beta blockers, low-kV, and the use of dose reduction software, as AIDR-3D.