Measuring Brain Volume by MR Imaging: Impact of Measurement Precision and Natural Variation on Sample Size Requirements

R.G. Steen; R.M. Hamer; J.A. Lieberman

doi:10.3174/ajnr.A0537

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Table 1:

Volume (milliliters) of whole brain and brain lobes in 52 control subjects, measured at baseline and again at 12 weeks postbaseline*

Structure	Baseline		Week 12		Mean Volume Difference		Percent Difference	Pearson Correlation	Concord
Structure	Mean	SD	Mean	SD	Mean	SD	Mean	Pearson Correlation	Concord
Whole brain	1190.4	127.0	1192.6	126.9	2.2	25.2	0.22	0.98	0.98
L frontal GM	157.2	16.8	158.6	14.8	1.4	4.7	1.08	0.96	0.95
R frontal GM	160.5	17.7	161.3	16.5	0.8	4.1	0.65	0.97	0.97
L occipital GM	59.3	6.9	59.1	6.4	−0.2	2.1	−0.20	0.95	0.95
R occipital GM	59.4	6.8	59.4	6.3	0.0	2.2	0.11	0.95	0.95
L parietal GM	62.7	7.1	62.4	6.8	−0.3	2.0	−0.34	0.96	0.96
R parietal GM	63.7	7.4	63.6	7.0	−0.1	2.5	−0.01	0.94	0.94
L temporal GM	67.8	8.1	68.0	7.4	0.2	3.0	0.59	0.93	0.92
R temporal GM	68.6	8.7	68.8	7.5	0.2	3.3	0.72	0.93	0.91
L frontal WM	141.9	14.5	142.4	13.4	0.6	5.3	0.58	0.93	0.93
R frontal WM	142.1	16.0	142.3	14.6	0.2	5.3	0.37	0.94	0.94
L occipital WM	17.1	2.9	16.9	2.9	−0.2	1.4	−0.64	0.89	0.88
R occipital WM	17.5	2.6	17.3	2.5	−0.2	1.3	−0.77	0.88	0.88
L parietal WM	46.1	5.7	45.6	5.6	−0.4	1.7	−0.87	0.96	0.95
R parietal WM	46.0	5.0	45.6	4.8	−0.3	1.8	−0.62	0.93	0.93
L temporal WM	36.0	4.9	36.0	4.1	0.0	2.1	0.55	0.90	0.89
R temporal WM	36.1	5.1	36.1	4.3	0.1	2.1	0.71	0.91	0.90
L caudate	4.3	0.6	4.3	0.6	0.0	0.5	1.46	0.58	0.58
R caudate	4.8	0.7	4.7	0.7	0.0	0.6	−0.21	0.57	0.56

Note:—Concord indicates concordance; L, left; R, right; GM, gray matter; WM, white matter.
* None of the mean volume differences were significant by paired-sample t test, and the Pearson correlation between week 0 and week 12 was generally quite high. Concordance is also a measure of the degree to which values at week 0 and week 12 are correlated, unlike the Pearson, which does not take account of the sample mean, concordance is sensitive to changes in sample mean.

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Table 2:

Brain volume (milliliters) differences between values at week 0 and week 12, expressed in several ways*

Structure	Absolute Difference		Max Volume Change		Max Percent Change
Structure	Mean	SD	Decrease	Increase	Decrease	Increase
Whole brain	17.9	17.7	−68.3	77.3	−5.7	6.5
L frontal GM	3.3	3.6	−6.7	19.8	−4.3	12.6
R frontal GM	3.0	2.9	−6.3	17.8	−3.9	11.1
L occipital GM	1.6	1.4	−5.2	7.4	−8.8	12.5
R occipital GM	1.4	1.6	−5.1	6.8	−8.6	11.4
L parietal GM	1.6	1.2	−5.9	4.8	−9.4	7.7
R parietal GM	1.7	1.8	−7.1	8.5	−11.1	13.3
L temporal GM	2.2	2.1	−6.0	9.4	−8.9	13.9
R temporal GM	2.4	2.3	−7.7	10.7	−11.2	15.6
L frontal WM	3.8	3.7	−9.7	21.3	−6.8	15.0
R frontal WM	3.9	3.6	−11.1	21.1	−7.8	14.8
L occipital WM	1.0	1.0	−3.4	3.6	−19.9	21.1
R occipital WM	1.0	0.8	−2.9	3.2	−16.6	18.3
L parietal WM	1.3	1.2	−7.1	3.6	−15.4	7.8
R parietal WM	1.5	1.1	−5.1	3.5	−11.1	7.6
L temporal WM	1.5	1.5	−6.5	5.7	−18.1	15.8
R temporal WM	1.6	1.4	−5.0	6.5	−13.9	18.0
L caudate	0.4	0.3	−1.2	2.1	−27.9	48.8
R caudate	0.4	0.4	−2.1	1.8	−43.8	37.5

Note:—L, left; R, right; Max, maximum; GM, gray matter; WM, white matter.
* The absolute difference is unaffected by the sign of the change from week 0 to week 12. The maximal decrease and increase from week 0 to week 12 is expressed as volume (with units of milliliters) and as percent change (with respect to baseline).

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Table 3:

Minimal sample size required to detect a 5% change in volume with at least 80% statistical power, under a variety of assumptions about study design*

Structure	Cross-Sectional		Longitudinal		Longitudinal
	2 Groups		1 Group		2 Groups (change)
	No.	Power	No.	Power	No.	Power
Whole brain	146	80.3	4	86.7	10	90.2
L frontal GM	146	80.2	5	80.0	14	82.3
R frontal GM	156	80.4	5	90.0	12	86.5
L occipital GM	170	80.1	7	86.0	20	83.3
R occipital GM	166	80.4	7	85.6	20	82.9
L parietal GM	166	80.3	6	87.1	16	84.0
R parietal GM	172	80.4	7	80.3	22	81.3
L temporal GM	184	80.4	9	83.2	28	81.0
R temporal GM	204	80.1	10	82.2	32	80.1
L frontal WM	134	80.2	7	83.4	20	80.4
R frontal WM	162	80.2	7	83.8	20	80.9
L occipital WM	362	80.1	23	81.1	84	80.2
R occipital WM	280	80.3	19	81.4	68	80.3
L parietal WM	192	80.0	7	85.0	20	82.2
R parietal WM	150	80.1	8	86.2	22	80.3
L temporal WM	234	80.1	13	80.7	46	80.9
R temporal WM	256	80.3	13	80.4	46	80.5
L caudate	210	80.1	47	80.5	180	80.0
R caudate	244	80.3	54	80.4	208	80.0

Note:—L, left; R, right; GM, gray matter; WM, white matter.
* Estimated statistical power is shown for a sample size that was calculated to yield at least 80% power.

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Table 4:

Minimal sample size required to detect a 2% change in volume with at least 80% statistical power, under a variety of assumptions about study design*

Structure	Cross-sectional		Longitudinal		Longitudinal
	2 groups		1 group		2 Groups (change)
	No.	Power	No.	Power	No.	Power
Whole brain	896	80.0	11	80.5	38	80.8
L frontal GM	898	80.0	20	81.5	72	80.4
R frontal GM	956	80.0	15	80.8	54	80.9
L occipital GM	1050	80.0	28	80.6	106	80.6
R occipital GM	1016	80.0	28	80.2	106	80.2
L parietal GM	1022	80.1	22	81.2	80	80.2
R parietal GM	1056	80.1	33	81.2	124	80.6
L temporal GM	1130	80.1	42	80.5	162	80.5
R temporal GM	1262	80.0	49	80.3	190	80.3
L frontal WM	824	80.1	30	80.5	114	80.5
R frontal WM	998	80.0	30	81.0	112	80.3
L occipital WM	2246	80.0	130	80.1	514	80.1
R occipital WM	1730	80.0	105	80.3	412	80.1
L parietal WM	1190	80.1	29	80.9	108	80.2
R parietal WM	926	80.0	33	80.2	126	80.2
L temporal WM	1448	80.0	70	80.5	272	80.2
R temporal WM	1580	80.0	70	80.2	274	80.1
L caudate	1298	80.0	280	80.0	1116	80.1
R caudate	1506	80.0	324	80.1	1290	80.1

Note:—L, left; R, right; GM, gray matter; WM, white matter.
* Estimated statistical power is shown for a sample size that was calculated to yield at least 80% power.