Questions

Answers

1. What intracellular ion does thallium (Tl) most closely mimic?

1. potassium

2. What is the half-life of ²⁰¹Tl?

2. 73 hours

3. How does ²⁰¹Tl decay?

3. electron capture

4. What element does ²⁰¹Tl decay to?

4. ²⁰¹Hg (mercury-201)

5. True or false: The photons used for ²⁰¹Tl imaging come from the decay of ²⁰¹Tl.

5. false (They come from the decay of ²⁰¹Hg.)

6. What are the energies and abundances of the photons used for ²⁰¹Tl imaging?

6. 69–83 keV (95% abundance), 135 keV (3.5% abundance), 167 keV (10% abundance)

7. What percentage of ²⁰¹Tl entering the coronary circulation is extracted by the myocardium?

7. 88%

8. What is the relationship between ²⁰¹Tl extraction by the myocardium and coronary blood flow at physiologic flow rates?

8. a linear increase in ²⁰¹Tl uptake proportional to coronary blood flow

9. What happens to the extraction efficiency of ²⁰¹Tl at very high flow rates?

9. It increases.

10. What happens to the extraction efficiency of ²⁰¹Tl at very low flow rates?

10. It decreases.

11. How much of an injected dose of ²⁰¹Tl remains in the blood five minutes after injection?

11. 5%

12. When does peak uptake in the myocardium occur after injection of ²⁰¹Tl?

12. Peak uptake occurs in 10 to 20 minutes.

13. How much of the intravenous ²⁰¹Tl dose localizes in the myocardium in a normal patient?

13. 5%

14. What is the difference between early and delayed ²⁰¹Tl uptake?

14. Early uptake represents blood flow; delayed uptake represents redistribution to equilibrium.

15. What are the two causes of decreased uptake on early ²⁰¹Tl images?

15. reduced blood flow or absence of viable myocardium (scar)

16. What is the cause of reduced uptake on delayed ²⁰¹Tl images?

16. scar

17. What is the cause of reduced uptake on post-stress images with fill-in on delayed images?

17. myocardial ischemia

18. What patient preparation is recommended for ²⁰¹Tl imaging?

18. The patient should fast for four hours before injection.

19. Why should ²⁰¹Tl injection be as close as possible to a direct venous injection?

19. to prevent adsorption to intravenous tubing or to venous structures being exposed to medications

20. What is the usual initial dose of ²⁰¹Tl for myocardial imaging?

20. 2.0 to 3.5 mCi (74.0 to 129.5 MBq)

21. When is imaging begun after a dose of ²⁰¹Tl for myocardial imaging?

21. 2–5 minutes

22. What type of collimator is used for ²⁰¹Tl for planar myocardial imaging?

22. low-energy, high-resolution, or low-energy all-purpose collimator

23. What are the choices for energy windows for imaging of ²⁰¹Tl for myocar-dial imaging?

23. a 20% to 25% asymmetric window centered at 80 keV or a 20% symmetric window centered at 69 to 83 keV, and an optional 20% window centered at 167 keV

24. What is the advantage of using an asymmetric window centered at 80 keV for ²⁰¹Tl imaging?

24. This reduces measurement of scattered higher-energy K-α ²⁰¹Hg X-rays scattered in the energy range of the lower energy ²⁰¹Hg K-β X-rays.

25. What is the advantage of using a 20% window centered at 167 keV for ²⁰¹Tl imaging?

25. It will increase the count rate by 10%.

26. What type of collimator is used for ²⁰¹Tl in single-photon emission computed tomography (SPECT) myocar-dial imaging?

26. low-energy, high-resolution, or low-energy all-purpose collimator

27. What are typical details of a SPECT acquisition for ²⁰¹Tl myocardial imaging?

27. 180-degree arc from 45-degree right anterior oblique to 135-degree left posterior oblique, with imaging completed in 20 to 35 minutes (Acquisition details differ among camera systems.)

28. What is apical thinning on ²⁰¹Tl myocardial imaging?

28. physiologically reduced uptake in the apex

29. What kind of attenuation artifact is commonly seen in women on ²⁰¹Tl myocardial imaging?

29. anterior wall reductions in uptake, due to breast attenuation

30. What artifact does diaphragmatic attenuation cause on imaging?

30. decreased uptake in the inferior wall

31. What is the only organ of the body that does not take up ²⁰¹Tl?

31. the brain (Thallium does not cross the blood–brain barrier.)

32. Which organs (other than the heart) normally accumulate ²⁰¹Tl?

32. salivary glands, thyroid gland, skeletal muscles, kidneys

33. How is ²⁰¹Tl myocardial imaging used for risk stratification and prognosis after myocardial infarction?

33. Patients with larger infarcts and/or marginal ischemia are at higher risk of mortality.

34. What is stunned myocardium?

34. injured myocardium that is distal to a lysed thrombus that has normal ²⁰¹Tl uptake but is akinetic on wall motion studies

35. What is hibernating myocardium?

35. chronically ischemic myocardium that is viable but appears cold on immediate ²⁰¹Tl images and is akinetic on wall motion studies

36. What is the rationale for using exercise in ²⁰¹Tl myocardial perfusion imaging?

36. Normal coronary arteries dilate during exercise and flow increases, while stenotic vessels do not dilate and flow does not increase to the same degree as in normal myocardium.

37. How does one calculate the maximum predicted heart rate?

37. 220 minus age (in beats per minute)

38. What percentage of maximal predicted heart rate is considered an adequate stress test?

38. 85%

39. How do you calculate the double product of an exercise stress test?

39. maximum systolic blood pressure multiplied by maximum heart rate

40. What is the most common reason for a false-negative ²⁰¹Tl stress test?

40. patient’s failure to exercise adequately

41. What should be done with cardiac drugs before a ²⁰¹Tl stress test?

41. They should be stopped at the discretion of the patient’s attending physician.

42. What medications interfere with cardiac stress testing, and how long before the cardiac stress test should they be stopped?

42. beta (β) blockers (72 hours), calcium channel blockers (48 to 72 hours), and long-acting nitrates (12 hours)

43. For a stress myocardial perfusion study, how long should patients continue to exercise after injection of ²⁰¹Tl?

43. 30 to 90 seconds

44. How much time should patients wait between injections of ²⁰¹Tl for a stress myocardial perfusion study, and why do they not wait for longer or shorter times?

44. 10 minutes. Longer periods may miss rapid redistribution, whereas shorter periods cause “cardiac creep” artifact.

45. How do planar stress ²⁰¹Tl myocar-dial images differ from rest images?

45. On the stress images, the target-to-background ratio is higher, right ventricular activity is commonly seen, and less activity is seen in the liver and abdominal structures.

46. What is the meaning of normal ²⁰¹Tl myocardial uptake on rest and post-stress images?

46. normal myocardium

47. What is the meaning of reduced ²⁰¹Tl myocardial uptake on the post-stress images and normal uptake on the rest images?

47. ischemic myocardium

48. What is the meaning of reduced ²⁰¹Tl myocardial uptake on the post-stress images and improved uptake on the rest images?

48. ischemic myocardium, possibly with scar

49. What is the meaning of reduced ²⁰¹Tl myocardial uptake on the post-stress images and the same uptake on the rest images?

49. scar

50. Which three items should be described for myocardial perfusion defects?

50. location, size, and severity

51. What is the term for reduced ²⁰¹Tl myocardial uptake on the rest images and the same uptake on the post-stress images?

51. reverse redistribution

52. Which areas of the myocardium are served by the left anterior descending coronary artery?

52. the anterior wall and septum

53. Which areas of the myocardium are served by the left circumflex coronary artery?

53. the lateral and posterior walls

54. Which areas of the myocardium are served by the right coronary artery?

54. the inferior wall, the inferior septum, and the right ventricle

55. What two findings, other than myocardial defects, should be described on ²⁰¹Tl myocardial perfusion imaging?

55. left ventricular dilation and pulmonary uptake

56. What is the upper limit of normal for the quantitative lung-to-heart ratio on ²⁰¹Tl myocardial perfusion imaging?

56. 0.5

57. What are the possible causes of increased lung uptake on ²⁰¹Tl myocar-dial perfusion imaging?

57. left ventricular dysfunction, severe coronary artery disease, and heavy tobacco smoking

58. What is “bulls-eye analysis” of ²⁰¹Tl myocardial perfusion imaging?

58. a quantitative presentation of myocardial uptake and washout

59. Why is the specificity of ²⁰¹Tl myocardial perfusion imaging difficult to measure?

59. Only patients with positive or equivocal myocardial perfusion imaging are sent for cardiac catheterization.

60. What is the sensitivity of ²⁰¹Tl myocardial perfusion imaging for the detection of coronary artery disease among patients without a previous history of coronary artery disease who exercise adequately?

60. 80% to 85%

61. How is ²⁰¹Tl myocardial perfusion imaging used to manage the patient with an acute myocardial infarction?

61. Patients with a single fixed defect are given medical therapy; all others are considered for more invasive evaluation and therapy, because they are at higher risk for subsequent cardiac events and death.

62. How is ²⁰¹Tl myocardial perfusion imaging used to assess a patient with a previous coronary artery bypass or angioplasty?

62. Successful revascularization results in no areas of ischemia, scars will not be affected by revascularization, and perioperative infarction of an ischemic area will appear as a fixed defect.

63. What is the rationale for reinjection of ²⁰¹Tl before delayed myocardial perfusion imaging?

63. 15% to 35% of ischemia segments do not fill in or normalize by four to five hours.

64. What are the alternatives to exercise for performing stress ²⁰¹Tl myocar-dial perfusion imaging?

64. dipyridamole, adenosine, and dobutamine

65. How much can dipyridamole and adenosine increase coronary blood flow?

65. four-fold to five-fold

66. What enzyme does dipyridamole inhibit?

66. adenosine deaminase

67. What is the main advantage of adenosine over dipyridamole?

67. Adenosine has a shorter plasma half-life than dipyridamole.

68. What are the main symptoms and side effects of adenosine and dipyridamole?

68. chest pain, nausea, vomiting, dizziness, headache, shortness of breath, hypotension

69. What is the antidote of dipyridamole?

69. aminophylline, 125 to 250 mg IV

70. What additional patient preparation is required for patients undergoing stress testing with adenosine or dipyridamole?

70. Patients must be off methylxanthines (e.g., theophylline) and caffeine.

71. Where in the myocardial cell does ^99mTc-sestamibi localize?

71. the mitochondrion

72. How does the myocardial extraction of ^99mTc-sestamibi compare with ²⁰¹Tl?

72. At resting flows, the extraction fraction of ^99mTc-sestamibi is approximately half that of ²⁰¹Tl and decreases with increasing flow.

73. How much of a dose of ^99mTc-sestamibi remains in the blood at 10 minutes after injection?

73. approximately 5%

74. What is the clearance time of ^99mTcsestamibi from the myocardium?

74. five hours

75. Why should ^99mTc-sestamibi myocar-dial imaging be delayed until at least 30 minutes after injection?

75. to allow for clearance of the tracer from the lungs and liver

76. How is ^99mTc-sestamibi excreted?

76. by hepatobiliary and renal routes

77. What is the usual time between injection of ^99mTc-sestamibi and myocardial imaging?

77. 30 to 90 minutes

78. What is the usual dose of ^99mTc-sestamibi for myocardial perfusion imaging?

78. 10 to 30 mCi (370 to 1110 MBq)

79. What two types of additional myocar-dial imaging can be performed with ^99mTc-sestamibi but not with ²⁰¹Tl?

79. gated SPECT and first-pass radionuclide angiography

80. What is the advantage of performing gated SPECT and first-pass radionuclide angiography with ^99mTc-sestamibi myocardial perfusion imaging?

80. Both ventricular function and myocardial perfusion can be assessed with a single dose of the radiopharmaceutical.

81. What doses of ^99mTc-sestamibi are used for a one-day rest/stress myocardial perfusion imaging protocol?

81. approximately 10 mCi (370 MBq) for rest imaging, followed by 30 mCi (1110 MBq) for stress imaging

82. What doses of ^99mTc-sestamibi are used for a two-day rest/stress or stress/rest myocardial perfusion imaging protocol?

82. approximately 30 mCi for both rest and stress imaging

83. How do the diagnostic criteria for infarction and ischemia differ between ²⁰¹Tl and ^99mTc-sestamibi?

83. In fact, they do not differ.

84. What is the difference between first-pass study cardiac ventriculography and equilibrium cardiac ventriculography?

84. In first-pass studies, all data collection occurs during the initial transit of tracer through the heart; in equilibrium studies, data are collected over many cardiac cycles using the tracer that remains in the blood pool.

85. What is the radiopharmaceutical of choice for equilibrium-gated blood pool imaging?

85. ^99mTc-labeled red blood cells (RBCs)

86. What are the names of the three different methods for labeling red blood cells (RBCs) with ^99mTc?

86. in vivo, modified in vivo, and in vitro

87. How is in vivo labeling of RBCs with ^99mTc performed?

87. The patient is injected with cold stannous pyrophosphate; 15 to 30 minutes later the patient is injected with ^99mTc-pertechnetate.

88. Where in the RBC does ^99mTc bind?

88. to the beta (β) chain of hemogloblin

89. What is the labeling yield of the in vivo method for labeling RBCs with ^99mTc?

89. approximately 80% and as low as 60% to 65%

90. What are five causes of poor in vivo RBC labeling?

90. drug interactions, circulating antibodies, incorrect amount of stannous ion, excess carrier ^99mTc, inappropriate procedure

91. What are six drugs that can cause poor in vivo RBC labeling?

91. heparin, iodinated contrast media, doxyrubicin, methyldopa, hydralazine, quinidine

92. How can poor labeling of RBCs be detected?

92. by excessive gastric, thyroid, and soft tissue uptake

93. How is modified in vivo labeling of RBCs with ^99mTc performed?

93. The patient is injected with cold stannous pyrophosphate; 15 to 30 minutes later, 3 to 5 mL of blood are withdrawn into an anticoagulated syringe containing ^99mTc-pertechne-tate. The syringe is incubated for 10 minutes; the patient is then injected with the contents of the syringe.

94. What is the labeling yield of the modified in vivo method for labeling RBCs with ^99mTc?

94. approximately 90% or greater

95. How is in vitro labeling of RBCs with ^99mTc performed?

95. Blood is withdrawn into an anticoagulated syringe; the blood is added to a reaction vial containing stan-nous chloride; after incubation, sodium hypochlorite is added to the vial and incubated; the patient is injected with the contents of the vial.

96. What is the labeling yield of the in vitro method for labeling RBCs with ^99mTc?

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