Introduction

Cruciate retaining knee replacements are only implanted into patients with “healthy” ligaments. However, partial anterior cruciate ligament (ACL) tears are difficult to diagnose with conventional MRI. Variations of signal intensity within the ligament are suggestive of injury but it is not possible to confirm damage or assess the collagen alignment within the ligaments. The potential use of Magic Angle Directional Imaging (MADI) as a collagen contrast mechanism is not new, but has remained a challenge. In theory, ligament tearing or joint degeneration would decrease tissue anisotropy and reduce the magic angle effect. Spontaneous cruciate ligament rupture is relatively common in dogs. This study presents results from ten canine knees.

Purpose

Collagen-rich structures of the knee are prone to damage through acute injury or chronic “wear and tear”. Collagen becomes more disorganised in degenerative tissue e.g. osteoarthritis. An alignment index (AI) used to analyse orientation distribution of collagen-rich structures is presented.

Background: Conventional magnetic resonance pulse sequence echo times (TEs) produces no signal of cortical bone. In this pilot study we wished to explore the value of a novel pulse sequence with an ultrashort echo time (UTE), which is able to detect signal from cortical bone and periosteum (Ref.). The signal obtained using an UTE sequence from cortical bone reflects the soft tissue component of cortical bone including its vasculature. We hypothesized that conditions, which alter the soft tissue component and vascularity of bone, show a change in signal. We have examined the lower limb in patients and volunteers of different age and at different time points following fracture of the tibia.

Subjects and Methods: Seven volunteers (aged 29 – 85 years) and eight patients with acute fractures of the tibia (aged 18 – 56 years) were examined at different time points (2 days – 16 weeks) following fracture, in three of the patients serial scans were obtained. Three patients were examined years following bone injury: one patient with a hypertrophic mal-union at 5 years, one patient with polio 14 years following a tibial osteotomy and one patient 28 years following a tibial fracture. Ultra-short echo time pulse sequences (TE = 0.07 or 0.08 ms) were used with and without preceding fat suppression and / or long T2 component suppression pulses. Intravenous gadolinium (0.3 mmol/kg) was administered to one volunteer and three of the patients. Mean signal intensity (AU) was plotted against time following contrast enhancement. T1 and T2* values for cortical bone were determined and T1 was plotted against age.

Results A signal was obtained of cortical bone, periosteum and callus in all subjects. The injection of contrast enhanced the signal in all of these tissues. Distribution curves of gadolinium in cortical bone showed enhanced signal intensity following fracture. The signal was dependent on the type and severity of fracture and the time following fracture. There was a marked increase in signal in a hypertrophic mal-union 5 years following fracture and a moderate increase in signal was still detectable 28 years following fracture. Osteoporosis associated with polio reduced volume and signal of bone. T1 echo times ranged from 140 – 260 ms and increased significantly with age (P < 0.01). T2* ranged from 0.42 – 0.50 ms. Fat suppression and long T2 suppression increased the conspicuity of the periosteum.

Conclusion: Magnetic resonance imaging using UTE sequences is able to detect a signal from cortical bone for the first time. Cortical bone, callus and adult periosteum show a distinct signal following fracture with a characteristic time course. Measurements reflect the organic matrix rather than the inorganic crystals of bone. The T1 of cortical bone is very short and changes with age. The distribution curve of gadolinium can be established in cortical bone and is understood to reflect changes in blood flow. We present a pilot study to introduce a new MRI sequence, which at present a research tool, has potential for selected clinical application.

Introduction: Normal adult periosteum and cortical and produces no signal with typical bone has a short T₂ Magnetic Resonance pulse sequence echo times available in clinical practice. We wished to assess the value of using pulse sequences with a very short echo time to detect signal from periosteum and cortical bone.

Materials and Methods: Ultrashort echo time (UTE) pulse sequences (TE = 0.08 msec) were used with and without preceding fat suppression and/or long T₂ component suppression pulses. Later echo images and difference images produced by subtracting these from the first echo image were also obtained. Two volunteers and ten patients were examined, four of whom had contrast enhancement with intravenous Gadodiamide. Two sheep tibiae were also examined before and after stripping of the periosteum. The separated periosteum was also examined.

Results: The periosteum was seen on the sheep tibiae before stripping but there was only a faint signal adjacent to cortical bone afterwards and the removed tissue produced a high signal when examined separately. High signal regions were observed adjacent to cortical bone in the femur, tibia, spine, calcaneus, radius, ulna and carpal bones. Fat suppression and long T₂ suppression generally increased the conspicuity of these regions. The high signal regions were more obvious with contrast enhancement. Periosteum could generally be distinguished from susceptibility artifacts on difference images by its high signal on the initial image and its failure to increase in extent with images with increased TE’s. Signal in cortical bone was detected with UTE sequences in normal adults and patients. This signal was usually made more obvious by subtracting a later echo image from the first provided that the SNR was sufficiently high. Normal mean adult T₁’s ranged from 140 msec to 260 msec, and mean T₂’s ranged from 0.42 to 0.50 msec. Increased signal was observed after contrast enhancement in a normal volunteer and in all three patients in whom it was administered. Changes in signal in short T₂ components were seen in acute fractures in cortical bone and after fracture malunion. In a case of osteoporosis, bone volume and signal were reduced. Furthermore, in fractures increased signal was seen in the periosteum and this showed marked enhancement. Three weeks after fracture, tissue with properties consistent with periosteum was seen displaced from the bone by callus.

Discussion: The normal adult periosteum and cortex can be visualized with ultrashort TE sequences. Conspicuity is usually improved by fat suppression and the use of difference images. Use of subtraction images was useful for selectively demonstrating periosteal and cortical contrast enhancement and separating this from enhancement of surrounding blood. Obvious periosteal and cortical enhancement was seen after fractures. This novel MRI sequence images for the first time the soft tissue component of cortical bone and enables visualization of different haemodynamic situations.