Single-photon emission computed tomography (SPECT)

• Uses include
  • Stress testing
    • If patient cannot exercise, i.e. requires pharmacologic stress, then PET is likely a better option
  • PYP imaging for ATTR cardiac amyloidosis screening
• SPECT acquisition
  • Elliptical orbit
    • Heart distance changes for each rotation
    • Resolution variations can occur between projections
    • May cause image artifacts in the apex (defect), base and septum (decreased counts)
  • Circular orbit
    • Heart is near the center of the rotation
    • Resolution approximately the same for all projections
    • Some projections further away from body
• Acquisition Parameters
  • Matrix: 64x64 or 128x128 - generally accepted
  • Pixel Size: 6 - 6.5 mm (Typical Extrinsic Resolution)
  • Projections: 32 to 64
  • Time per projection: 25 to 50 seconds
  • 180° or 360° arc obtained
• Solid State SPECT Imaging systems (Digital SPECT)
  • Spectrum Dynamics
    • We have the Veriton now
  • GE Discovery
  • Phantom Data Anger vs Digital Single-photon emission computed tomography (SPECT)
    • Veriton has much better spatial resolution → you can discern many more of the black dots in the figure below with the Veriton
    • 

Who is suitable for SPECT?

• Lower likelihood patients
• Lower risk patients
• No prior history of HFrEF or prior MI
• Able to exercise to an adequate level
• Relatively normal body habitus and BMI
• Previous study had good quality images
• McGhie: “If I’m going to go with pharmacologic, I would prefer PET, but sometimes insurance won’t pay for it. I don’t order very many stress echo or stress SPECT.”

Collimators

• Collimators are the “focusing” device of a SPECT camera
• Collimators limit the number of detected photons by absorbing photons outside a range of incident angles.
• Made up of holes (usually parallel)
• Exclude photons not traveling parallel to the hole
• Rated by sensitivity and resolution
  • inverse relationship (↑ resolution, ↓ sensitivity)
• See Eric’s slides for diff flavors of collimators

Cadmium Zinc Telluride (CZT)

Protocols

• 2-day protocols are often used for obese patients because of weight-adjusted dosing. For same-day protocols, you will administer 3x the initial dose for the second injection (e.g. 10 mCi + 30 mCi). For obese patients, 3x the initial dose will often exceed the daily dose limit.
• Two-day protocols require that patients sit around for at least 4-5 hrs between image acquisition. We switched to two day protocol using stress-first during COVID and have kept it up for patient convenience.
• Advantage of “stress first”/“stress only” protocols is that if the stress images are normal, then rest images may not be needed, i.e. you’re spared the radiotracer injection/dose to the patient.
  • Also, because “stress first” is relatively lower dose, you also have the advantage of potential lower radiation exposure.
  • 📝 Because you’re using lower dose of radiotracer with “stress dose”, you will typically need to have Attenuation Correction (AC), i.e. need this ability.
• Early 📄 on “stress only” versus rest from Methodist group ¹
  • “Our results support that additional rest imaging is not required in patients who have a normally appearing initial stress study. A significant reduction in radiation exposure can be achieved with such an approach.”
  • 61% lower radiopharmaceutical dose because stress images were obtained with low-dose one-/two-day protocol.

Stress-Rest

Figure source

Rest-Stress

Figure source

Combined Low-level Exercise and Pharmacologic Stress

🌟 Recommended by ASNC based on clinical experience

• Advantages
  • Improves imaging quality
    • ↓ tracer uptake in the liver/gut, improving signal-to-noise ratio
  • ↓ side effects (frequency and severity)
  • Improves patient comfort during and following exam
  • Possibly increases ischemic detection
  • Can improve prognostic value of exams
• See EXERRT trial ² that Dr. Randy Thompson was senior author on

Footnotes

1. Chang SM, Nabi F, Xu J, Raza U, Mahmarian JJ. Normal stress-only versus standard stress/rest myocardial perfusion imaging: similar patient mortality with reduced radiation exposure. J Am Coll Cardiol. 2010 Jan 19;55(3):221-30. doi: 10.1016/j.jacc.2009.09.022. Epub 2009 Nov 13. PMID: 19913381. ↩

2. Thomas GS, Cullom SJ, Kitt TM, Feaheny KM, Ananthasubramaniam K, Gropler RJ, Jain D, Thompson RC. The EXERRT trial: “EXErcise to Regadenoson in Recovery Trial”: A phase 3b, open-label, parallel group, randomized, multicenter study to assess regadenoson administration following an inadequate exercise stress test as compared to regadenoson without exercise for myocardial perfusion imaging using a SPECT protocol. J Nucl Cardiol. 2017 Jun;24(3):788-802. doi: 10.1007/s12350-017-0813-3. Epub 2017 Feb 21. PMID: 28224449; PMCID: PMC5491644. ↩