Tissue identification with micro-magnetic resonance imaging in a caprine spinal fusion model

Nonunion is a major complication of spinal
interbody fusion. Currently X-ray and computed tomography
(CT) are used for evaluating the spinal fusion process.
However, both imaging modalities have limitations in
judgment of the early stages of this fusion process, as they
only visualize mineralized bone. Magnetic resonance
imaging (MRI) could be of great value as it is able to discriminate
between different types of tissue. A feasibility
study was performed in nine animals from a goat spinal
fusion study, to evaluate the detection capacity of different
tissues with micro-MRI. In this study bioresorbable polylactic
acid cages were used. Six- and 12-months follow-up
specimens were scanned in a 6.3 T micro-MRI scanner.
After scanning, the specimens were processed for histology.
Different types of tissue as well as the degradable cage
material were identified in the fusion zone and designated as
regions of interest (ROIs). Subsequently, the location of
these ROIs was determined on the corresponding micro-
MRI image, and average signal intensities of every individual
ROI were measured. An excellent match was seen
between the histological sections and micro-MRI images.
The micro-MRI images showed quantifiable differences in
signal intensity between bone with adipose marrow, bone
with hematopoietic marrow, fibrocartilage, fibrous tissue,
and degradable implant material. In time the signal intensity
of bone with adipose marrow, bone with hematopoietic red
marrow, and of fibrous tissue remained relatively constant.
On the other hand, the signal intensity of the degradable
implant material and the fibrocartilage changed significantly
in time, indicating change of structure and
composition. In conclusion, in our model using bioresorbable
cages the MRI provides us with detailed information
about the early fusion process and may therefore, allow
early diagnosis of non-union.