The accuracy of image-guided knee replacement based on computed tomography

The Effect of Patient-Specific Instrumentation Incorporating an Extramedullary Tibial Guide on Operative Efficiency for Total Knee Arthroplasty

This retrospective study was to determine if patient-specific instrumentation (PSI) for total knee arthroplasty (TKA) leads to shortened surgical time through increased operating room efficiency according to different tibial PSI designs. 166 patients underwent primary TKA and were categorized into three groups as follows: PSI without extramedullary (EM) tibial guide (group 1, n = 48), PSI with EM tibial guide (group 2, n = 68), and conventional instrumentation (CI) group (group 3, n = 50). Four factors were compared between groups, namely, operative room time, thickness of bone resection, tibial slope, and rotation of the component. The mean surgical time was significantly shorter in the PSI with EM tibial guide group (group 2, 63.9 ± 13.6 min) compared to the CI group (group 3, 82.8 ± 24.9 min) (P < 0.001). However, there was no significant difference in the PSI without EM tibial guide group (group 1, 75.3 ± 18.8 min). This study suggests that PSI incorporating an EM tibial guide may lead to high operative efficiency in TKA compared to CI. This trial is registered with KCT0002384.

A novel registration method for computer-assisted total knee arthroplasty using a patient-specific registration guide

Total knee arthroplasty (TKA) is a surgical method for replacing a degenerated or diseased knee joint that can no longer perform daily functions with an artificial knee implant. In TKA, the artificial knee implant should be inserted such that it aligns well with the mechanical axis of the leg. Thus, precise bone cutting is essential. To improve TKA outcomes, a registration process is performed to locate the predetermined bone cutting area by calculating the position and posture of the femur and tibia. In this article, we propose a patient-specific registration guide that is able to significantly reduce registration time and effort without loss of accuracy. Furthermore, the patient-specific registration guide can be implemented with real-time registration, allowing continuous surgical information to be provided without the insertion of any tracking devices. The precision and accuracy of the proposed registration guide were confirmed through animal tests with a digitizer, stereo camera, and linear motion generator. The error of our registration method, including measurement and guide attachment errors, reached a maximum of 0.321 mm for one pair of cow legs.

Accuracy of image-free computer navigated total knee arthroplasty is not compromised in severely deformed varus knees

In severe varus knee deformity, image-free computer navigated total knee arthroplasty (TKA) may result in a malaligned knee. The aim of this study was to compare the results of 17 severe varus knees (≥ 20°) and 81 varus knees (< 20°) that underwent image-free computer navigated TKA and analyze postoperative malalignment. Computer navigated TKA was performed according to standard protocol, and component angles and mechanical axes were evaluated postoperatively with weight bearing full-length standing radiographs. All severe varus knees were corrected to within 3° of neutral lower limb alignment despite having a mean preoperative varus deformities of 22.4°. Neutral alignment was obtained in 88.9% of the varus group (mean preoperative varus deformity of 11.7°), without significant difference between the two groups. No significant difference was found in either the femoral or tibial component angles, or in the frequency of complications. Severity of varus deformity did not affect the accuracy of image-free computer navigated TKA.

More accurate component alignment in navigated total knee arthroplasty has no clinical benefit at 5-year follow-up

BACKGROUND AND PURPOSE

Computer navigation in total knee arthroplasty is somewhat controversial. We have previously shown that femoral component positioning is more accurate with computed navigation than with conventional implantation techniques, but the clinical impact of this is unknown. We now report the 5-year outcome of our previously reported 2-year outcome study.

METHODS

78 of initially 84 patients (80 of 86 knees) were clinically and radiographically reassessed 5 (5.1-5.9) years after conventional, image-based, and image-free total knee arthroplasty. The methodology was identical to that used preoperatively and at 2 years, including the Knee Society score (KSS) and the functional score (FS), and AP and true lateral standard radiographs.

RESULTS

Although a more accurate femoral component positioning in the navigated groups was obtained, clinical outcome, number of reoperations, KSS, FS, and range of motion were similar between the groups.

INTERPRETATION

The increased costs and time for navigated techniques did not translate into better functional and subjective medium-term outcome compared to conventional techniques.

Upsizing the femoral component increases patellofemoral contact force in total knee replacement

In posterior stabilised total knee replacement (TKR) a larger femoral component is sometimes selected to manage the increased flexion gap caused by resection of the posterior cruciate ligament. However, concerns remain regarding the adverse effect of the increased anteroposterior dimensions of the femoral component on the patellofemoral (PF) joint. Meanwhile, the gender-specific femoral component has a narrower and thinner anterior flange and is expected to reduce the PF contact force. PF contact forces were measured at 90°, 120°, 130° and 140° of flexion using the NexGen Legacy Posterior Stabilized (LPS)-Flex Fixed Bearing Knee system using Standard, Upsized and Gender femoral components during TKR. Increasing the size of the femoral component significantly increased mean PF forces at 120°, 130° and 140° of flexion (p = 0.005, p < 0.001 and p < 0.001, respectively). No difference was found in contact force between the Gender and the Standard components. Among the patients who had overhang of the Standard component, mean contact forces with the Gender component were slightly lower than those of the Standard component, but no statistical difference was found at 90°, 120°, 130° or 140° of flexion (p = 0.689, 0.615, 0.253 and 0.248, respectively). Upsized femoral components would increase PF forces in deep knee flexion. Gender-specific implants would not reduce PF forces.

Accuracy of image-free navigation for severely deformed knees

This study evaluated the accuracy of an image-free navigated total knee arthroplasty (TKA) system when used to align deformed tibia bone models. The accuracy was assessed in normal, 10 degrees varus, 20 degrees varus, 10 degrees valgus, and 20 degrees valgus tibia bone models (a total of five tibial models) by direct measurement of the navigated cutting guide. The mean angular errors in the tibial mechanical axes of the normal, 10 degrees, and 20 degrees varus models, respectively, were 0.0 degree, 0.7 degree varus, and 2.4 degrees varus. Thus, the errors seen with the two varus models were significantly larger than that associated with the normal model. The mean angular errors were 0.1 degree varus and 0.4 degree valgus in the 10 degrees and 20 degrees valgus models, respectively. These errors were not significantly different from those obtained with the normal model. These results suggest that in varus-deformed knees, image-free navigation has a tendency to cut the tibia in varus. This fact is considered to be one of the reasons for the lack of superiority of TKA alignment in severely deformed knees when using image-free navigation. Therefore, special attention must be paid when using image-free navigation TKA in such cases.

Three-dimensional analysis of computed tomography-based navigation system for total knee arthroplasty: the accuracy of computed tomography-based navigation system

We evaluated the postoperative alignment of 37 primary total knee arthroplasties performed using a computed tomography-based navigation system (Vector Vision Knee 1.5; Brain Lab, Germany) with a new 3-dimensional analysis. The mean coronal femoral angle was 89.0 degrees +/- 1.4 degrees (85.5 degrees -92.8 degrees ), and the coronal tibial component was 89.2 degrees +/- 1.0 degrees (87.4 degrees -91.6 degrees ). The hip-knee-ankle angle was observed to be 178.2 degrees +/- 1.5 degrees (173.9 degrees -181.8 degrees ). The external rotational alignment of the femoral component relative to the surgical epicondylar axis was -0.5 degrees +/- 1.7 degrees (-3.2 degrees to 3.4 degrees ). The results demonstrated that a computed tomography-based navigation system provided a reasonably satisfactory component alignment. The discrepancy between the 2-dimensional and 3-dimensional evaluations was 1.0 degrees +/- 0.9 degrees (0.1 degrees -3.4 degrees ). Three-dimensional analysis is necessary to evaluate the accuracy of the navigation system.

Minimally invasive computer-navigated total knee arthroplasty

Modern computerized knee navigation systems aid surgeons both in the conventional and in the minimally invasive approach to optimize mechanical and rotational alignments of the components in all three planes to avoid any malrotation and/or any errors in coronal, sagittal, and axial alignments. The advantages of minimally invasive total knee arthroplasty can be achieved without loss of accuracy. There is increasing evidence of a positive correlation between accurate mechanical alignment after total knee arthroplasty and functional as well as quality-of-life patient outcomes.

The accuracy and precision of computer assisted surgery in the assessment of frontal plane deviations of the lower extremity: a femoral fracture model

OBJECTIVES

Mechanical axis deviation of the lower extremity as a result of malreduction or malunion of fractures plays an important role in the development of arthritis. Therefore it is crucial to restore the limb alignment as accurate as possible. The purpose of this study was to evaluate the accuracy and precision of navigation in assessing isolated frontal plane (varus/valgus) deviations of the lower limb in a simulated fracture model of the femur.

MATERIALS AND METHODS

Three fracture models with ten specimens in each were created in femoral synthetic composite bones to simulate a subtrochanteric (AO/OTA 31-A1), mid-diaphyseal (AO/OTA 32-A3), and supracondylar (AO/OTA 33-A1) femur fracture. Each specimen was mounted on a custom holding device and registered with the navigation system. Eight custom-made aluminum wedges of varying angles (5 degrees -26 degrees ) were used to create varus/valgus angulations at the fracture site. After wedge placement, the frontal plane deformity was recorded and registered by the navigation system. The means and standard deviations for each navigated wedge angle were calculated and compared to the actual wedge angle using a one sample t test. A single factor ANOVA test was subsequently performed to see if the differences between the navigated mean angles in each fracture group were statistically significant. The level of significance was defined as P < 0.05.

RESULTS

None of the navigated mean angles were found to be significantly different from the actual wedge angles (P = 0.05-1.00). More specifically, the differences between the navigated mean angles and the actual wedge angles ranged from 0 degrees to 0.7 degrees . Furthermore, the differences between the navigated mean angles in each angle group were found to be statistically insignificant (P = 0.53-0.99).

CONCLUSION

The high accuracy and precision of navigation systems in determining frontal plane deformities of long bones can make them an invaluable tool for the exact reduction and realignment of lower extremity fractures.

Mechanical accuracy of navigated minimally invasive total knee arthroplasty (MIS TKA)

This study was designed to provide evidence that computer-navigated minimally invasive total knee arthroplasty (MIS CN-TKA) enables identical mechanical accuracy as conventional computer navigated total knee arthroplasty (CN-TKA) while reducing rehabilitation time and hospital stay of the patients. Two groups of 20 patients requiring total knee arthroplasty due to degenerative or posttraumatic knee osteoarthritis were included. Twenty consecutive patients received conventional CN-TKA and 20 consecutive patients received minimally invasive CN-TKA. Mechanical and rotational alignments were measured preoperatively and 6 months postoperatively on long-standing radiographs, on conventional coronal and sagittal views and on CT-scans of the knee. Length of skin incision, operating time, blood loss, length of hospital stay, postoperative ROM and HSS as well as KSS scores were determined. Postoperative mechanical axis improved significantly in both groups. Coronal and sagittal component positioning were accurate in both groups without significant differences. Rotational alignment showed the desired reproducible values without significant differences between the two groups. The posterior slope of the tibial component was significantly reconstructed to match the preoperative condition in both groups. The coronal alignment of the femoral and tibial components showed accurate reproducible results for implantation of both components in both groups. Length of skin incision was significantly shorter in the MIS CN-TKA. Duration of hospital stay was significantly reduced in the MIS CN-TKA group. Operating time and blood loss were similar in both groups. Postoperative ROM after the first 3 months was significantly higher in MIS CN-TKA, but after 6 months differences were minimal. Clinical outcome scores were identical for both groups 6 months after surgery. The advantages of CN-TKA are well known. Performing computer navigated TKA in combination with a minimally invasive approach in this study lead to a reduction of hospital stay and an initially increased ROM without differences in operating time and blood loss. Computer navigation in TKA preserves accurate coronal, sagittal and rotational components alignment even with a minimally invasive approach.

The effect of oxidative aging on the wear performance of highly crosslinked polyethylene knee inserts under conditions of severe malalignment

First-generation crosslinked polyethylene developed for total hip arthroplasty has not gained wide acceptance for knee arthroplasty because of the increased potential for failure under high stresses and the increased risk for oxidative damage caused by free radicals. Sequentially crosslinked polyethylene (SQXL) is a second-generation crosslinked polyethylene that is reported to reduce the level of free radicals and preserve mechanical properties. Three groups of ultrahigh molecular weight polyethylene inserts were wear tested after artificial aging as per ASTM F2003: gamma-irradiated in air (GA-aged), sequentially crosslinked (SQXL-aged), and electron-beam irradiated and remelted (EBeam-aged). Inserts were tested in an AMTI knee wear simulator under malalignment conditions that were two standard deviations from the mean reported for computer-navigated and conventionally aligned techniques. GA-aged inserts delaminated by 500,000 cycles, and were severely damaged after 1 million cycles. None of the highly crosslinked inserts (SQXL-aged or EBeam-aged) delaminated or showed any signs of severe wear. Mean wear rate for GA-aged inserts was 124.6 +/- 49.4 mg/million cycles. Mean wear rates for SQXL-aged and EBeam-aged inserts were significantly lower (1.74 +/- 0.3 and 4.72 +/- 0.7, respectively). These results support the low levels of free radicals and preservation of mechanical properties reported in second-generation crosslinked ultrahigh molecular weight polyethylene.

The evaluation of post-operative alignment in total knee replacement using a CT-based navigation system

We compared the alignment of 39 total knee replacements implanted using the conventional alignment guide system with 37 implanted using a CT-based navigation system, performed by a single surgeon. The knees were evaluated using full-length weight-bearing anteroposterior radiographs, lateral radiographs and CT scans. The mean hip-knee-ankle angle, coronal femoral component angle and coronal tibial component angle were 181.8 degrees (174.2 degrees to 188.3 degrees), 88.5 degrees (84.0 degrees to 91.8 degrees) and 89.7 degrees (86.3 degrees to 95.1 degrees), respectively for the conventional group and 180.8 degrees (178.2 degrees to 185.1 degrees), 89.3 degrees (85.8 degrees to 92.0 degrees) and 89.9 degrees (88.0 degrees to 93.0 degrees), respectively for the navigated group. The mean sagittal femoral component angle was 85.5 degrees (80.6 degrees to 92.8 degrees) for the conventional group and 89.6 degrees (85.5 degrees to 94.0 degrees) for the navigated group. The mean rotational femoral and tibial component angles were -0.7 degrees (-8.8 degrees to 9.8 degrees) and -3.3 degrees (-16.8 degrees to 5.8 degrees) for the conventional group and -0.6 degrees (-3.5 degrees to 3.0 degrees) and 0.3 degrees (-5.3 degrees to 7.7 degrees) for the navigated group. The ideal angles of all alignments in the navigated group were obtained at significantly higher rates than in the conventional group. Our results demonstrated significant improvements in component positioning with a CT-based navigation system, especially with respect to rotational alignment.

Interobserver and intra-observer errors in obtaining visually selected anatomical landmarks during registration process in non-image-based navigation-assisted total knee arthroplasty

This study investigated the errors of obtaining visually selected anatomic landmarks for use in the registration process in a passive optical non-image-based computer-assisted total knee arthroplasty system in 5 fresh frozen cadavers. The projected maximum errors in the femoral mechanical axis (due to registration errors of the center of the distal femur) were 0.7 degrees in the coronal and 1.4 degrees in the sagittal plane. The projected maximum errors in the tibial mechanical axis arising from registration errors of the center of the proximal tibia were 1.3 degrees in the coronal and 2 degrees in the sagittal plane. The projected maximum errors in the transepicondylar axis were 9.1 degrees (registration errors of the medial femoral epicondyle) and 7.2 degrees (registration errors of the lateral femoral epicondyle). It should be noted that the results may be partly related to the use of the particular system in this experiment.

Postoperative Calculation of Acetabular Cup Position Using 2-D–3-D Registration

A method to accurately measure the position and orientation of an acetabular cup implant from postoperative X-rays has been designed and validated. The method uses 2-D-3-D registration to align both the prosthesis and the preoperative computed tomography (CT) volume to the X-ray image. This allows the position of the implant to be calculated with respect to a CT-based surgical plan. Experiments have been carried out using ten sets of patient data. A conventional plain-film measurement technique was also investigated. A gold standard implant position and orientation was calculated using postoperative CT. Results show our method to be significantly more accurate than the plain-film method for calculating cup anteversion. Cup orientation and position could be measured to within a mean absolute error of 1.4 mm or degrees.

Very low-dose computed tomography for planning and outcome measurement in knee replacement

Surgeons need to be able to measure angles and distances in three dimensions in the planning and assessment of knee replacement. Computed tomography (CT) offers the accuracy needed but involves greater radiation exposure to patients than traditional long-leg standing radiographs, which give very little information outside the plane of the image.

There is considerable variation in CT radiation doses between research centres, scanning protocols and individual scanners, and ethics committees are rightly demanding more consistency in this area.

By refining the CT scanning protocol we have reduced the effective radiation dose received by the patient down to the equivalent of one long-leg standing radiograph. Because of this, it will be more acceptable to obtain the three-dimensional data set produced by CT scanning. Surgeons will be able to document the impact of implant position on outcome with greater precision.

Computer-assisted navigation in TKA: where we are and where we are going

When compared to standard intramedullary and extramedullary referencing systems, computer-assisted navigation systems have been shown in multiple randomized studies to increase the accuracy of bone resections in total knee arthroplasty. Accuracy to within 1 degrees in the coronal plane resections can routinely be obtained. Recent modifications of the software programs address the problem of proper soft tissue balance. Newer techniques of bone morphing have obviated the use of preoperative imaging in most cases. Despite these advances, the systems remain somewhat cumbersome to use and costly to acquire. As these limitations are addressed, computer-assisted navigation may become a valuable part of the knee surgeon's armamentarium, especially for the patient with a deformed femur or tibia in whom conventional navigation instruments are difficult to use accurately.

Is restricted femoral navigation sufficient for accuracy of total knee arthroplasty?

A total knee arthroplasty performed with navigation results in more accurate component positioning with fewer outliers. It is not known whether image-based or image-free-systems are preferable and if navigation for only one component leads to equal accuracy in leg alignment than navigation of both components. We evaluated the results of total knee arthroplasties performed with femoral navigation. We studied 90 knees in 88 patients who had conventional total knee arthroplasties, image-based total knee arthroplasties, or total knee arthroplasties with image-free navigation. We compared patients' perioperative times, component alignment accuracy, and short-term outcomes. The total surgical time was longer in the image-based total knee arthroplasty group (109 +/- 7 minutes) compared with the image-free (101 +/- 17 minutes) and conventional total knee arthroplasty groups (87 +/- 20 minutes). The mechanical axis of the leg was within 3 degrees of neutral alignment, although the conventional total knee arthroplasty group showed more (10.6 degrees ) variance than the navigated groups (5.8 degrees and 6.4 degrees , respectively). We found a positive correlation between femoral component malalignment and the total mechanical axis in the conventional group. Our results suggest image-based navigation is not necessary, and image-free femoral navigation may be sufficient for accurate component alignment.

Präzision in der orthopädischen Computernavigation

Navigation has become increasingly integrated into orthopaedic surgery, especially in the area of endoprosthetic procedures. Simplification of the instrumentation along with the use of imageless systems has increased the ease of use for the orthopaedic surgeon. Principle navigation systems enable an accuracy of corrections and alignments within intervals of 1 mm or 1 degrees . Consequently, potential intra- and interobserver failures during the registration procedure typically range within a few millimetres or degrees. Analysis of the actual algorithms used for the registration process of the lower extremity mechanical axis and the articular surfaces reveal valid and reproducible results. With the help of navigation, it is possible to achieve a higher degree of precision in total hip and knee implant placement, including a distinct reduction in variance as compared to conventional techniques. Similarly, application of navigation during a high tibial osteotomy or at the osteotomy of the distal radius also enables a more precise correction of the axis of the affected extremity, in addition to improved reproducibility. Despite these promising early results, large prospective clinical studies comparing conventional techniques versus computer assisted navigation are thus far only available for total knee arthroplasty. Whether navigated prosthesis placement can truly extend the longevity of an implant will require continued observation in the years to come. In addition, further prospective studies are required to determine the benefit of navigation in other orthopaedic procedures.

Two-year outcomes of computed tomography-based and computed tomography free navigation for total knee arthroplasties

Optimal component position in all planes and well-balanced soft tissues facilitate a good clinical outcome and long-term survival after total knee arthroplasties. We investigated the accuracy of implantation of navigated total knee arthroplasties at 3 months followup and the influence on the clinical outcome at 2 years followup. Forty-four patients (44 procedures) were enrolled in our prospective study. One half of the surgeries were performed using a computed tomography-based navigation system, and half were performed with imageless navigation. Outcomes were based on the Insall knee score parameters, anterior knee pain, patient satisfaction, feeling of instability, and step test. The radiographic parameters were the mechanical axis, tibial slope, lateral distal femoral angle, and medial proximal tibial angle. The radiographic measurements were similar in both groups (patients within +/- 3 degrees inaccuracy range in computed tomography-based/imageless groups; mechanical axis 86%/81%, tibial slope 95%/91%, lateral distal femoral angle 95%/91%, medial proximal tibial angle 91%/95%). The imageless system provided equal radiographic results, but we found improved ligament balancing in the computed tomography free group. The computed tomography-based approach has a good pre-operative planning procedure, but is more expensive and time consuming.

LEVEL OF EVIDENCE

Therapeutic Study, Level II. See the Guidelines for Authors for a complete description of levels of evidence.

Cadaver validation of intensity-based ultrasound to CT registration

A method is presented for the rigid registration of tracked B-mode ultrasound images to a CT volume of a femur and pelvis. This registration can allow tracked surgical instruments to be aligned with the CT image or an associated preoperative plan. Our method is fully automatic and requires no manual segmentation of either the ultrasound images or the CT volume. The parameter which is directly related to the speed of sound through tissue has also been included in the registration optimisation process. Experiments have been carried out on six cadaveric femurs and three cadaveric pelves. Registration results were compared with a "gold standard" registration acquired using bone implanted fiducial markers. Results show the registration method to be accurate, on average, to 1.6 mm root-mean-square target registration error.

Intraobserver errors in obtaining visually selected anatomic landmarks during registration process in nonimage-based navigation-assisted total knee arthroplasty: a cadaveric experiment

This study investigated the intraobserver errors in obtaining visually selected anatomic landmarks that were used in registration process in a nonimage-based computer-assisted total knee replacement (TKR) system. The landmarks studied were center of distal femur, medial and lateral femoral epicondyle, center of proximal tibia, medial malleolus, and lateral malleolus. Repeated registration in the above sequence was done for 100 times by a single surgeon. The maximum combined errors in the mechanical axis of the lower limb were only 1.32 degrees (varus/valgus) in the coronal plane and 4.17 degrees (flexion/extension) in the sagittal plane. The maximum error in transepicondylar axis was 8.2 degrees. The errors using the visual selection of anatomic landmarks for the registration technique of bony landmarks in nonimage-based navigated TKR did not introduce significant error in the mechanical axis of the lower limb in the coronal plane. However, the error in the transepicondylar axis was significant in the "worst-case scenario."

Cadaver Validation of Intensity-Based Ultrasound to CT Registration

A method is presented for the registration of tracked B-mode ultrasound images to a CT volume of a femur or pelvis. This registration can allow tracked surgical instruments to be aligned with the CT image or an associated preoperative plan. Our method requires no manual segmentation of either the ultrasound images or the CT volume. The CT and US images are processed to produce images where the image intensity represents the probability of the presence of a bone edge. These images are then registered together using normalised cross-correlation as a similarity measure. The parameter which represents the speed of sound through tissue has also been included in the registration optimisation process. Experiments have been carried out on six cadaveric femurs and three cadaveric pelves. Registration results were compared with a "gold standard" registration acquired using bone implanted fiducial markers. Results show the registration method to be accurate, on average, to 1.7 mm root-mean-square target registration error.

Kniegelenksendoprothetik

Today the most common application in the field of computer-assisted surgery is navigated total knee arthroplasty. During the last 5 years the imageless kinematic navigation systems have gained wide acceptance. As several prospective randomized studies could show, the standard deviation of the mechanical axis is reduced significantly by these techniques. However, the direction of the mechanical axis is only one factor which influences the long-term results of total knee arthroplasty. Further important factors are ligament balancing and position of the femoral and tibial components in all three planes. Up to now no studies have been able to show a significantly better functional result, a more rapid recovery, or a decreased complication rate. Drawbacks of the navigation systems are the additional costs and the additional operation time between 15 and 20 min. Therefore, navigated total knee arthroplasty is not yet a standard procedure, but this technique is well on the way to becoming the gold standard in the future.

Image-Guided Cancer Therapy Using PET/CT

Functional imaging with positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of malignant disease, image-guided therapy planning, and treatment monitoring. PET scanning with the radiolabeled glucose analogue (18)F-fluorodeoxyglucose ((18)FDG) is a relatively recent addition to the clinically available technology for imaging cancer, complementing the more conventional anatomical imaging modalities of computed tomography (CT) and magnetic resonance (MR). These modalities are complementary in the sense that CT provides accurate localization of organs and lesions while PET maps both normal and abnormal tissue function. When combined, the two modalities can identify and localize functional abnormalities. Attempts to align CT and PET data sets with fusion software are generally successful in the brain, whereas the remainder of the body is more challenging owing to the increased number of possible degrees of freedom between the two scans. Recently these challenges have been addressed by the introduction of the combined PET/CT scanner, a hardware-oriented approach to image fusion. With this device, accurately registered anatomical and functional images can be acquired for each patient in a single scanning session. Currently, over 400 combined PET/CT scanners are installed in medical institutions worldwide, almost all of them for the diagnosis and staging of malignant disease. However, the real impact of this technology undoubtedly will be for cancer therapy, where PET/CT images will be used to guide biopsies and assist in surgical intervention, to define target volumes for radiation therapy and optimize dose, and to monitor response to chemotherapy and establish individualized patient treatment strategies.