Yearly Rates of Relative Atrophy

Yearly rates of Relative Atrophy (yrRA) are useful to compare the progression of the atrophy of a given brain structure in comparison with the progression of global brain atrophy. These methods are useful in the diagnosis and follow up of several neurodegenerative conditions, such as Alzheimer's disease (1) and Multiple Sclerosis (2). Thus, yrRA can be referenced to the ventricular system or referenced to the whole brain:

Global brain atrophy can be assessed well directly, measuring the loss of volume of the whole brain, well indirectly, measuring the enlargement of the volume of the ventricular system that correlates with the extent of brain atrophy. Thus, yearly rates of relative atrophy can be referenced to the ventricular system or referenced to the whole brain:

VENTRICULAR-REFERENCED

For telencephalic structures, one can compute the yearly rate of relative atrophy, comparing the change in the volume of ROIs, with the change of volume in the lateral ventricle system

yrR*A= (A1-A2) x 1200 / (B2-B1) x (#months between MRI studies)
where * is the ROI
A1 is the volume of the ROI in the first MRI and A2 the volume of the ROI in the second MRI
B1 is the volume of the ventricular system in the first MRI and B2 the volume of the ventricular system in the second MRI

These methods can be computed for each hemisphere separately, or for both brain hemispheres together (taking the addition of volumes of A on both hemispheres and the addition of volumes of B on both hemispheres).

For diencephalic structures, one can compute the yearly rate of relative atrophy, comparing the change in the volume of ROIs, with the change of volume in the lateral ventricles plus the third ventricle

yrR*A= (A1-A2) x 1200 / (B2-B1) x (#months between MRI studies)
where * is the ROI
A1 is the volume of the ROI in the first MRI and A2 the volume of the ROI in the second MRI
B1 is the volume of the ventricular system in the first MRI and B2 the volume of the ventricular system in the second MRI

For structures in the posterior fossa, one can compute the yearly rate of relative atrophy, comparing the change in the volume of ROIs, with the change of volume in the fourth ventricle.

yrR*A= (A1-A2) x 1200 / (B2-B1) x (#months between MRI studies)
where * is the ROI
A1 is the volume of the ROI in the first MRI and A2 the volume of the ROI in the second MRI
B1 is the volume of the forth ventricle in the first MRI and B2 the volume of the forth ventricle in the second MRI

WHOLE BRAIN-REFERENCED

For telencephalic and diencephalic structures, one can compute the yearly rate of relative atrophy, comparing the change in the volume of ROIs, with the change of volume in the brain hemisphere.

yrR*A= (A1-A2) x 1200 / (B2-B1) x (#months between MRI studies)
where * is the ROI
A1 is the volume of the ROI in the first MRI and A2 the volume of the ROI in the second MRI
B1 is the volume of the the brain hemisphere in the first MRI and B2 the volume of the the brain hemisphere in the second MRI

These methods can be computed for each hemisphere separately, or for both brain hemispheres together (taking the addition of volumes of A on both hemispheres and the addition of volumes of B on both hemispheres).


For structures in the posterior fossa, one can compute the yearly rate of relative atrophy, comparing the change in the volume of ROIs, with the change of volume in parenchyma located in the posterior fossa, including the cerebellum and the brain stem.

These methods can be computed for each side separately: 

yrR*A= (A1-A2) x 1200 / (B1-B2) x (#months between MRI studies)
where * is the ROI
A1 is the volume of the ROI in the first MRI and A2 the volume of the ROI in the second MRI. 
B1 is the volume of the parenchyma located in the posterior fossain the first MRI and B2 the volume of the parenchyma located in the posterior fossa in the second MRI

Table 1. Most useful ROI

Most useful telencephalic ROI:

• Forebrain Parenchyma: yrRFPA
• Whole Cortical Gray Matter: yrRwCGMA
• Lobe/Gyrus Cortical Gray Matter: yrR#CGMA, where # is the name of the Lobe/Gyrus
• Hippocampus: yrRHA
• Amygdala: yrRAmyA
• Caudate: yrRCauA
• Putamen: yrRPuA
• Striatum (Caudate+Putamen): yrRStA
• Pallidum: yrRPaA
• Lenticularis (Putamen+Pallidum): yrRLenA

Most useful diencephalic ROI:

• Thalamus: yrRTA 

Most useful ROI in the posterior fossa:

• Whole Cerebellum: yrRwCerA
• Left Hemispheric Cerebellum: yrRlHCerA
• Right Hemispheric Cerebellum: yrRrHCerA
• Mesencephalon/Midbrain: yrRMidA
• Pons: yrRPonsA
• Medulla: yrRMedA

References

1. Menéndez-González M. Volumetric indices and rates of atrophy for the assessment of medial temporal lobe atrophy. Journal of Neurology and Neuroscience 2014;5:2(4)
2. Menéndez-González M, Salas-Pacheco JM and Arias-Carrión O (2014). The yearly rate of Relative Thalamic Atrophy (yrRTA): a simple 2D/3D method for estimating deep gray matter atrophy in Multiple Sclerosis. Frontiers in Aging Neuroscience 6:219