Chapter IV Instrumentation Sections: 7, 9 ,10 :Chapter IV Instrumentation Sections: 7, 9 ,10 By: Natasha, Jessenia and Matthew


Objectives :Objectives Discuss how to determine energy resolution (Full-width at half maximum) Describe how to calculate well counter efficiency. Explain how to determine the energies that will be accepted in a symmetrical pulse height analyzer window with a center line and percent window setting.


Dose Calibrator :Dose Calibrator Measures the amount of Radionuclides in units of curies, millicuries or in the SI unit Becquerel It is a shielded ion chamber with settings for specific isotopes which can give an approximate reading of the activity of that specific isotope.


Energy resolution :Energy resolution The energy resolution test evaluates the sharpness of a photo peak produced when scintillation instrument is exposed to a mono-energetic nuclide . A sharp Bell-shaped peak means proper functioning of the equipment. Widening of the bell means malfunction of some system component. In order to evaluate energy resolution a long lived mono-energetic source , like Cs137 is used Its gamma photo-peak occurs at 662kev.


Cont’d :Cont’d According to the standard of practice, the percent resolution should fall between 6.5% and 12% for Cs137. The width of the resulting peak is measured at half the peaks height.


The Formulas :The Formulas For Energy resolution: Half –Maximum = Maximum counts / 2 Full-width at half maximum: % energy resolution = FWHM in kev / energy of radionuclide in kev x 100%


What the graph looks like: :What the graph looks like:


Example :Example The energy range at Half- maximum for a Cs137 source is from 635 kev to 689 kev. What is the percent energy resolution and is it acceptable? 689kev-635kev = 54kev 54kev / 662kev x 100 = 8.2 % 8.2 % This value IS within acceptable range.


Well Counter Efficiency :Well Counter Efficiency A calculation of well counter efficiency determines the % of emissions produced by a radioactive source that are actually detected by the instrument % efficiency= counts per unit time/(disintegration per unit time )(mean number per disintegration) X 100%


Convert dps to dpm :Convert dps to dpm 1 uCi= 3.7 x 104 dps To convert dps to dpm you must: (3.7 x 104 dps)(60 sec)= 2.22 x 106 dpm or 2,220,000 dpm/uCi  2,220,000 dpm/uCi is a constant


Example :Example The current activity of Cs137 stick source is 0.2 uCi. Cs137 has an abundance at 662 keV. A 1 minute acquisition produces 175,000 counts. What is well counter effciency? 175,000 cpm/(.2 uCi)(2,220,000 dpm/uCi)(.85) X 100%= 46%


Window calculations: Centerline plus percent window :Window calculations: Centerline plus percent window The pulse height analyzer (which is an instrument that accepts signals from a detector and categorizes the pulses on basis of signal strength). Is usually set as a percent window around the centerline The centerline is the energy of the photon being imaged, and the percent window is a percent of that energy.


This formula will determine the energies accepted in a symmetrical pulse height analyzer window with a centerline and percent window setting :This formula will determine the energies accepted in a symmetrical pulse height analyzer window with a centerline and percent window setting Energies within window = centerline energy+- Energy in KeV x percent window/ 2


Example: :Example: If a pulse height analyzer is set for a 20% window for Tc99m, what energies will be accepted? Tc99m has an energy of 140 KeV. 140 KeV(+-) 140 KeV x 0.20 / 2 = 140 KeV (+-) 14 KeV or 126 KeV to 154 KeV


Example :Example When a 30% window is used for the 364 KeV gamma photo peak of I131, which energies are imaged? A 20% window? A 10% window? 364kev (+-) 364x0.30 / 2 = 55kev or 309 to 419kev 364kev (+-) 364x0.20 / 2 =36kev or 328 to 400kev 364kev (+-) 364x0.10 / 2 = 18kev or 346 to 382kev


Question 1 :Question 1 If the energy spread for Cs137 is 630kev to 694kev at half maximum, does the instrument have an acceptable energy resolution?


Answer 1 :Answer 1 694kev-630kev = 64 kev 64 / 662 x 100 = 9.7 kev 9.7 is acceptable, because it falls between 6.5% and 12%


Question 2 :Question 2 What energies are included if a 20% window is used for the 93 kev peak of Ga67? The 185 kev window?


Answer 2 :Answer 2 93kev (+-) 93kev x 0.20 / 2 = 9kev (+-) = 84 to 102kev 185kev (+-) 185kev x 0.20 /2 = 18kev (+-) = 167 to 203


Question 3 :Question 3 What energies will be accepted by a 15% window set around the 140kev Tc99m peak?


Answer 3 :Answer 3 140kev (+-) 140kev x 0.15 / 2 = 10 (+-) = 130 to 150kev


Question 4 :Question 4 If a 20% window is set around the 296kev peak of Ga67, what energies will be accepted by the pulse height analyzer?


Answer 4 :Answer 4 296kev (+-) 296kev x 0.20 / 2 = 30(+-) 266kev to 326


Question 5 :Question 5 A Co57 source with a known activity of .18 uCi produces a counting rate of 256,600 cpm. If it has an abundance of 87% for a 122 keV gamma ray, what is the efficiency of the instrument?


Answer 5 :Answer 5 256,600 cpm/ (0.18 uCI)(2,220,000)(.87) x 100%= 256,600/347652= 0.74 0.74 x 100%= 74%


Question 6 :Question 6 A 0.3uCi source of Cs137gives a counting rate of 340,000 cpm in a well counter. What is the counter efficiency? Cs137 has an 85% abundance at 662 keV.


Answer 6 :Answer 6 cpm / (.3 uCi)(2,220,000)(.85) X 100%= 340,000/566,100= 0.60 0.60 x 100%= 60%


Question 7 :Question 7 A Cs137 source containing 0.2 uCi activity provides 140,000 cpm. What is the well counter efficiency? Note: Cs137 has an abundance of 85%


Answer 7 :Answer 7 140,000 cpm/ (.2 uCi)(2,220,000)(.85) X 100% 140,000/ 377400= 0.37 0.37 X 100%= 37%


Question 8 :Question 8 If an I131 source with an activity of .95 uCi produced a counting rate of 335,700 cpm, what would be the well counter efficiency be? I131 has an 83.8% abundance at 364 keV.


Answer 8 :Answer 8 335,700cpm/(.95 uCi)(2,220,000)(.838) x 100% 335,700/ 1767342= 0.19 0.19 x 100%= 19%


Question 9 :Question 9 Determine the % energy resolution for Cs137 when the energy spread at the half-maximum is 600 keV to 720 keV. Is it in acceptable range?


Answer 9 :Answer 9 keV – 600 keV=120 keV 120 keV/ 662 keV=0.18 0.18 X 100%= 18% No it is not in acceptable range


Question 10 :Question 10 The energy range at half maximum for a Cs 137 source is from 68.5kev to 63.7kev what is the percent energy resolution? And is it acceptable?


Answer :Answer 68.5 – 63.7 x 10 = 48 kev 48kev/ 663kev x 100% = 7.25% Yes it is acceptable due to its range


Summary :Summary With the energy resolution test we are able to evaluate the sharpness of a photo-peak, to help us determine whether the equipment was functioning properly or malfunctioning. With the well counter efficiency we were able to determine the emissions produced by a radioactive source. We are now also able to determine the energies that will be accepted in a symmetrical pulse height analyzer window with a centerline and percent window setting.


References :References Patricia Wells, Martha Pickett (1999). Practical Mathematics: In Nuclear Medicine technology. Virginia: society of nuclear medicine. Edited by Paul E. Christian, Kristen M. Waterstram-Rich (2007). Nuclear Medicine and PET/CT: Technology and Techniques (6th edition). Missouri: Mosby Elsvier Radiological Society of North America, Inc (2008). General nuclear medicine : Equipments used in Nuclear medicine. Retrieved May 20, 2008, from http://www.radiologyinfo.org/en/info.cfm?pg=gennuclear