Quantification of [11C]MP4A PET studies

Analysis methods used in literature

Sato et al. (2004) have made an excellent comparison of most of the proposed methods for a closely related PET tracer, [11C]MP4P.

Compartment model for [11C]MP4A

For the quantification of AChE activity in the brain, a three-compartment (two-tissue compartment) model is applied (Iyo et al., 1997; Nagatsuka et al., 1998; Namba et al., 1999; Herholz et al., 2000; Shinotoh et al., 2000; Shiraishi et al., 2005). The rate constant k3 represents the rate of hydrolysis of [11C]MP4A by AChE. The diffusion of [11C]methylpiperidinol ([11C]MP4OH), the radioactive metabolite of [11C]MP4A, through the blood-brain-barrier is negligible during the PET study, thus k4=0 in the brain, but not in other organs.

Very close correlation between K1, k2 and cerebral blood flow (CBF) have been found (Herholz et al., 2000). Linear regression of K1 on CBF gave a slope of about one 1.1 and intercept close to zero, suggesting that the first-pass extraction rate for MP4A is very high (Herholz et al., 2000).

Reference tissue -based analysis

In the cerebellum and striatum, hydrolysis is extremely rapid, i.e. k3 >> k2, therefore

Nagatsuka et al. (2001) suggested using cerebellum as a positive reference region for a linearized calculation model, and validated its results against standard non-linear fitting with plasma input; the TRTM method performed better than shape analysis (see below). Marcone et al (2012) applied the model to computation of parametric images. For CLI software, see fit_trtm and lhtrtm for regional analysis and imgtrtm for calculation of parametric k3 images.

Herholz et al. (2001) used putamen and caudate nucleus as reference region, but a quite different mathematical method for estimation of k3. For software, see hm4mpa. This method leads to negative bias and it was later developed further and applied to produce parametric k3 images by the same group (Zündorf et al., 2002), and used in later studies (Haense et al., 2012; Richter et al., 2014, 2017, and 2018).

Radioactivity ratio of cortical regions to the cerebellum 30 to 40 min after tracer administration was calculated by Ota et al. (2004) to develop a simple analysis method for future SPECT tracer. The [11C]MP4A ratios were correlated better with k3 (r=0.69) than with K1 (r=0.33). The diagnostic sensitivity for AD was 92%. Coefficient of variance of the [11C]MP4A ratio was small compared with k3 (Ota et al., 2004). In this method it is assumed that extraction fraction is similar in all brain regions. Also blood flow in cortical regions and in cerebellum (ratio) affect the [11C]MP4A ratio, but in degenerative diseases this may even increase the sensitivity for detecting abnormal change (Ota et al., 2004).

None of the reference tissue input methods can be applied to AChE inhibition studies, if marked inhibition is seen in positive reference regions, because in that case the assumption k3 >> k2 does not hold.

Shape analysis

Shape analysis (Koeppe et al., 1999) was first introduced to analyze another AChE tracer, [11C]MP4P, also called [11C]PMP, which has somewhat lower specificity for AChE in the human brain. The shape analysis method was also used to analyze [11C]MP4A studies and found to be sensitive technique to detect cortical AChE changes in patients with dementia (Tanaka et al., 2001) but less sensitive in detecting changes in Alzheimer’s disease than the reference region input method (Nagatsuka et al., 2001).

Effect of age

AChE activity, measured as [11C]MP4A k3 or ratio in the cerebral cortex does not change significantly or at all with age (Namba et al., 1998 and 1999; Ota et al., 2004).

Suggested analysis method for TPC

Image processing

Movement correction may have to be applied to dynamic PET images (Östberg et al, 2011).

PET images are summed over frames, coregistered with MRI, and regions of interest are defined as usual. For regional analysis, ROI TACs are calculated from the dynamic PET image. TACs should be weighted before fitting.

Without arterial blood sampling

[11C]MP4A ratio

Ota et al. (2004) suggested the use of regional [11C]MP4A ratio for diagnostics of Alzheimer’s disease. Ota et al. also proposed using the ratio for monitoring the effects of AChE inhibitors, but this approach has not yet been validated.

Use CLI programs dftratio and imgratio for computing the regional ratio or ratio image, respectively, from 30 to 40 min after injection.

Reference-tissue compartmental model

Since in putamen k3 >> k2, putamen TAC can be used as positive reference region in TRTM method. This method has been used in both regional analysis and to calculate parametric k3 images (Östberg et al, 2011). Regional analysis can be done using program fit_trtm and imgtrtm for calculation of parametric k3 images.

With arterial blood sampling

Compartmental model fit method (Iyo et al., 1997; Namba et al., 1999) with arterial plasma input has been the recommended method for analysis of acetylcholinesterase inhibition studies, but due to the challenges in measurement of arterial plasma input function, and especially in metabolite correction, reference-tissue input methods are considered preferable.

Pre-processing plasma input

Necessary data files:

  1. On-line blood sampler data file
  2. Count-rate curve
  3. Plasma curve from manual sampling
  4. Plasma parent fractions

Then, follow the instructions on input data processing. Note that for [C-11]MP4A:

Before proceeding, make sure that both the plasma and tissue data are in the same calibration units.

Notice that group differences in metabolite fractions may occur. For example, the brain β-amyloid load, as measured with [11]PIB, has been shown to correlate with plasma AChE activity (Alkalay et al., 2013). Blood cells, including erythrocytes, contain AChE, which may lead to further metabolism after blood sampling.

Regional AChE activity (k3)

After all the previous steps have been done successfully, the enzyme activity k3 can be calculated using fitk3. Program fitk3 allows constraining K1/k2 to a value determined as the mean estimate across cortical regions of all subjects from the unconstrained fits: this has been suggested for related tracer [11C]PMP (Koeppe et al., 1999; Kuhl et al., 1999). If K1/k2 is not constrained, you should consider reporting (K1/k2)×k3 as an index of enzyme activity, instead of k3.

Constraints for model parameters can be set with a text file with the following contents:

K1_lower := 0 
K1_upper := 2 
K1k2_lower := 1 
K1k2_upper := 15 
k3_lower := 0 
k3_upper := 1.5 
Vb_lower := 0 
Vb_upper := 0.10 

To constrain K1/k2 to a predetermined value, set the lower and upper limit to that value. This file is then given to fitk3 with option -lim. For example:

fitk3 -lim=constraints.set ua1807ap_parent_delay.kbq ua1807ab_delay.kbq ua1807.dft 999 ua1807k3.res

See also:



References:

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Created at: 2008-06-10
Updated at: 2018-01-12
Written by: Vesa Oikonen