Role Of Imaging In The Diagnosis Of Osteomyelitis Biology Essay

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This assignment will be discussing the role of imaging in the diagnosis, management, treatment and monitoring of a patient with osteomyelitis. Several imaging techniques used for osteomyelitis will be discussed such as Conventional Radiography, Magnetic Resonance Imaging (MRI), Scintigraphy, Ultrasound (U/S), Computerised Tomography (CT) and recent developments in the imaging of osteomyelitis.

Osteomyelitis can be defined as an infection of bone and/or bone marrow (Offiah, 2006). This infection results in the inflammatory destruction of bone, bone necrosis, and new bone formation (Brady, Leid, Costerton & Shirtliff, 2006). The causative factors usually depend on the patient's age (Offiah, 2006). However, the commonest causative organism is Staphylococcus aureus (Restrepo, Vargas, Riascos & Cuellar, 2005). Other organisms can also include Escherichia coli and Group-B Streptococci (Offiah, 2006).

Pathogenesis

There are a number of ways that infection can spread to bone. The three main routes are haematogenous spread, direct contamination and indirect contamination. Haematogenous spread is the commonest route in neonates and involves the spread of infection via the bloodstream. Direct contamination is commoner in older children and adults and is associated with things such as penetrating or puncture wounds and surgery. A further route is indirect contamination from a nearby infection such as cellulitis (Offiah, 2006). Adult osteomyelitis is said to be commonly caused by open fractures and diabetic foot infections, or surgery (Pineda, Vargas & Rodriguez, 2006). An infected prosthesis is also a common cause of osteomyelitis in adults (Lew & Waldvogel, 2004). The bones and joints of the lower limb are commonly affected (Offiah, 2006).

Stages Of The Infection

Osteomyelitis clinically is divided into three categories or stages, acute, subacute and chronic. These categories are important to be understood in order to provide the most appropriate imaging to aid diagnosis. Acute Osteomyelitis is seen commonly in paediatrics and involves marrow edema, cell infiltration and vascular engorgement which can lead to necrosis and the formation of abscesses. If there is no complete resolution or elimination of the infection in the acute phase, it can result in subacute osteomyelitis. This involves the formation of Brodie's abscess. The chronic stage of osteomyelitis involves a low grade infection that is ongoing and is characterized by necrotic bone and the formation of new bone (Restrepo, Vargas, Riascos & Cuellar, 2005). The role of imaging may differ in each stage of the infection in order to diagnose, manage, treat & monitor a patient with osteomyelitis.

Signs & Symptoms

Signs and symptoms of a patient with suspected osteomyelitis vary in terms of the patient's age and the stage of the disease (acute, subacute or chronic). In a patient with acute osteomyelitis, symptoms may include; swelling, pyrexia, pain, loss of movement and erythema of the area infected. In the subacute and chronic stage, symptoms include the affected body part appearing deformed, loss of movement or instability, drainage of pus or fluid from the affected part and localised bone pain (NHS, 2009).

Laboratory Tests

Laboratory tests in the acute phase such as blood tests are helpful in the diagnosis of osteomyelitis. A raised C - reactive protein (CRP) and white blood cell count (WBC) can be indicative of an infection (Baker & Macnicol, 2007). Imaging however plays a key role in diagnosis and monitoring of osteomyelitis (Pineda, Vargas & Rodriguez, 2006).

Conventional Radiography & Osteomyelitis

Conventional radiography is inexpensive and readily available, making it the initial imaging investigation of choice for the diagnosis of osteomyelitis (Restrepo, Vargas, Riascos & Cuellar, 2005, & Royal College of Radiologists referral guidelines, 2007). Plain radiographs are used to make a correct diagnosis, exclude other pathologies, and provide signs for other pathological conditions (Pineda, Vargas & Rodriguez, 2006). The earliest radiographic changes in osteomyelitis in the acute phase are soft-tissue swelling and blurring of adjacent fat planes (Palestro, Love & Miller, 2006).These can be detected 48 hours after the onset of the infection (Offiah, 2006). However, more accurate changes in radiographic appearances take 10-21 days to become apparent because there must be a 30-50% reduction in bone density for radiographic signs to be visible (Pineda, Vargas & Rodriguez, 2006). After this period radiographs may show lysis of medullary trabeculae, focal loss of cortex, and periosteal reaction, shown in figure 1 (Palestro, Love & Miller, 2006). In the subacute phase of osteomyelitis, Brodie's abscesses appear on x-rays as well-circumscribed with lytic lesions, surrounded by a sclerotic rim that is seen well-defined inside and ill defined outside (Sammak et al, 1999). The chronic phase displays thick, irregular sclerotic bone combined with radiolucencies, elevated periosteum and draining sinuses (Nawaz Khan, 2009). This suggests that x-rays maybe useful for the diagnosis and monitoring of a patient with osteomyelitis.

Figure 1: Displays osteomyelitis of the fourth toe which shows a radiographic appearance of cortical and trabecular bone destruction of the proximal phalanx (black arrow) and soft tissue swelling (white arrows) (Palestro, Love & Miller, 2006).

Figure 1: Palestro, C.J., Love, C. & Miller, T.T. (2006). Imaging of musculoskeletal infections. Best Practice & Research Clinical Rheumatology, 20(6), 1197-1218.

The sensitivity of plain radiography in the detection of osteomyelitis is between 43%-75% and a specificity of 75%-83%. Therefore, although positive findings are useful, negative ones cannot exclude the presence of the infection (Palestro, Love & Miller, 2006). Subsequently, because of its insensitivity for the early diagnosis of acute osteomyelitis, an alternative imaging modality such as MRI is used to confirm findings (Pineda, Vargas & Rodriguez, 2006). X-rays do however have a role in the diagnosis of osteomyelitis, but this is usually in the subacute or chronic stage of the infection. This diagnosis can aid in the management and treatment of the patient in terms of if they require antibiotics or surgery (Moon & Moon, 2000).

MRI & Osteomyelitis

MRI is one of the best imaging modalities in a case of suspected osteomyelitis, providing excellent soft tissue and bone marrow contrast as well as multiplanar images (Restrepo, Vargas, Riascos & Cuellar, 2005). It's therefore considered by the Royal College of Radiologists referral guidelines (2007) as a first line imaging investigation of choice for diagnosing osteomyelitis. It can be helpful in unclear situations where other pathologies are difficult to exclude (Carek et al, 2001). MRI has a high sensitivity for the early detection of osteomyelitis as it's able to provide superb contrast between normal bone marrow and the abnormal areas (Restrepo, Vargas, Riascos & Cuellar, 2005). It's seen to be more sensitive than both conventional radiography and Scintigraphy in recognizing sites of infection with a sensitivity of 82%-100% and a specificity of 75%-96% (Impallomeni et al, 2002, & Restrepo, Vargas, Riascos & Cuellar, 2005). This is supported by studies that have provided evidence of MRI being superior at diagnosing osteomyelitis than other imaging modalities such as Scintigraphy, CT and conventional radiographs (Morrison et al, 1993, Berquist et al, 1985, & Unger et al, 1987). In practice, however, MRI is not often used at the acute stage as an initial imaging investigation (Brady et al, 2006). The main reasons for this are said to be the cost implications, waiting times and availability for having an MRI scan on the NHS.

The sequences used in MRI for the detection of osteomyelitis are T1-weighted and T2-weighted scans. The most suitable sequences for the imaging of acute osteomyelitis are Short TI Inversion Recovery (STIR) or a T2 fat-suppressed fast spin-echo sequence which provides excellent visualization of the inflammatory edema. Suppression of fat signal in MRI is done to improve the dynamic range of tissue contrast and remove interfering signal of fat on a T1 and T2-weighted scan and in post gadolinium contrast injection (Pineda, Vargas & Rodriguez, 2006).

"Depending on the pulse sequences used, major differences can be noted on the signal intensity and appearance of normal and abnormal tissues" (Pineda, Vargas & Rodriguez, 2006, p.810).

Acute osteomyelitis on a T1-weighted scan shows as ill-defined low signal intensity because of the edema in the medullary space, whereas on a T2-weighted scan and STIR or fat suppressed sequences, areas of infection appear as high signal intensity seen on figure 2. The surrounding soft tissue appears abnormal with ill-defined planes (Pineda, Vargas & Rodriguez, 2006).These changes in image appearances can be detected 1-2 days after the onset of the infection (Palestro, Love & Miller, 2006).

Acute Fig.2 Subacute Fig.3 Chronic Fig.4

Chronic osteomyelitis- Figure 4: T1-weighted scan showing thickening of the cortical bone correlating with the low signal intensity seen in the bone marrow (Pineda, Vargas & Rodriguez, 2006).

Pineda, C., Vargas, A. & Rodriguez, V.A. (2006). Imaging of osteomyelitis: current concepts. Infectious Disease Clinics of North America, 20(4), 789-825.

Acute osteomyelitis- Figure 2: Abnormalities in the bone marrow are shown as focal areas of high signal intensity, indicative of acute osteomyelitis (Pineda, Vargas & Rodriguez, 2006).

Pineda, C., Vargas, A. & Rodriguez, V.A. (2006). Imaging of osteomyelitis: current concepts. Infectious Disease Clinics of North America, 20(4), 789-825.

Subacute osteomyelitis- Figure 3: MEDPIX. (2006). Multi organism osteomyelitis. Retrieved, January 2nd, 2010, from http://rad.usuhs.edu/medpix/medpix.html?mode=single&recnum=4466&th=-1.

3a 3b

In the subacute stage of osteomyelitis (Figure 3) where brodie's abscesses are common, they are shown as low signal intensity on a T1-weighted scan (Figure 3a). On a STIR T2-weighted image, the abscess is seen as high signal intensity which is displayed on figure 3b. In the chronic stage of osteomyelitis (Figure 4), a low signal intensity can be seen in the bone marrow, instead of the high intensity usually seen on a T2-weighted scan in the bone marrow (Pineda, Vargas & Rodriguez, 2006). This suggests that MRI has a useful role for the diagnosis and monitoring of osteomyelitis.

Among the main advantages of using MRI for the diagnosis of osteomyelitis is the excellent anatomical detail produced, which can subsequently aid in the surgical planning of a patient with subacute or chronic osteomyelitis. This means that MRI can aid in the management and treatment of the patient (Palestro, Love & Miller, 2006). MRI can also be used in image guided biopsies, thus aiding diagnosis and management; though CT is more commonly used for this technique (Sutton, 2003). However, due to the long scan times associated with MRI, children with suspected osteomyelitis may require sedation for the prevention of motion artefacts (Baker & Macnicol, 2007). Furthermore, there can be difficulties in imaging areas that have metallic implants such as joint prostheses (Palestro, Love & Miller, 2006). Nevertheless MRI has a very important role in the diagnosis, management, and monitoring of a patient with osteomyelitis.

Bone Scintigraphy & Osteomyelitis

Bone Scintigraphy is considered as one of the primary investigative tools for the imaging of osteomyelitis, being readily available and relatively inexpensive (Carek et al, 2001, & Palestro, Love & Miller, 2006). However, although often used to image osteomyelitis, it's considered as a specialised imaging investigation of choice and not an initial one (Royal College of Radiologist referral guidelines, 2007). Scintigraphy is more sensitive than radiography in the detection of osteomyelitis in the early stages of the infection (Offiah, 2006). This is due to the time taken for acute osteomyelitis to be detectable on x-rays. Scintigraphy is useful in the diagnosis, as it can confirm the disease as early as 48 hours after the onset of the infection, even if clinical signs are unclear. Subsequently, Scintigraphy can aid in the early treatment and management of the infection, thus preventing the destruction of bone (Sutton, 2003).

In a case of suspected osteomyelitis a three phase bone scan is usually performed. The first (flow) phase is performed the first minute after the injection of the tracer. The second (blood pool) phase follows with images taken between 2 and 10 minutes. The third (skeletal) phase is performed by taking delayed images 2 and 4 hours after injection (Restrepo, Vargas, Riascos & Cuellar, 2005).This final phase has the advantage of distinguishing osteomyelitis from cellulitis (Sutton, 2003). This is because increased activity in the first and second phase occurs in both osteomyelitis and cellulitis. These two pathologies are important to distinguish between in order to determine what management and treatment the patient may require (Armstrong & Wastie, 1998). Image appearances of a patient with osteomyelitis in the three phase scan will show as focal hyper- perfusion, focal hyperemia, and focal bone uptake in the areas where infection is present (Palestro, Love & Miller, 2006). If a positive uptake is found in all three phases, the sensitivity for diagnosing osteomyelitis is 73%-100%, therefore enabling a good diagnosis (Restrepo, Vargas, Riascos & Cuellar, 2005). However, the use of bone scans has the major disadvantage of lacking specificity. This is because Scintigraphy is not able to distinguish from other pathological conditions, such as soft tissue infections, a tumour, gout, or a healing fracture, as these pathologies mimic the similar appearances seen in osteomyelitis on a scan (Carek et al, 2001). This may explain the high rates of false positives found in bone scans (Restrepo, Vargas, Riascos & Cuellar, 2005). Therefore further imaging is required to add a more accurate diagnosis (Pineda, Vargas & Rodriguez, 2006).

White blood cell scans are another useful imaging technique in Scintigraphy for osteomyelitis which allow us to diagnose the infection and can be said to have a higher sensitivity as well as specificity than a three phase bone scan (Lew & Waldvogel, 2004, & Restrepo, Vargas, Riascos & Cuellar, 2005). This technique is mainly used to diagnose acute osteomyelitis, paediatric patients and infected prostheses. Images appear with increased uptake in the areas where infection is. This scan has the advantage of taking less time for the images to be acquired as well as having a smaller radiation dose and a better image quality. However, the main disadvantage is the superimposition of the bone marrow uptake (Restrepo, Vargas, Riascos & Cuellar, 2005).

Both the three phase and white blood cell scans can be said to have a role in the diagnosis of acute osteomyelitis when there are no signs and symptoms but osteomyelitis is suspected (Royal College of Radiologists referral guidelines, 2007). Furthermore, both the scans are useful for the monitoring of patient's with subacute or chronic osteomyelitis to decide if management and treatment is required and to monitor if the treatment given is being effective (Nawaz Khan, 2009). Therefore, Scintigraphy can be used as an alternative to MRI, but shouldn't be seen to take its place or value at the acute stage for the diagnosis of the disease (Royal College of Radiologists referral guidelines, 2007).

U/S & Osteomyelitis

U/S is fast, cheap, readily available and lacks any ionizing radiation dose making it an appropriate imaging investigation for osteomyelitis (Santos, 2006). U/S can be provided as a tool for determining if there is a need for other more costly imaging examinations such as MRI, Scintigraphy and CT (Sammak et al, 1998). Changes in a patient with acute osteomyelitis on a U/S scan are visualised several days earlier than in conventional radiographs. These changes can be seen as early as 48 hours after the onset of the infection (Restrepo, Vargas, Riascos & Cuellar, 2005). Although considered as a useful tool, the use of U/S in the diagnosis of osteomyelitis is limited (Tehranzadeh et al, 2001).

The earliest signs on an ultrasound in a patient with acute osteomyelitis are juxtacortical soft tissue swelling and early periosteal thickening. In the subacute and chronic stages U/S findings are increased periosteal thickening, which may also include a layer of subperiosteal exudate and abscess formation. In the chronic stage cortical erosion can also be apparent (Chau & Griffith, 2004). Therefore U/S can have a use for the diagnosis and monitoring of a patient with osteomyelitis.

The Royal College of Radiologists referral guidelines (2007) state that U/S is a specialised imaging investigation for the diagnosis of osteomyelitis, which is useful in demonstrating subperiosteal abscesses. The sensitivity and specificity for the use of U/S in imaging osteomyelitis has not yet been determined, however it's not considered to be as high as MRI and Scintigraphy (Chau & Griffith, 2004).

In the subacute and chronic stage of osteomyelitis, U/S is not only considered to have a role in making a diagnosis, but also has a role in the management and treatment of the patient by assessing involvement of adjacent soft tissues to guide the placement of a percutaneous drainage catheter (Figure 5). An abscess which develops because of the infection is identified on the U/S as an anechoic collection of fluid (A). When the U/S has identified the abscess, a needle is placed into the collection (B). This collection is then aspirated. The final result shows a reduction in the abscess size (C) (Pineda, Vargas & Rodriguez, 2006). This suggests that U/S has a role in the diagnosis, management, treatment and monitoring of a patient with osteomyelitis.

Figure 5: U/S guided aspiration. Pineda, C., Vargas, A. & Rodriguez, V.A. (2006). Imaging of osteomyelitis: current concepts. Infectious Disease Clinics of North America, 20(4), 789-825.

U/S has the advantage compared to other imaging modalities that it can be used in areas that are complicated by orthopaedic instrumentation, which can cause problems in MRI and CT (Palestro, Love & Miller, 2006). U/S is more tolerated by paediatric patients compared to other imaging modalities (Pineda, Vargas & Rodriguez, 2006). In addition, U/S guided biopsies can be performed to aid diagnosis and management (Sutton, 2003). However, U/S is operator dependent and can have high rates of false negatives and false positives. Furthermore U/S has the disadvantage of not being able to image beyond the bones cortex (Palestro, Love & Miller, 2006).

CT & Osteomyelitis

CT is seen to have a valuable role in the subacute and chronic stage rather than the acute stage of osteomyelitis (Sutton, 2003). In the acute stage, image appearances of a patient with osteomyelitis will show increased density of the medullary canal as it's replaced by infectious edema and blurring of fat planes. Although these changes can be visualized earlier than on conventional radiographs, it's less advantageous than MRI due to the decreased soft tissue contrast and the ionizing radiation dose associated with it (Palestro, Love & Miller, 2006). The Royal College of Radiologists referral guidelines (2007) suggest that CT is valuable for guiding biopsies and visualizing sequestra.

Image appearances in the subacute and chronic stage on a CT image are abnormal cortical bone thickening, sclerosis, bony destruction, drainage tracts, abscesses and sequestra (Restrepo, Vargas, Riascos & Cuellar, 2005). In the chronic stage, CT is said to be superior to MRI for the demonstration of gas and cortical destruction (Fayad, Carrino & Fishman, 2007). In addition, CT has an important role of aiding in image-guided biopsies or percutaneous aspiration of collections (Restrepo, Vargas, Riascos & Cuellar, 2005). CT can also aid in the surgical planning of a patient with osteomyelitis. This suggests that CT can be said to have a role in the diagnosis, management, treatment and monitoring of a patient with subacute or chronic osteomyelitis.

Research/Developments In The Imaging Of Osteomyelitis

Positron Emission Tomography (PET) Imaging

PET has drawn attention lately in the imaging of osteomyelitis and is considered to have many advantages over Scintigraphy with scans taking less time and images being of a higher resolution (Restrepo, Vargas, Riascos & Cuellar, 2005, & Palestro, Love & Miller, 2006).It's considered useful for chronic osteomyelitis (Royal College of Radiologists referral guidelines, 2007). The way this imaging technique works is by assessing glucose metabolism in the body. Subsequently, in infections there is an increase in glucose consumption which will therefore demonstrate osteomyelitis on the PET image. However, inflammatory pathologies and tumours also have an increased metabolism uptake and therefore PET maybe said to be non-specific (Restrepo, Vargas, Riascos & Cuellar, 2005). Furthermore PET scanners have a limited availability in hospitals and therefore aren't considered at the initial stage of the infection for a role in the diagnosis, management, treatment and monitoring of a patient with osteomyelitis (Pineda, Vargas, Rodriguez, 2006).

PET/CT Fused Imaging

Scanners have been produced which integrate both PET and CT technology. This subsequently means that functional information (PET) as well as high resolution anatomical information (CT) can be provided at the same time. This has the advantage of improving anatomic localization and the ability to distinguish osteomyelitis from soft tissue infections (Restrepo, Vargas, Riascos & Cuellar, 2005).

In Vivo Leukocyte Labeling & Radiolabeled Antibiotics

These two fairly new techniques are being used in Scintigraphy for the imaging of osteomyelitis and are said to have advantages over conventional Scintigraphy with a higher sensitivity and specificity (Restrepo, Vargas, Riascos & Cuellar, 2005).

Conclusion

In conclusion, it can be said that all the imaging modalities considered have a particular role in the diagnosis, management, treatment and monitoring of a patient with osteomyelitis. The imaging modality used should be appropriate for the patient's outcome. MRI is considered as the gold standard for the diagnosis of osteomyelitis and should therefore be used more often in the initial stage of the disease than it is currently used.

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