"Low Radiation Dose, Prospectively Gated Coronary CTA"
Low Radiation Dose, Prospectively Gated Coronary CTA David A. Dowe, MD Atlantic Medical Imaging Galloway, NJ Cardiac CT with Gating What is gating? • It is a technique where the image reconstruction is centered to a given phase % along the R-R interval along with being centered in the Z-axis plane How do we achieve this? • The patient is attached to an ECG monitor which is in turn attached to the CT gantry. The ECG waveform is collected during image acquisition. ECG Trace - Optimizing cardiac image quality, minimizing dose Red Segment shows R-Peak detection White segment shows trigger prescription Lightspeed Retrospective Gating: • Continuous data acquisition through entire R-R interval • ECG waveform used to reconstruct images at given phases for the R-R interval along with given Z-locations • Multiple phase reconstruction capability • Variable speed imaging based on patients heart rate (HR) during acquisition • LightSpeed Pro16 – 0.4 sec • LightSpeed VCT – 0.35 sec LightSpeed Retrospective Gating Continued X-ray on Recon Delay Time ( Above example for 16 slice system ) Radiation occurs throughout the cardiac cycle. LightSpeed Gating Types of Gating: •Prospective •Retrospective Prospective Gating: • X-ray on/off is triggered by ECG R-Peak with user selectable time off-set from the R- Peak • Up to 64 contiguous images are acquired within the same cardiac cycle depending on your scanner’s detector • Ability to generate 1-5 phases per slice • Primarily used for Calcium Scoring (SmartScore) LightSpeed Prospective Gating z - Axis Location 16 Axial Images 16Axial Images Delay X-ray Off On On On Time Radiation occurs in a variable portion of the cardiac cycle. Pro-Gated Injection Technique RG-CCTA PG-CCTA Identical! Phase 1 40cc Contrast @ 40cc Contrast @ 5cc/sec 5cc/sec Phase 2 30cc Contrast + 30cc Contrast + 20cc Saline @ 20cc Saline @ 5cc/sec 5cc/sec Phase 3 30cc Saline @ 30cc Saline @ 5cc/sec 5cc/sec PG CCTA Technique Parameters RG-CCTA PG-CCTA Detector 64 x 0.625mm 64 x 0.625mm Kvp 100-120 100-120 mA Based on BMI Based on BMI Pitch Based on HR None Gantry Speed .35 sec .35 sec FOV Cardiac Medium Cardiac Medium %RR int. radiated 100% RR interval Center variable with EKG mod. window at 75% Window Padding Centered at 75% Dynamic padding done By CT scan unless you over-ride Over-ride to 10 msec Window Padding Centered at 75% At a HR= 60bpm the RR interval is 1000 msec in length. Using the rate determined computer padding we would irradiate 200 msec of the RR interval or 20% in addition to the 75% phase. Using the override padding we would radiate only 20 msec of the RR interval or only 2% in addition to the 75% phase!! Early Cases Revealed 52-74% radiation dose savings when using prospectively gated CCTA rather than retrospectively gated CCTA. LAD & RCA Plaque with Positive remodeling HR 50-52 Family hx CAD, High Cholesterol & smoker 21.94 mSv Cardiac Helical(did not show) V90 8.04 mSv Axial Step & Shoot 64%dose saving (no ECG gating) LMA & RCA calcified and non-calcified plaque HR 54-57bpm Lt side Chest Pain Family history CAD 23.02 mSv Cardiac Helical 6.97 mSv Axial Step & Shoot 70% reduction LAD Plaque HR 51-55 Family HX of CAD, ex smoker 23.15 mSv Cardiac Helical 8.22 mSv Axial Step & Shoot 64% dose reduction HR 38-44 Hypertension 19.86 mSv Cardiac Helical 9.5 mSv Axial Step and Shoot 52% dose reduction Figure 7. A&C Prospective gating B&D Retrospective gating Figure 8. A&C Prospective gating B&D Retrospective gating Figure 9. Prospective Gating Retrospective Gating Prospectively gated, 100kvp Coronary CT Angiography Further Dose Savings by Decreasing Kvp 120 kvp 100 kvp Decrease Dose by 38% in Addition to Pro-gated Savings Of 52-74% Prospective Gated CCTA Technique BMI Kvp mA mSv <25 100 450 1.6 25-30 100 550 2.0 30-32 100 650 2.3 32-34 120 650 3.7 >34 120 800 4.6 CAC Score 120 300 1.8 For most patients the dose is now less than or in the range of a Calcium Score! Tips for Using 100 kvp CCTA Lowering kvp increases image contrast. Routinely image coronary arteries with a W/L of 1200/200 instead of the 800/100 used at 120kvp. Like all Pro-gated CCTA, HR must be <65bpm. Cannot do functional CT. If BMI range is on the border between two steps on the chart be conservative and go with the slightly higher dose range. Tips for Using 100 kvp CCTA W/L=800/100 W/L=1200/200 “Skinny Minnie” 100 kvp 450 mA 10 msec padding Prospective gating Less radiation than a calcium score! Less radiation than a coronary cath!! 90% less radiation than a SPECT Sestamibi!!! 52 yo, wm asymptomatic. Hypercholesterolemia. Left Main Equivalent Left Main Equivalent 50 yo, wm, history of MI 16 years ago. Asymptomatic. +FHx, hypercholesterolemia, HTN. RCA atherosclerotic aneurysms Diffuse Coronary Ectasia 39 yo wm with chest pain. Stress test normal. +Fhx, hypercholesterolemia and HTN. High grade LAD stenosis High grade LAD stenosis. Normal LCX and RCA. 80 yo wm with recurrent chest pain and SOB. S/P CABG in 1992 and 2002. S/P PCI 2004 and 2006 Patent LIMA and LAD Stent Thrombosis within SVG to RCA 62 yo wf with chest pain. Abrupt Caliber Change RCA LAD and LCX patent. What’s that in the RLL? Bronchiectasis with Left to Right Systemic-Pulmonary Collaterals RIMA and Bronchial Arteries to RLL RCA to RLL Left to Right Shunt Thank you.