Iterative Reconstruction

Transcription

1 RECENT ADVANCES IN CT RADIATION DOSE REDUCTION TECHNIQUES Iterative Reconstruction Kalpana Kanal, PhD, FSCBTMR, FACR, FAAPM Professor and Director, Diagnostic Physics Section University of Washington Seattle, WA SCBT-MR

2 DISCLOSURE NONE 2

3 TALKING POINTS Iterative Reconstruction (IR) How it works? Different types of IR Examples from Literature IR and Full/Half Dose IR and Spatial Resolution IR and Lesion Detection IR and Radiation Dose Clinical Implementation of IR Advantages and Disadvantages of IR 3

4 INTRODUCTION IR reconstruction is not a new concept It was first used in the 1960s for SPECT Also used in first CT in 1972 Limited by computational capabilities Re-emergence in last few years 4

5 HOW DOES ITERATIVE RECONSTRUCTION (IR) WORK? Ref: Beister et al, Physica Medica, 2012, 28:

6 DIFFERENT TYPES OF IR Pure IR (no modeling) Statistical IR (modeling of photon counting statistics) Raw data domain (sinogram domain) Volumetric images (image domain) During IR process Model based IR (go beyond statistical IR) Try to model acquisition process as accurately as possible Ref: Beister et al, Physica Medica, 2012, 28:

7 GEOMETRIC MODELING Ref: Beister et al, Physica Medica, 2012, 28:

8 VARIOUS AVAILABLE IR ALGORITHMS IN THE USA Ref: Raman et al, JACR 2013, 10:37-41 GE ASIR-V Siemens ADMIRE Ref: Beister et al, Physica Medica, 2012, 28:

9 GENERATIONS OF ITERATIVE RECONSTRUCTION (GE) ASiR ASiR-V Veo Physics Noise Object Noise Object Optics Real-time performance (up to 35 ips) Reduce dose up to 82% Profound image quality under 1 msv Full model-based IR Longer reconstruction times Slide courtesy: GE Medical Systems Ref: Beister et al, Physica Medica, 2012, 28:

10 ASIR - V Slide courtesy: GE Medical Systems 10

11 SIEMENS ADMIRE Five image noise and sharpness levels Strength 1 Strength 4 Strength 5 There is an incremental reduction of noise in each of the images at a natural image impression. Slide courtesy: GE Medical Systems 11

12 FULL DOSE VS. HALF DOSE Images acquired with half dose have 1.4 times SD values without ASIR (28.57 vs 20.39) Reconstructing images with 30% ASIR for half-dose acquisitions produce images with noise nearly equivalent to that of full dose images without ASIR Degree of dose reduction is greater for patients with a lower BMI 57 yr old woman, BMI = 18 Ref: Hara et al, AJR, Sep kv, 3.75 mm, CTDI = 8 mgy, no ASIR 120 kv, 3.75 mm, CTDI = 8 mgy, ASIR 140 kv, 3 mm, CTDI = 22 mgy 12

13 SPATIAL RESOLUTION Spatial resolution has been shown to degrade with IR. Slightly decreased sharpness of cyst edges and a mildly irregular or jagged margin of solid organs 120 kv, 3.75 mm, CTDI = 11 mgy, no ASIR 120 kv, 3.75 mm, CTDI = 11 mgy, ASIR 140 kv, 3 mm, CTDI = 20 mgy 13

14 IR AND LESION DETECTION Ref: Jensen et al, Eur Radiol, July

15 IR and Rad Dose Ref: Padole et al, AJR,

16 CLINICAL IMPLEMENTATION OF IR How does one go about implementing IR? Section buy-in is important, have all radiologists on board Start at vendor recommendations based on clinical application, then fine-tune? Important to reduce dose upfront by at least 20-25% before using IR Be prepared for resistance to new technology/process Continuous iterative improvement process 16

17 ADVANTAGES OF IR Reduce image noise and artifacts depending on degree of modeling By modeling cause of artifacts during reconstruction, IR methods represent more intuitive and natural way of image reconstruction Potential for significant patient dose reduction Using model based IR has better lesion conspicuity than FBP Reading off thinner slices as low noise, improves lesion detection Using statistical based IR does not show significant lesion conspicuity 17

18 DISADVANTAGES OF IR Artificial appearance of image Increased computational effort Evaluation of image noise and resolution is more complex Statistical modeling for low dose CT electronic noise more dominant Difficult to adopt by radiologists as noise patterns and artifacts look different Are we missing diagnostically important information? Spatial resolution degrades slightly at high IR levels 18

19 ARE WE MISSING DIAGNOSTICALLY IMPORTANT INFORMATION? Hypodense seam along the stent indicating proliferation of the vessel intima which may ultimately lead to stent stenosis Hypodense seam not visible in IR image Hypodense seam confirmed by interventional angiography Ref: Beister et al, Physica Medica, 2012, 28:

20 CONCLUSIONS Iterative reconstruction methods are available on modern scanners Adoption and implementation varies depending on facility Faster computational speeds will make model-based IR more practical to use Radiologist adoption due to different image quality (less noise, more artificial look) and confidence level (are we missing something) varies 20

21 THANKS! TUESDAY 11:20 12:05: Workshop (flipped classroom) Recent Advances in CT Radiation Dose Reduction Techniques William Shuman, MD, FSCBTMR Kalpana Kanal, PhD, FSCBTMR Reduction-Techniques 21

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23 ASiR-V Slide courtesy: GE Medical Systems 23

24 SIEMENS ADMIRE (Advanced Modeled Iterative Reconstruction) Potential to lower radiation Superb details, delineation and sharpness of organ borders Positive impact on the image quality of e.g. streak artifacts in the shoulder region Slide courtesy: Siemens Medical Systems 24