Handbook of MRI Technique. Catherine Westbrook

entire length of femur Axial perpendicular to long axis of the femur prescribed to extend from well below, to well above lesions seen on coronal or sagittal images entire thigh to skin surfaces Knee Coronal parallel to posterior surfaces of femoral condyles femoral condyles to anterior margin of patella superior edge of patella to inferior edge of tibial tuberosity Sagittal parallel with ACL lateral to medial collateral ligaments superior edge of patella to below tibial tuberosity Axial perpendicular to posterior surfaces of femoral condyles superior surface of patella to tibial tuberosity entire knee to skin surfaces Tibia and fibula Coronal parallel to interosseous ligament posterior to anterior skin surfaces of calf whole of tibia and fibula to skin surfaces Sagittal perpendicular to the interosseous ligament left to right skin margins of calf whole of tibia and fibula to skin surfaces Axial perpendicular to long axis of the tibia well above and below lesions seen in sagittal and coronal planes whole calf to skin surfaces Ankle Coronal parallel to transmalleolar line Achilles tendon to base of proximal metatarsals inferior border of calcaneum to distal portion of tibia Sagittal parallel to mortise axially, to distal tibia coronally from the lateral to medial aspects of the ankle distal tibia to the sole of the foot and the tarsometatarsal joints Axial perpendicular to long axis of distal tibia superior margin of tibiofibular margin to bottom of calcaneum and base of fifth metatarsal entire ankle joint to skin surfaces Foot Coronal proximally parallel to bases of the first to fourth metatarsals metatarsophalangeal joints to tarsometatarsal joints whole foot to skin surfaces Sagittal perpendicular to plane joining base of first to fourth metatarsals lateral to medial aspects of foot sole of foot to distal tibia Axial perpendicular to metatarsals metatarophalangeal joints to tarsometatarsal joints whole foot to skin surfaces

CONCLUSION

      To use this book:

       Find the required anatomical region and then locate the specific examination.

       Study the categories under each section. It is possible that all the categories are relevant if the examination is being performed for the first time. However, there may be occasions when only one item is appropriate. For example, there could be a specific artefact that is regularly observed in chest examinations, or image quality is not up to standard in lumbar spine protocols. Under these circumstances, read the subsection entitled Protocol optimization.

       If the terms used, or concepts discussed in Part 2 are unfamiliar, then turn to Part 1 and read the summaries described there.

Part 1 Theoretical and Practical Concepts

2 Protocol Parameters and Trade‐offs

        Introduction

        Signal-to-noise ratio (SNR)

        Contrast-to-noise ratio (CNR)

        Spatial resolution

        Scan time

        Decision strategies

        Conclusion

INTRODUCTION

      This section refers mainly to the Technical issues subheading discussed under the Protocol optimization heading considered for each examination in Part 2. Only a brief overview is provided here. For a more detailed explanation, please refer to Chapter 7 of the fifth edition of MRI in Practice or an equivalent text.

      The main considerations of protocol optimization are:

       signal‐to‐noise ratio (SNR)

       contrast‐to‐noise ratio (CNR)

       spatial resolution

       scan time.

Each factor is controlled by certain protocol parameters, and each ‘trades off’ against the other. This section summarizes these parameters and their trade‐offs. Suggested protocol parameters are outlined in Table 2.1 and should be universally acceptable on most systems. However, weighting parameters are field strength dependent, and therefore some modification may be required for extremely low‐ or high‐field systems.

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