Quantitative imaging of 124I and 86Y with PET

Abstract

The quantitative accuracy and image quality of positron emission tomography (PET) measurements with (124)I and (86)Y is affected by the prompt emission of gamma radiation and positrons in their decays, as well as the higher energy of the emitted positrons compared to those emitted by (18)F. PET scanners cannot distinguish between true coincidences, involving two 511-keV annihilation photons, and coincidences involving one annihilation photon and a prompt gamma, if the energy of this prompt gamma is within the energy window of the scanner. The current review deals with a number of aspects of the challenge this poses for quantitative PET imaging. First, the effect of prompt gamma coincidences on quantitative accuracy of PET images is discussed and a number of suggested corrections are described. Then, the effect of prompt gamma coincidences and the increased singles count rates due to gamma radiation on the count rate performance of PET is addressed, as well as possible improvements based on modification of the scanner's energy windows. Finally, the effect of positron energy on spatial resolution and recovery is assessed. The methods presented in this overview aim to overcome the challenges associated with the decay characteristics of (124)I and (86)Y. Careful application of the presented correction methods can allow for quantitatively accurate images with improved image contrast.

Fig. 1

Simplified decay schemes of ¹²⁴I, ⁸⁶Y, ⁸⁹Zr and, for comparison, the ‘standard’ PET isotope ¹⁸F. Only radiation with abundance >5% is shown. Energy shown for positrons is maximum energy. Data based on [39]

Fig. 2

Degrading effects in PET, from left to right random coincidences, scattered radiation and prompt gamma coincidences where one of the annihilation photons is detected in coincidence with a prompt gamma photon. Reprinted from [4] with permission from IOP Publishing

Fig. 3

Normalized line source profiles derived from sinograms of ¹²⁴I or ¹⁸F line sources inside a cylindrical phantom filled with water. Reprinted from [10] with permission from Elsevier

Fig. 4

Images of a NEMA 1994 phantom with cold Teflon (top), water (left) and air (right) inserts. The measurements were done with an ECAT Exact HR+ (Siemens/CTI, Knoxville, TN, USA) PET scanner in 3-D acquisition mode. There is a clear bias in the Teflon and water inserts for ⁸⁶Y and, to a lesser extent, for ¹²⁴I. Reprinted with permission from [16], ©2008 Edizione Minerva Medica

Fig. 5

Projections of a torso phantom filled with ⁷⁶Br and ¹⁸F, along its short axis, as measured on an ECAT Exact HR+ scanner in 3-D mode. The decay scheme of ⁷⁶Br resembles that of ⁸⁶Y. The difference between both projections shows the contribution of gamma coincidences. The red line indicates the linear fit used to correct for prompt gamma coincidences, corresponding to 80% of the counts in the outermost bins. Adapted from [23]

Fig. 6

⁸⁶Y-DOTATOC images as measured with the ECAT Exact in 2-D mode, without (top row) and with (bottom row) subtraction of a uniform sinogram background. The corrected image shows a lower background in the liver and a considerable reduction of radioactivity concentration in the spine. Reprinted from Fig. 4 in [17] with kind permission from Springer Science+Business Media

Fig. 7

Images of a 20-cm diameter phantom containing one 3-cm, one 1.5-cm and three 1-cm diameter spheres. From left to right: ¹⁸F, ¹²⁴I without offset correction and ¹²⁴I with offset correction in the scaling of the scatter estimate. Reprinted with permission from [22], ©2009 IEEE

Fig. 8

Projections of a 10-cm off-centre cylindrical phantom with cold inserts filled with ⁷⁶Br (solid black line) and ¹⁸F (solid grey line) as well as the prompt gamma contribution for ⁷⁶Br (dashed line) as measured with an ECAT Exact HR+ in 3-D mode (a) and corresponding delayed coincidence projections (b). Shapes of delayed coincidence and prompt gamma coincidence projections are approximately similar. Reprinted from [23]

Fig. 9

NEC rates for a 20-cm diameter phantom filled with ¹²⁴I (solid line) and ¹¹C (dashed line) for the Gemini TF PET/CT scanner. Data for ¹¹C were based on Surti et al. [40] with radioactivity concentrations divided by 0.225 to account for the difference in positron abundance between ¹²⁴I and ¹¹C [31]

Fig. 10

a NEC rates, which are a measure of the signal to noise ratio, of the Gemini TF-64 with ¹¹C using the standard 440–665 keV energy window (black), for ¹²⁴I using this same window (blue) and for ¹²⁴I using a narrow 440–560 keV window (red). Activity concentrations were normalized for positron abundance. b Improvement in recovery of ¹²⁴I using the narrower energy window [31]

Fig. 11

PET images of a patient with metastatic thyroid cancer at 24 h after administration of 37 MBq ¹²⁴I acquired on a Gemini TF-64 PET/CT scanner (a) 440–665 keV and (b) 440–560 keV energy window. The narrower energy window results in a 15% improvement in image contrast in the largest metastasis (arrow) due to the decreased image background [31]

Fig. 12

Recovery for ¹²⁴I and ¹⁸F as measured with an ECAT Exact HR+ scanner in 3-D mode. Reprinted from Fig. 2 in [38] with kind permission from Springer Science+Business Media