Table 1 Brief description of the study design, sampling method, data collection, and MRI procedure.

Study design

Cross-sectional study

Sampling method

In all, 625 40-yr olds born in Denmark and living in the county of Funen were randomly selected from the population register (every ninth person), of which 412 accepted to participate

Data collection

Information on low back pain, type of work, height and weight were collected in a questionnaire and through MRI during the period of June 2000 till February 2001. The type of work was self-reported and divided into four main groups, of which one was described as "hard physical work". An experienced radiologist described all images according to a set protocol. The interpretation of MRI images was made independently of the present study subject.

MRI procedure

MRI was performed with an open low field 0.2 T MR unit (Magnetom Open Viva, Siemens AG, Erlangen, Germany). T1- and T2-weighted sagittal and T1-weighted sagittal sequences were used. 1) A localizer sequence of five images, 40/10/40 degrees (TR/TE/flip angle) consisting of two coronal and three sagittal images in orthogonal planes, 2) Sagittal T1 weighted spin echo, 621/26 (TR/TE), 144 × 256 matrix, 300 mm field of view, and 4 mm section thickness, 3) Sagittal T2 weighted turbo spin echo 4609/134 (Tr/effective TE), 210 × 256 matrix, 300 mm field of view, and 4 mm section thickness, and 4) Axial T2 weighted turbo spin echo 6415/134 (TR/effective TE), 180 × 256 matrix, 250 mm field of view, and 5 mm section thickness in the plane of the five lower discs.

Ethics

The local ethics committee and the Danish Data Protection Agency approved of the study. All participants received written information and signed an informed consent.