Feasibility of MRI-based reference images for image-guided radiotherapy of the pelvis with either cone-beam computed tomography or planar localization images

Figures & data

Figure 1. Example transversal slices of 3D reference images for IGRT with CBCT localization; a T₁/T₂*-weighted in-phase MR image (A), a heterogeneous pseudo-CT that was constructed from the MR image by the dual model HU conversion technique (B), a bulk pseudo-CT (C), and a standard CT (D). Contrast in the images is not comparable (different windowing properties and scaling).

Figure 2. Example reference DRRs reconstructed from different 3D image series; the heterogeneous pseudo-DRRs (A) and (B), the bulk pseudo-DRRs (C) and (D), and the CT-DRRs (E) and (F) (PA and RL, respectively).

Table I. Differences in automatic patient position corrections between those obtained by MRI-based reference images and those achieved with standard reference CT images registered against localization CBCTs as mean ± SD (recalculated by excluding major registration outliers, i.e. > 3 SD).

		CT vs. CBCTn – Pseudo^a vs. CBCTn			CT vs. CBCTn – MRI^b vs. CBCTn
Registration method	Direction	Included registrations	Translation (mm)	Rotation (degrees)	Included registrations	Translation (mm)	Rotation (degrees)
Gray-value 6D with ‘ClipBox’	RL IS PA	100%	0.3 ± 1.0 0.2 ± 0.4 −0.3 ± 0.7	0.1 ± 0.4 −0.1 ± 0.7 0.0 ± 0.4	92%	0.6 ± 1.0 −0.6 ± 0.6 −1.4 ± 0.7	0.0 ± 0.5 −0.4 ± 0.8 −0.3 ± 0.4
Gray-value 6D with ‘Mask’	RL IS PA	92%	0.8 ± 1.4 −0.9 ± 1.6 −0.4 ± 2.2	1.1 ± 3.3 0.1 ± 2.4 −0.5 ± 2.3	90%	0.5 ± 1.3 −0.8 ± 1.6 −0.1 ± 2.5	1.6 ± 3.6 0.3 ± 2.6 0.2 ± 2.0
Gray-value 3D with ‘ClipBox’	RL IS PA	98%	0.2 ± 0.9 0.2 ± 0.5 −0.3 ± 0.6	– – –	90%	0.5 ± 1.2 −0.7 ± 0.7 −1.5 ± 0.6	– – –
Gray-value 3D with ‘Mask’	RL IS PA	92%	0.5 ± 1.3 −1.2 ± 1.9 −0.4 ± 2.0	– – –	90%	0.5 ± 1.5 −1.1 ± 1.6 −0.3 ± 2.4	– – –
Bone 6D with ‘ClipBox’	RL IS PA	100%	0.3 ± 0.9 0.1 ± 0.4 −0.3 ± 0.7	0.1 ± 0.3 −0.1 ± 0.7 −0.1 ± 0.3	–	– – –	– – –

^aThe heterogeneous pseudo-CT image series was constructed by the dual model HU conversion technique; ^bThe T₁/T₂*-weighted in-phase MR image series.

Table II. Differences in manual patient position corrections between those obtained by reference pseudo-DRRs and those achieved with reference CT-DRRs registered against planar localization images as mean ± SD (range).

Localiz. images	Transl. direct.	DRRCT^a vs. planarn − DRRHETER.^b vs. planarn (mm)	DRRCT^a vs. planarn − DRRBULK^c vs. planarn (mm)	t- and F-tests (pt and pF)
kV	RL	0.1 ± 0.5 (−0.8–1.5)	0.2 ± 0.6 (−1.0–1.4)	0.55 and 0.36
	IS	0.2 ± 0.9 (−1.6–2.8)	0.2 ± 1.0 (−1.9–2.7)	0.72 and 0.14
	PA	−0.3 ± 1.0 (−2.5–1.5)	−0.5 ± 1.4 (−3.9–4.3)	0.33 and 0.01
MV	RL	−0.2 ± 0.6 (−1.5–1.7)	−0.1 ± 0.7 (−2.2–1.2)	0.06 and 0.23
	IS	0.0 ± 1.1 (−2.9–3.0)	0.1 ± 1.2 (−3.6–2.2)	0.25 and 0.40
	PA	0.3 ± 1.7 (−5.2–5.1)	−0.3 ± 2.1 (−6.4–6.1)	0.04 and 0.12

^aDRR_CT was reconstructed from standard CT image series; ^bDRR_HETER. was reconstructed from heterogeneous pseudo-CT image series; ^cDRR_BULK was reconstructed from bulk pseudo-CT image series.