What is an "Unpaired Bland-Altman plot"?

Question

I found this quote:

Unpaired Bland-Altman plots were used to check systematic or bias errors in the SPECT myocardial uptake coefficients with respect to the reference standard PET data.

in the following paper:

ALHASSEN, Fares, et al. Myocardial blood flow measurement with a conventional dual-head SPECT/CT with spatiotemporal iterative reconstructions-a clinical feasibility study. American journal of nuclear medicine and molecular imaging, 2014, 4.1: 53.

My knowledge of "Bland-Altman plot" is the following:

I have $n$ distinct objects and two instruments, A and B; the instruments measure a property $w$ of the objects.

$\mathbf{w}_A\in{\mathbb{R}}^n$, $\mathbf{w}_A=[w_{1A},w_{2A},\ldots,w_{nA}]$ and $w_{iA}$ is the measurement of the $i$-th object made with instrument A.

$\mathbf{w}_B\in{\mathbb{R}}^n$, $\mathbf{w}_B=[w_{1B},w_{2B},\ldots,w_{nB}]$ and $w_{iB}$ is the measurement of the $i$-th object made with instrument B.

The plot is made by the $n$ points $p_i(x_i,y_i)$ where

$\mathbf{x}\in{\mathbb{R}}^n$, $\mathbf{x}=\frac{\mathbf{w}_A+\mathbf{w}_B}{2}=[x_1,x_2,\ldots,x_n]$

$\mathbf{y}\in{\mathbb{R}}^n$, $\mathbf{y}=\mathbf{w}_A-\mathbf{w}_B=[y_1,y_2,\ldots,y_n]$

So, what is an "Unpaired Bland-Altman plot" and how it's made?

Answer 1

I have never heard of this name.

In fact, the plot looks just like any Bland Altman plot I have seen, other than there are two sets of data overlaid on the plot. I guess the "unpaired" indicates that you cannot tell which MLEM and ST-MLEM data are coming from the same patient, because there is no linkage between the blue and the pink data points on the plot.