T1/T2*-weighted MRI provides clinically relevant pseudo-CT density data for the pelvic bones in MRI-only based radiotherapy treatment planning

Figures & data

Figure 1. Example of a small normalization ROI (upper white circle) chosen within a muscle that is located next to the bone. The edge of the bone (lower white contour) is clearly visible enabling segmentation of the bone edge in the MR image (LAVA Flex® in-phase image).

Figure 2. Relation between MRI intensity (in-phase image) and HU number obtained for 400 randomly chosen voxels within the pelvic bones. The second order polynomial fit is expressed as a solid line.

Figure 3. Relation between the measured and calculated HU-values for 400 data points obtained from the validation group. Straight solid line with slope equal to one illustrates ideal calculation.

Figure 4. DRR image generated from A) actual CT image and B) pseudo-CT image with HU-values inside the bones converted from the MRI. The field size is equal in the both images. Lateral images were chosen to demonstrate clear visibility of bone edges through whole transversal section of the pelvic bones.

Table I. Maximal dose calculation differences (in percentages) between pseudo2CT and actual CT for five randomly chosen patients (equal MU counts were applied for both images).

		HU information used inside the pelvic bones
Energy	Fraction of PTV	Water only	Bulk value of 337 corresponding group average	Individual bulk value obtained from MRI	Regional HU obtained from MRI	Regional HU from MRI with body corrected^a
6 MV	V95%	1.8%	− 1.9%	− 1.1%	− 1.2%	− 0.6%
	V50%	2.0%	− 1.9%	− 0.9%	− 1.0%	− 0.4%
	V5%	1.7%	− 1.9%	− 1.2%	− 1.2%	− 0.5%
	all points within	± 3.9%	± 3.2%	± 2.2%	± 2.4%	± 1.3%
15 MV	V95%	1.9%	− 1.4%	− 0.9%	− 0.9%	− 0.4%
	V50%	1.6%	− 1.4%	− 0.7%	− 0.8%	− 0.4%
	V5%	1.5%	− 1.2%	− 0.8%	− 0.8%	− 0.4%
	all points within	± 3.7%	± 2.7%	± 2.2%	± 2.2%	± 1.2%

^aHU value for the body outside the bones was corrected for fat content (on the average to about 220) to demonstrate differences related mainly to the conversion of HU values inside the bones.