Introduction
Nuclear medicine has been engaged in considerable transformational change;
this is particularly true in the current decade. In part, this is due to developing
technology, software algorithms, disease management pathways and models,
and evidence based medicine. The merging of functional and anatomical
technologies (‘hybrid imaging’, for example, SPECT-CT) is beginning to provide a
more focused approach to disease management.
Within current clinical practice, various types of computed tomography (CT)
configurations are available for purchase with a SPECT system, and the workload
of a department as well as its financial constraints will have an influencing factor
on the final specifications of such a purchase
1
. Historically, low resolution/dose,
single slice CT units (eg GE Hawkeye) provided an initial platform to undertake
routine clinical hybrid SPECT-CT imaging; more recently the advent of multi slice
high resolution (diagnostic quality) CT has resulted in nuclear medicine centres
utilising this technology in routine SPECT-CT clinical practice.
This article focuses on several aspects of SPECT-CT that appear to have particular
importance within clinical practice. First, through case illustrations, an indication
of where SPECT-CT has value is given; a debate is then raised about the added
radiation dose that is incurred through CT. Building on this, the scope of practice
of radiographers is considered in light of practitioner and referrer status
2
for
the CT component of SPECT-CT and, finally, education and training and business
planning matters are considered.
Areas in which SPECT-CT has value
Figure 1. Myocardial
and abdomen has made diagnosis difficult. Non-homogenous attenuation can
perfusion scan, rest
affect sensitivity and specificity. To counteract this, AC can be used to counteract
Attenuation Correction
images only. Row A =
no AC; row B = CT AC
attenuation artefacts. Attenuation is an exponential process (linear attenuation
An obese 43 year-old female who had experienced left sided chest pain
applied.
coefficient); when measured, the value depends upon the proportion of scatter
underwent an exercise tolerance test (with ECG); no ST depression was noted. included in the measurement
4
. In SPECT imaging, the spatial distribution of the
The origin of her cardiac related pain was required and, consequently, the patient linear attenuation coefficient data and impact upon the raw projection data is
underwent stress and rest myocardial perfusion imaging (MPI; see figure 1). The unknown, and additional information (such as the information provided from CT) is
images shown are rest only; row A is not attenuation corrected (AC); row B is required to correct for the effects of attenuation and/or scatter
3
.
with CT AC applied. On comparing rows A and B, the attenuation induced photon
deficient artefact (on the non AC images) is corrected on the CT AC images, so Evidence suggests a necessity to interpret attenuation corrected studies as part
ruling out potential myocardium infarction (the stress study demonstrated similar of an overall quality control process
3
. This may include various steps but the
photon deficient areas on the non AC images). This is an interesting case as the added value of attenuation corrected images should be considered as part of the
resting CT AC images rule out any problem in the inferior wall but pinpoint the complete imaging process, and to aid the overall clinical decision making process.
problem in the anterior wall. This case illustrates that the use of attenuation In terms of clinical studies which have utilised AC for cardiac imaging, the majority
correction in MPI should only be used as a tool to support decision making in include small to medium sample sizes. Although Ficaro et al’s study in 1995
5
relation to establishing the presence of attenuation artefacts
3
. included a limited number of patients (n=10), lateral-to-posterior and basal-to-
apical wall ratios of near unity value were demonstrated using AC. The use of AC is
For certain SPECT studies non-homogeneous photon attenuation within the chest further supported in more recent research, also undertaken by Ficaro et al in 2002
6
.
2009
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