Improvements to HEVC Intra Coding for Lossless Medical Image Compression

Victor Sanchez†, Francesc Aulí-Llinàs‡, Joan Bartrina-Rapesta‡ and Joan Serra-Sagristà‡ †Dept. of Computer Science

University of Warwick, United Kingdom

‡Dept. of Information and Communications Engineering, Universitat Autònoma de Barcelona, Spain

This works focuses on the High Efficiency Video Coding (HEVC) standard as a compression method to be potentially adopted by the Digital Imaging and Communications in Medicine (DICOM) standard. We are particularly interested in improving the lossless compression efficiency of the intra coding process for grayscale anatomical medical images. We focus on intra coding due to its low complexity and outstanding compression results, as well as the fact that it allows coding high-dimensional medical images on a slice-by-slice basis. This is especially advantageous for cases when only a small set of slices needs to be accessed without the need to decode the entire data set.

Based on the characteristics of grayscale anatomical medical images, specifically their large amount of edge information and frequent number of patterns depicted on various directions, we propose improvements to HEVC intra coding based on sample-by-sample (SbS) differential pulse code modulation (DPCM) with equal displacements so the density of prediction modes is constant in all directions, as depicted in Fig. 1. Performance evaluations over MRI, CT and X-ray angiography sequences show that the proposed improvements outperform current HEVC lossless intra coding, achieving average coding gains of 6% (see Table 1).

(a) (b)

Figure 1. (a) Proposed SbS-DPCM angular modes. (b) Prediction principle. Px,y is computed by using a set ofneighboring samples at positions {a, b, c, d, e}, according to the prediction mode used. Rx,y denotes reference sample atposition (x,y) as used in the original HEVC intra coding process.

Table 1. Lossless compression ratios; bit-rate savings (%) are with respect to the original HEVC intra coding process.

Modality - (slices:pixels per slice:bpp) Proposed SbS-DPCM HEVC intra coding

1.MRI-(11:512×512:8) 2.56 (14.79%) 2.23

2.MRI-(50:512×512:8) 5.13 (4.69%) 4.90

3.CT-(596:512×512:12) 2.42 (6.60%) 2.27

4.CT-(637:512×512:12) 2.43 (8.96%) 2.23

5.Angio-(151:512×512:12) 2.35 (3.98%) 2.26

6.Angio-(186:512×512:12) 2.32 (4.03%) 2.23

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

1/8 pixel fraction

1/8

pixe

l fra

ctio

n 14

16

15

17

10

12

11

13

6

8

7

9

2

4

3

5

R0,0 R1,0 R2,0 … RN,0 RN+1,0

R0,1

R0,2

R0,N

R0,N+1

a

b

c d e

P2,2

…

padded samples

P1,N … … PN,N

This work has been partially funded by the Spanish Government, under Grants TIN2012-38102-C03-03 and RYC-2010-05671.

2014 Data Compression Conference

1068-0314/14 $31.00 © 2014 IEEE

DOI 10.1109/DCC.2014.76

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