OtherCONTINUING EDUCATION

SPECT/CT Physical Principles and Attenuation Correction

James A. Patton and Timothy G. Turkington
Journal of Nuclear Medicine Technology March 2008, 36 (1) 1-10; DOI: https://doi.org/10.2967/jnmt.107.046839

Article Figures & Data

Figures

  • FIGURE 1. 
    FIGURE 1. 

    (A) Conventional SPECT techniques use scintillation camera to acquire projection images around patient in orthogonal geometry. (B and C) Count profiles are filtered and backprojected into image space (B) to produce reconstructed arrays of counts representing slices of activity from region of interest (C). (D) These images are not accurate representations of actual distribution because of attenuation effects.

  • FIGURE 2. 
    FIGURE 2. 

    (A) Plot of percentage of transmitted photons as function of depth in 20-cm cylindric phantom measured in single planar projection image for radionuclides commonly used in nuclear medicine. (B) Plot of percentage of transmitted photons from 99mTc source as function of depth in lung, water, and bone.

  • FIGURE 3. 
    FIGURE 3. 

    Plot of transmitted photons as function of depth in 20-cm cylindric phantom calculated from data in Figure 2A by summing 2 planar projection images separated by 180°.

  • FIGURE 4. 
    FIGURE 4. 

    (A and B) CT data are acquired in fanbeam geometry where individual rays represent transmitted photon intensities from multiple projections around patient. (C and D) These data can be reformatted into orthogonal geometry similar to that used for SPECT.

  • FIGURE 5. 
    FIGURE 5. 

    Transmitted intensities can be used to solve for attenuation coefficients (μ) by using unattenuated intensity (Io) by attenuation equation (I = Io e−μx). (A and B) Using filtered backprojection, array of attenuation coefficients for each anatomic slice can be determined (A) and converted to array of CT numbers for display purposes (B).

  • FIGURE 6. 
    FIGURE 6. 

    Array of attenuation correction factors (B) can be determined from attenuation coefficient measurements determined from CT scan and used to correct emission counts from uncorrected SPECT scan (A) to provide final attenuation-corrected SPECT scan (C).

  • FIGURE 7. 
    FIGURE 7. 

    Typical energy spectrum of x-rays from x-ray tube. Filtered curve shows effects of filtration (beam hardening), which is used for CT. These data can be applied for attenuation correction of single-photon emitters such as 99mTc using bilinear model shown in Figure 8.

  • FIGURE 8. 
    FIGURE 8. 

    Bilinear model commonly used for converting measured CT numbers to attenuation coefficients for specific radionuclide such as 99mTc.

  • FIGURE 9. 
    FIGURE 9. 

    (A) Anterior and lateral planar 99mTc-sestamibi images of patient with parathyroid adenoma. SPECT/CT study with GE Infinia-Hawkeye demonstrated anatomy of region of interest with CT (B) in transverse and coronal images. (C) Corresponding transverse and coronal images of emission distribution with SPECT clearly identified presence of adenoma. (D) Fused images of 2 datasets provided precise anatomic location of lesion in left mediastinum. Availability of multiple fused images from transverse, sagittal, and coronal planes provided surgeon with accurate anatomic roadmap for use in operating room.

  • FIGURE 10. 
    FIGURE 10. 

    (A–C) Phantom configuration used for accuracy verification of CT attenuation correction techniques. (D) Count profiles of uncorrected and attenuation-corrected data with 99mTc.

  • FIGURE 11. 
    FIGURE 11. 

    Plot of ratio of uncorrected and attenuation-corrected activity measurements for radionuclides used in attenuation correction validation experiment.

  • FIGURE 12. 
    FIGURE 12. 

    Phantom configurations using 99mTc spheric sources for evaluation of accuracy of attenuation correction as function of lesion location. Arrows show lesion location in 20-cm cylindric phantom (A) and thorax phantom (B).

  • FIGURE 13. 
    FIGURE 13. 

    Plot of ratio of uncorrected and attenuation-corrected activity measurements for phantom configurations shown in Figure 12 using 99mTc.

  • FIGURE 14. 
    FIGURE 14. 

    (A) CT angiogram of patient demonstrating multiple calcium deposits, patent left internal mammary artery graft, previously diagnosed subtotal occlusion of LAD, and newly identified probable stenosis of distal LAD. (B) Attenuation-corrected rest and stress perfusion study showing new region of ischemia in apex (arrows). Rows 1 + 3 and 2 + 4 are short-axis stress and rest images, respectively. Rows 5 and 6 are vertical long-axis stress and rest images, respectively. Rows 7 and 8 are horizontal long-axis stress and rest images, respectively. (C) Three-dimensional fusion of CT angiogram and stress perfusion studies demonstrating correlation between probable stenosis of distal LAD and region of ischemia in apex.

  • FIGURE 15. 
    FIGURE 15. 

    Front (A) and back (B) views of SPECT/CT registration phantom and (C) example of error evaluation performed with this phantom.

  • FIGURE 16. 
    FIGURE 16. 

    Demonstration of attenuation correction artifacts in perfusion study (A) resulting from misalignment of CT and SPECT datasets (B) and removal of these artifacts (C) after realignment of datasets (D) using registration quality assurance software.

Tables

  • TABLE 1

    Collimator and Radionuclide Combinations for CT Attenuation Correction Evaluation

    RadionuclideEnergyCollimator
    201Tl70 keVLow-energy, high-resolution
    99mTc140 keVLow-energy, high-resolution
    111In172 and 247 keVMedium energy
    131I364 keVMedium energy
    131I364 keVHigh energy
    18F511 keVHigh energy
Back to top

In this issue

Print
Download PDF
Article Alerts
Email Article
Citation Tools
Bookmark this article

Jump to section