Background When applying pulsed radiofrequency on dorsal root ganglia for treating chronic lower back pain, maximum efficiency can be expected when a needle is placed 1-2 cm peripheral to the dorsal root ganglion. The object of this study is to analyze images taken after adding contrast to transforaminal epidural injection, categorize root ganglia according to anatomical position, and provide a reference for efficient needle positioning in applying pulsed radiofrequency on dorsal root ganglia. Methods From January 2008 to January 2009, 457 patients who visited our hospital for root pain or radiculopathy were treated with transforaminal epidural injection on the nerve roots based on the dermatome of the painful area. Anteroposterior views were taken after injection of contrast. A virtual line was made by connecting the internal and external parts of the spinal pedicle from the contrast images. Then the dorsal root ganglia were categorized as intraspinal (IS), intraforaminal (IF), or extraforaminal (EF). Results In the fourth lumbar spine, dorsal root ganglia positions were 48% IF, 41% IS, and 6% EF. In the fifth lumbar spine, dorsal root ganglia positions were 75% IF, 10% IS, and 6% EF. In the first sacral spine, dorsal root ganglia locations were 8% IF and 83% IS. Conclusions Positional categorization of dorsal root ganglia according to contrast images was proven to be good anatomical references for effective radiofrequency or blocking of dorsal root ganglia.