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344 Myocardial Imaging Products : Continuing Evolution For The Better

content in the delayed image, though it was a mis- lipophilic) in planar imaging could also be avoided
nomer; the difference was due to differential wash- to a large extent by SPECT. The rest is history, with
out of the tracer between high and low blood flow nearly 40-50% of all the large volume imaging done
regions. With the advent of SPECT, there was an with 99m Tc being that of myocardium. It is said that
important change for the better, superior resolution one 99m Tc based imaging study is being done every
image slices of myocardial perfusion became a rou- second somewhere in the world, and thus, one myo-
tine feature, with dose of 201 TlClincreased (up to 130 cardial imaging study using 99m Tc is being done once
MBq (3.5 mCi)). 201 Tl has been a good physiological every 2 to 2.5 seconds. Further, the ability to acquire
tracer for MPI, but is not ideal from the point of view ECG-gated images (when required), with the larger
of image quality and higher absorbed radiation dose amount of injected dose possible in the case of 99m Tc
to the patients. Medical cyclotrons (MC) of 30 MeV products, render it possible for acquiring ECG-gat-
protons (of a Belgium company) became highly pop- ed-data and deriving cardiac pumping parameter (left
ular for deployment for production of 201 Tl. 201 Tl was ventricular ejection fraction, LVEF), in addition to MP
produced by (p,3n) reaction on enriched 203 Tl target, imaging.
followed by a 2-stage processing, using automated PET tracers - NH , Rb , F: The fastest growing
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radiochemical processing module, and rendering reli- imaging modality since 2001is PET-CT, and this has
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able availability of TlCl for medical usefor well over turned the focus on PET tracers for MPI. PET modality
201
two decades, and, in turn, made nuclear cardiology has the advantage of attenuation correction leading
a regular feature of clinical NM.
to superior resolution images of the ‘defective’ re-
gions, as well as better quantitation feature. Sr- Rb
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Technetium-99m compounds: generator, and the use of Rb (K analogue) for PET
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The well-established merits of 99m Tc (6 h half-life; Eg imaging of myocardial perfusion, has increasingly at-
140.5 keV (89%)) for high quality imaging triggered tracted global attention, particularly after the 2008-
in late 1970s and early 1980s much R&D efforts 2010 supply crisis of Mo- 99m Tc. Sr with a half-life
99
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world over for developing 99m Tc-based myocardial of 25 days renders generators of shelf-life of about a
agent. The approach was to design and optimise month or two. Half-life of Rb (75 seconds) is favour-
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a moderately lipophilic, cationic complex of 99m Tc, able for repeat cardiac studies, wherever warranted,
to mimic the potassium/thallous ion, and facilitate as for example, for certain need-based physiologi-
uptake by the myocardium and fast clearance from cal and/or pharmacological intervention procedures.
the blood pool. This culminated in the development The other PET tracerto cite is NH (10 min half-life)
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and launch of Tc(I) hexakis isonitirle mono-cationic as NH for MPI, although it is not of large use due
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complex, called 99m Tc-sestamibi, by a US-based com- to requirement of on-site MC.Among the many PET
pany in 1986. This was subsequently supplemented radionuclides for use in nuclear cardiology, F with
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by another similar mono-cationic complex product, the lowest positron energy (0.635 MeV) and relatively
called 99m Tc-tetrofosmin, launched by a company in long half-life of F, 110 min, would be the best suit-
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UK. The 99m Tc-based products understandably did not ed for logistics reasons and to provide high resolu-
have the same desired features as Tl , and thus two tion images. Thus, F-flurpiridaz (structural analog of
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separate injections were necessary to elicit stress pyridaben), taken up by mitochondrial binding, is a
and rest perfusion pattern in patients. Further, their product in an advanced phase of clinical evaluation
myocardial extraction efficiency is 50-60% of that of and seems set to emerge as the winner, meeting all
201 Tl (compensated partly by the larger injected dose the key requirements, namely: uptake proportional to
of 99m Tc, up to 740 MBq (20 mCi); 1.5-2% uptake in blood flow; convenience of 110 min half-life of F;
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myocardium). Phasing out of 201 Tl did notoccur for low positron energy enabling high resolution images;
well over the next 10-15 years, despite the advantag- relatively lower cost (cf. Rb , NH ).
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es of 99m Tc in production logistics, patient dose and
cost; only since 2000, perceptible reduction in 201 Tl Quantitation advantage with PET tracers:
use has occurred.
Over time, the role of NM myocardial imaging moved
The arrival of 99m Tc based products for MPI and the beyond perfusion, responding to the importance and
entry of SPECT in NM took place more or less at role of absolute quantitation of myocardial flow, ex-
the same time. This led to MPI using 99m Tc products pressed in terms of Coronary Flow Ratio (CFR - ratio
being done with SPECT, and made the image quality of the myocardial blood flow at peak hyperemia to
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much superior to that obtainable with Tl . The inev- resting), in management of cardiac patients with cor-
itable liver shadow with 99m Tc products (being highly onary stenosis. To derive CFR values, tracers whose
GCDC 2017