Given the complex anatomy and function of the foot, the management of chronic pathologies of the ankle and foot remains a challenge. Imaging plays a crucial role and recent appropriateness criteria have been published for the use of the currently available investigating modalities, which are broadly classified into anatomical and functional imaging methods. The recent introduction of SPECT/CT scanners which are capable of combining functional and anatomical images is an exciting and important development. This article describes our clinical experience with SPECT/CT and discusses its potential applications in the imaging of complex foot and ankle pathologies.
by Dr H K Mohan, Dr G Gnanasegaran, Dr S Vijayanathan and Dr I Fogelman

Chronic disorders of the ankle and foot are a significant clinical challenge given the complex anatomy and function of the foot, which makes it difficult to localise origin of pain by routine clinical examination. In recent years, there has been a significant development of imaging techniques to aid the clinician in the management of this complex group of problems. Anatomical imaging (X-ray, MRI, Ultrasound and CT) and functional imaging (Bone scan, MRI and Ultrasound) techniques have been used in the management of patients with chronic foot pain.

The technique of co-registering anatomical and functional images of the feet has already been described [1] and more recently the clinical value of co-registering of bone single photon emission computed tomography (SPECT) images of the wrist with multislice CT images using software has been described [2,3]. CT images provide exquisite details of the bone anatomy whilst the bone scan provides the functional information. Combined SPECT/CT study proves to be an excellent technique for the evaluation of complex bony pathology in the feet although at present MRI remains the most widely used technique for the evaluation of chronic foot pain. In this article, we describe the current techniques available for imaging pathology in the feet, describe our clinical experience with the new SPECT/CT technique, and discuss its potential applications.

Conventional radiography (X-ray)
This remains the most widely used, accessible and cost effective imaging modality for evaluation and management of acute and chronic foot pain. This has a particularly important role in the management of acute trauma while its role in chronic injuries remains limited and at best complementary to other techniques.

Computed tomography
Multidetector computed tomography (MDCT) acquires data that may be reconstructed in any plane; advances in technology allows sub-millimetre resolution for the evaluation of the bones and related complications, articular cartilage lesions, and even limited assessment of tendon disease [4]. This enhances the ability to detect fractures, osteochondral lesions [4,5] and also provides high resolution images essential for presurgical planning. Whilst CT arthrography may prove to be superior to MR for the postoperative assessment of chondral repair [5], its role in the assessment of tendinous and ligamentous pathologies in the feet has not been proven and remains the main limitation when compared to MRI or ultrasound [6].

Magnetic resonance imaging (MRI)
This remains the imaging method of choice for complex ankle and foot pathology according to the appropriateness criteria published by the American College of Radiology. Advances in the strength of magnets (>1.5 T ) and the imaging protocols offer the potential of a better signal-to-noise ratio, improved resolution, and faster scan times [7]. MR imaging is routinely used in the evaluation of soft tissue pathology of the feet including tendinosis, bursitis, fasciitis and for the diabetic foot. It allows for the examination of the vasculature as well, which has been successfully used in the management of diabetes-related osteomyelitis. Optimal evaluation of tendinous pathology however remains elusive [8], and ultrasonography remains the investigation of choice in this population. The potential disadvantages of which one must be aware in MRI are metallic artifacts associated with hardware or micrometallic material in an operative bed, which are more pronounced in high field systems and worse with increasing field strength. Postoperative follow-up imaging in the presence of micrometallic artifacts may not be carried out using a high field system. Imaging parameters are not transferable from 1.5 to 3.0-T systems and must be modified to take advantage of the potential benefits of high field strength imaging [7,9]. Also, claustrophobia requiring the use of general anaesthesia has been seen in up to 10% of patients imaged in a closed high field MR system and obese patients frequently cannot be imaged in the constrained geometry of closed systems.

Ultrasonography
Ultrasound is used in patients with foot pain, to examine the soft tissue and is often the preferred imaging modality when Morton’s neuroma or Achilles tendinosis is suspected [8]. Advantages of sonography include direct correlation of findings with the patient’s symptoms, comparison to the asymptomatic ankle, dynamic imaging of the foot in multiple planes and positions, wide availability, and lower cost of and portability of the equipment to the bedside of an unwell patient. Disadvantages of ultrasound include unfamiliarity with the scanning technique and appearance of ultrasound pathology, operator dependence and limitation of imaging structures superficial to the bony cortex.

Nuclear medicine
A bone scan is a highly sensitive technique which has been used in the diagnosis and management of skeletal pathology for nearly three decades [10], and the role of bone scintigraphy in the management of specific conditions resulting in foot pain has been described [11]. Whilst the sensitivity of the bone scan in the diagnosis of bony pathology in the foot remains high, specificity remains suboptimal. SPECT studies have demonstrated improved sensitivity and specificity although only limited benefit has been shown in the evaluation of foot pathology.

Co registration of bone scan images with X-ray [1] and CT have been performed using software [3,12] and low dose CT systems [13]. More recently, hybrid systems capable of acquiring high-resolution multislice CT image sets that directly match SPECT findings in the same sitting have been developed. This is expected to further increase the diagnostic accuracy of this already highly sensitive but generally non
specific study [14].

Potential applications
Postoperative evaluation of joint fusion and related complications
Joint arthrodesis has long been used for the treatment of painful mal-alignment or arthritis of the hind foot [15]. Successful osseous union after joint arthrodesis is usually expected to occur within six months of the procedure [16] and is confirmed if no joint motion is detected on clinical examination and there is evidence of trabeculation across the arthrodesis site as observed on plain film radiographs. Delayed union is defined as a successful fusion 6 to 9 months after surgery. In patients who continue to suffer from pain following arthrodesis, non-union is suspected. Other complications include development of arthritis in the adjacent joints due to biomechanical overload in about 30% of patients [17] and infection in about 3-5% of patients [19].

Post-operative assessment of the success of fusion has been routinely evaluated using X-ray and CT techniques. However, exact localisation of the site of the origin of pain in these patients remains suboptimal on X-ray and CT [18]. MRI would be unsuitable in this situation due to in situ metal hardware or the presence of micrometallic artifacts and in diabetic patients in whom there is the additional risk of nephrogenic fibrosis associated with gadolinium contrast.
SPECT-CT would be a valuable technique for the evaluation of continuing pain in the context of arthodeses as sites of altered metabolic activity on the bone scan would allow a more focussed examination of the area on the CT study. This improves the accuracy of identifying non-union / malunion [Figure 1], subjacent arthritis or infection as the cause for continuing pain. A combination of SPECT/CT Bone scan / white cell study would be useful in confirming bone and soft tissue infection and also in monitoring response to treatment.

Talar osteochondral defect (OCD)
Osteochondral lesions are a result of acute and sub-acute injuries of the articular cartilage and underlying subchondral bone resulting in the aseptic necrosis observed on histopathology. Although uncommon, this remains one of the treatable causes of unexplained chronic ankle pain [20]. Medial talar dome OCD is more common and bilateral lesions may occur in approximately 10% of cases [20].

The role of bone scintigraphy in the diagnosis of talar OCD [21] has been described. CT scans provide high quality anatomical images for accurately assessing the location and size of the OCD lesions and the diagnostic value is very similar to an MRI study [22]. In addition CT provides information as to loose fragments within the lesion which makes surgical intervention a necessity [23]. The combination of highly sensitive bone SPECT images with a highly specific CT study would provide the clinician with an excellent tool for the diagnosis and management of talar OCD [Figure 2]. It would also be beneficial in the post operative assessment particularly where MRI images may be difficult to interpret.

Achilles tendonitis, bursitis and plantar fasciitis
MRI remains the imaging of choice in these conditions as it can demonstrate more anatomical detail including disruption of the soft tissue structures, associated soft tissue and reactive bone oedema [24]. Typically, plain radiography is not helpful, but is always done to rule out other conditions. Although the role of bone scintigraphy remains limited in this group of patients SPECT-CT may provide useful coincidental imaging information for the clinician. The SPECT study would demonstrate the metabolic abnormalities associated with the bone (edema/enthesophyte trauma etc) whilst the CT would be useful in demonstrating the associated bony (calcaneal spurs) and soft tissue abnormalities (plantar fascia, Kager’s fat pad and retrocalcaneal bursa) [Fig ure 4].

Stress fracture
Plain radiography although having poor sensitivity remains the first investigation that is ordered in patients with suspected stress fracture. Bone scan [25] and recently MRI have demonstrated high sensitivity in identifying early stress injury. [26] Interestingly Gaeta et al [27] in their recent article suggest that the earliest finding of abnormal repetitive stress may be osteopenia which can only be demonstrated by dedicated high resolution CT studies. In view of these recent findings, one would anticipate that SPECT/CT may find a more prominent role to play in the early diagnosis and management of patients with suspected stress fractures by combining the advantages of the two modalities [Figure 3].

Painful accessory bones
Potentially painful normal bony variants, such as accessory tarsal navicular and os trigonum, have been described with chronic foot pain [28]. The mechanism of pain in the presence of an accessory tarsal bone has been attributed to traumatic or degenerative changes at the synchondrosis or to soft-tissue inflammation. Symptomatic accessory tarsal bones have been studied with bone scanning and MRI. Symptomatic lesions are reported to show increased radiotracer uptake or marrow edema across the synchondrosis. SPECT/CT study provides an excellent technique for the evaluation of the cause of chronic pain in this situation with the SPECT study elegantly demonstrating any altered metabolic activity in the joint whilst the CT demonstrates the associated anatomical abnormalities [Figure 4].

Tarsal coalition
Tarsal coalition is a rare deformity [29] that results from abnormal bridging (fibrous / cartilaginous or osseous) across two or more tarsal bones resulting in painful deformity of the hind foot with restricted motion. Calcaneonavicular and talocalcaneal (middle facet at the level of the sustentaculum tali) coalition are the most common sites. CT, although often diagnostic and MRI are not the imaging studies of choice for tarsal coalition and plain radiography remains the main mode of investigation [30]. SPECT/CT may be an improved alternative to CT alone with addition of supplementary functional information from the bone scan [Figure 5]. The area of uptake on the SPECT study may also help guide intra articular injections.

Our experience
We have assessed the additional value of SPECT/CT in 16 patients referred from a specialist orthopedic clinic and it was observed that SPECT/CT provided additional information in 13/16 (81%) patients and was unhelpful in 3 (19%) cases. A specific diagnosis was made in 6/13 (46%) patients which included mal-union, osteochondral defect, osteomyelitis and inflammatory arthritis. More accurate localisation of degenerative or post surgical changes was observed in the remaining 7 patients. When compared to conventional bone scintigraphy, SPECT/CT provides more specific information (malunion / non-union / stress fractures/ impingement etc) as well as allowing more accurate localisation of the abnormalities detected [31,32]. It was found that in more than 50% of patients, management was changed following the findings of the SPECT/CT study and many patients did not undergo any further investigation. We have also demonstrated the value of SPECT/CT in the investigation of heel pain [33] in a patient with retrocalcaneal bursitis and plantar fasciitis.
The limitations of SPECT-CT imaging however are the additional radiation exposure and the increased cost compared to planar bone scintigraphy. The ACR has previously made recommendations regarding the appropriateness of each investigation in evaluating pathology of the foot and ankle. The role of a bone scan however remains limited with best use seen in identifying patients with reflex sympathetic dystrophy syndrome. However, with the combination of functional and structural information in a single study, SPECT/CT may prove to be a very useful technique for the evaluation of foot pain especially in patients with previous surgery or in situ metal work. This would also reduce the inconvenience of extra hospital visits for patients who previously may have had a standalone bone scan and / or a CT study performed. This would also result in a more efficient use of resources and likely to be cost effective for the health provider. We believe that the ACR recommendations will need to be revisited to incorporate the role of SPECT/CT as more evidence becomes available as to its benefits.

Conclusion
Imaging of chronic foot pain remains complex and challenging. Currently MR remains the favoured modality although with increasing evidence, SPECT/CT could prove to be a valuable addition to the imaging armamentarium, particularly in the evaluation of pathology following surgery or in patients not suitable for MR, and may also play a role in guiding intra-articular injections.

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The authors
Hosahalli K Mohan, M.D.1, Gopinath Gnanasegaran,M.D.1 Sanjay Vijayanathan, M.D. 2 Ignac Fogelman, M.D. 1
1Department of Nuclear Medicine,
2 Department of Radiology
Guys & St Thomas Hospitals NHS trust,
London,
United Kingdom

Correspondence to :
Dr. Hosahalli.K.Mohan
Department of Nuclear Medicine,
Ground Floor, New Guys House,
Guys Hospital,
St Thomas Street,
London, SE1 9RT.
UK.
Tel: +44 207 1887188
e-mail: mohanhk@hotmail.com