CT Teaching Manual E-Book

Matthias Hofer

• Herausgeber: Thieme
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 353

Veröffentlichungsjahr: 2021

Beschreibung

Ideal for residents starting in radiology and radiologic technologists, this concise manual is the perfect introduction to the physics and practice of CT and the interpretation of basic CT images. Designed as a systematic learning tool, it introduces the use of CT scanners for all organs, and includes positioning, use of contrast media, representative CT scans of normal and pathological findings, explanatory drawings with keyed anatomic structures, and an overview of the most important measurement data. Finally, self-assessment quizzes – including answers – at the end of each chapter help the reader monitor progress and evaluate knowledge gained.

New in this fifth edition: Recent technical developments such as dual source CT, protocols for CT angiography, and PET/CT fusion.

Leseprobe

Library of Congress Cataloging-in-Publication Datais available from the publisher.

Matthias Hofer, MD, MPH,Master of Medical Education (MME)Assistant Professor for Diagnostic Radiology,Director of Education, Institute for Diagnostic,Interventional and Pediatric Radiology (DIPR),Head: Professor Johannes Heverhagen, MDInselspital Bern, Bern University, Switzerland

Contributor: Ingrid Boehm, MD, Assistant Professorfor Diagnostic Radiology, (DIPR)Inselspital Bern, Bern University, Switzerland

PET/CT images from

Professor Gerald Antoch, MDDirector, Institute for Diagnostic,Pediatric and Interventional RadiologyHeinrich Heine University, Düsseldorf, Germany

Till-Alexander Heusner, MDProfessor for Diagnostic RadiologyHead of Radiology Dept.Sankt Elisabeth Hospital, Gütersloh, Germany

© 2021. Thieme. All rights reserved.Georg Thieme Verlag KGRüdigerstrasse 14, D-70469 Stuttgart, Germanywww.thieme.de

New parts translated byJohn Grossman, Schrepkow, Germanywww.john-grossman.com

Cover Design: Thieme Publishing GroupCover Image source- the cover image wascomposed by Thieme using the following images:Siemens Healthineers CT SOMATOM Force:  © Siemens Healthcare GmbH, 2020Cardiac Images: Deutsches Herzzentrum, München, GermanyHepatic PET/CT Images: Professor Gerald Antoch,Düsseldorf, Germany

Typesetting by Ramona Sprenger, Cologne, Germanywww.einraumapartment.de

Printed in Germany byDruckerei Steinmeier, Deiningen

DOI 10.1055/b000000534

ISBN 978-3-13-244263-4                  5 4 3 2 1

Also available as an ebook:eISBN (PDF):     978-3-13-244264-1eISBN (ePub):     978-3-13-244265-8

Important Note: Medicine is an ever-changing science undergoing continual development. Research and clinical experience are continually expanding our knowledge, in particular our knowledge of proper treatment and drug therapy. Insofar as this book mentions any dosage or application, readers may rest assured that the authors, editors, and publishers have made every effort to ensure that such references are in accordance with the state of knowledge at the time of production of the book.

Nevertheless, this does not involve, imply, or express any guarantee or responsibility on the part of the publishers in respect of any dosage instructions and forms of application stated in the book. Every user is requested to examine carefully the manufacturers’ leaflets accompanying each drug and to check, if necessary in consultation with a physician or specialist, whether the dosage schedules mentioned therein or the contraindications stated by the manufacturers differ from the statements made in the present book. Such examination is particularly important with drugs that are either rarely used or have been newly released on the market. Every dosage schedule or every form of application used is entirely at the user’s own risk and responsibility. The authors and publishers request every user to report to the publishers any discrepancies or inaccuracies noticed. If errors in this work are found after publication, errata will be posted at www.thieme.com on the product description page.

Some of the product names, patents, and registered designs referred to in this book are in fact registered trademarks or proprietary names even though specific reference to this fact is not always made in the text. Therefore, the appearance of a name without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain.

This book, including all parts thereof, is legally protected by copyright. Any use, exploitation or commercialization outside the narrow limits set by copyright legislation, without the publisher’s consent, is illegal and liable to prosecution. This applies in particular to photostat reproduction, copying, mimeographing or duplication of any kind, translating, preparation of microfilms, and electronic data processing and storage.

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Foreword

Computed tomography has become an integral and indispensable part of clinical diagnostics, with which special clinical questions can be answered with high accuracy in only a short time latency. The recent technical developments enable a spatial resolution in the submillimeter range for CT-supported angiographies, and with the help of dual source CT, also statements about the chemical composition of tissues or e.g. kidney stones. Above all, the development of PET/CT has made possible immense progress in oncology with regard to the diagnostic accuracy in the question of metastases or tumor recurrences. Modern imaging modalities are often not satisfactorily covered when teaching medical students in lectures and clinical courses. When leaving medical school, the knowledge gaps in this area can often be considerable.

All these aspects and recent developments have already been taken into account in this standard book on CT diagnostics, so that it offers the necessary basics for beginners, who are just familiarizing themselves with the subject, and is suitable for advanced users with a special radiological interest. The quiz cases, in particular, will hopefully arouse many readers` detective ambition to check their diagnostic skills themselves. The success story of 28 editions in 9 languages speaks for itself, and proves the broad acceptance of this book among German-speaking and international colleagues. I wish you a high learning benefit and lots of fun using this teaching manual!

Bern, January 2021

Professor Johannes Heverhagen, MDDept. Head of University Institute for Diagnostic,Interventional and Pediatric Radiology (DIPR)Inselspital Bern, Bern University, Switzerland

List of Abbreviations

Table of Contents

Key to Anatomic Structurespp. 26–73, 152–153(head and neck)

Some Practical Hints for Using this Manual

Schemata for CT drawings

Foreword and List of Abbreviations

Physical and Technical Fundamentals

General Principles of CT

Comparison of Conventional CT with Spiral CT

Spatial Resolution, Pitch

Section Collimation: Resolution along the Z-axis

Adaptive Detector Design

Reconstruction Algorithms

Effects of kV, mAs and Scan Time

Three-Dimensional Reconstruction Methods:

Maximum Intensity Projection (MIP)

Multiplanar Reconstruction (MPR)

Surface Rendering

Basic Rules for Reading CT Examinations

Anatomic Orientation

Partial Volume Effects

Distinguishing Nodular from Tubular Structures

Densitometry

Density Levels of Different Tissues

Documentation Using Different Window Settings

Preparing the Patient (contributed by Ingrid Boehm)

Medical History,

Renal Toxicity, Medication as Risk

Adverse CM-Reactions and their Prophylaxis

Oral Administration of Contrast Agents

Informing the Patient, Controlling Respiration,

Removal of All Metallic Objects

Administration of Contrast Agents (contributed by Ingrid Boehm)

Contrast-Enhanced CT-Scans and Contrast Media

Per oral CM-Application, The Procedure,

Additional Medication for an Optimal Imaging Result

Intravenous CM-Application

Preparing the IV-Line,

Inflow Phenomena

Hyperthyroidism (Type A Reactions)

Hypersensitivity Reactions (Type B Reactions)

Premedication

Treatment of Adverse CM Reactions to CM

Cranial CT

Selection of Image Plane

Systemic Approach to Interpretation

Checklist for Reading Cranial CT

Cranial CT: Normal Findings

Normal Cranial Anatomy

Test Yourself!

Normal Anatomy of the Orbit (Axial)

Normal Anatomy of the Facial Skeleton (Coronal)

Test Yourself!

Normal Anatomy of the Temporal Bone (Coronal/Axial)

Normal Variants of the Cranium

Typical Partial Volume Effects of the Cranium

Cranial Pathology

Intracranial Hemorrhage

Cerebral Infarcts

Cerebral Tumors and Metastases

Inflammatory Processes

Orbit

Facial Skeleton and Sinuses

Cervical CT

Selection of the Image Plane

Checklist for Reading Cervical CT Images

Normal Anatomy of the Neck

Cervical Pathology

Inflammatory Processes and Tumors

Thyroid Gland

Test Yourself!

Key to Anatomic Structurespp. 71, 74–149(thorax and abdomen)

Chest CT

Selection of the Image Plane

Systemic Sequential Approach to Interpretation

Checklist Thorax Readings

Normal Anatomy of the Chest

Test Yourself!

Chest CT Pathology

High-Resolution CT – Normal Anatomy

HRCT of the Lungs: Technique, Effects, Indications

Anatomic Variants of the Chest

Chest Wall

Abnormal Lymph Nodes

Breast, Bony Thorax

Mediastinum

Tumor Masses

Enlarged Lymph Nodes

Vascular Pathology

Heart

Lung

Intrapulmonary Nodules

Bronchial Carcinoma, Malignant Lymphangiomatosis

Sarcoidosis, Tuberculosis, Aspergillosis

Pleural Changes, Asbestosis

Silicosis, Pulmonary Emphysema

Interstitial Pulmonary Fibrosis

Test Yourself!

Abdominal CT

Selection of the Image Plane

Systemic Sequential Approach to Interpretation

Checklist Abdominal Readings

Normal Anatomy of the Abdomen

Normal Anatomy of the Pelvis (Male)

Normal Anatomy of the Pelvis (Female)

Abdominal Pathology

Anatomic Variants of the Abdomen,

Typical Partial Volume Effects

Abdominal Wall

Enlarged Lymph Nodes, Abscesses

Subcutaneous Heparin Injections

Abdominal Wall Metastases, Inguinal Hernias

Liver

Segmental Anatomy of the Liver

Examination Techniques:

Window Selection, Contrast Bolus, CT Portography

Cystic Hepatic Lesions

Hepatic Metastases

Solid Liver Tumors:

Hemangioma, Adenoma, Focal Nodular Hyperplasia

Diffuse Liver Changes:

Fatty Liver, Hemochromatosis, Hepatic Cirrhosis

Biliary Tract

Pneumobilia, Cholestasis

Gallbladder

Cholecystolithiasis,

Chronic Inflammatory Processes

Spleen

Contrast Enhancement, Splenomegaly

Focal Splenic Changes

Pancreas

Acute and Chronic Pancreatitis, Pancreatic Tumors

Adrenal Glands

Hyperplasia, Adenomas, Metastases, Carcinoma

Kidneys

Congenital Variants

Cystic Lesions, Hydronephrosis

Solid Tumors

Vascular Renal Changes

Urinary Bladder

Catheters, Diverticula, Solid Tumors

Reproductive Organs

Uterus

Ovaries, Prostate, Vas Deferens

Gastrointestinal Tract

Stomach, Inflammatory Bowel Diseases

Colon

Bowel Obstruction and Ileus

Test Yourself!

Retroperitoneum

Aneurysms

Venous Thrombosis

Enlarged Lymph Nodes

Skeletal Changes

Bony Pelvis: Normal Findings, Metastases

Fractures

Femoral Head Necrosis and Hip Dysplasia

Test Yourself!

Spinal Column: Skeletal Pathology

Cervical Spine: Normal Anatomy

Cervical Disk Herniation and Fractures

Thoracic Spine: Normal Findings and Fractures

Lumbar Spine:

Normal Findings and Lumbar Disk Herniation

Lumbar Fractures

Lumbar Spine: Tumors and Metastases

Lumbar Spine: Infection and Methods of Stabilization

Lower Extremity

Normal Anatomy of the Thigh

Normal Anatomy of the Knee

Normal Anatomy of the Lower Leg

Normal Anatomy of the Foot

Fractures of the Foot

Pelvis and Upper Leg: Inflammatory Processes

Knee – Fractures, Checklist Skeletal System: Fracture Diagnosis

CT-Guided Interventions

ABC-Primer of CT Evaluation

Radiation Safety

Radiation Dose and Cancer Risk

Automatic Bolus Tracking (BT)

Tube Current Modulation – Dose Reduction

CT Angiography

Intracranial Arteries

Venous Sinus Thrombosis

Carotid Arteries

Aorta

Heart: Coronary Arteries, Coronary Calcium Screening

Pulmonary Vessels (Pulmonary Embolism)

Abdominal Vessels

Iliofemoral Vessels

Vascular Prostheses, Outlook

Test Yourself!

Contrast Injectors

Dual Source CT

Introduction to PET/CT

Tumors of the Gastrointestinal Tract

Pulmonary Tumors

Tumors of the Prostate Gland

Neuroendocrine Tumors

Anatomy in Coronal MPRs

Anatomy in Sagittal MPRs

Solutions to Test Yourself!

Index

References

Key to Spinal and Lower Extremity Diagrams (pp. 152–167)

Key to Thoracic and Pelvic Diagrams (pp. 74–149)

Physical and Technical Fundamentals

General Principles of CT

Computed tomography is a special type of x-ray procedure that involves the indirect measurement of the weakening, or attenua-tion, of x-rays at numerous positions located around the patient being investigated. Basically speaking, all we know is

• what leaves the x-ray tube,

• what arrives at the detector and

• the position of the x-ray tube and detector for each position.

Simply stated, everything else is deduced from this information. Most CT slices are oriented vertical to the body`s axis. They are usually called axial or transverse sections. For each section the x-ray tube rotates around the patient to obtain a preselected section thickness (Fig. 6.1). Most CT systems employ the continuous rotation and fan beam design: with this design, the x-ray tube and detector are rigidly coupled and rotate continuously around the scan field while x-rays are emitted and detected. Thus, the x-rays, which have passed through the patient, reach the detectors on the opposite side of the tube. The fan beam opening ranges from 40° to 60°, depending on the particular system design, and is defined by the angle originating at the focus of the x-ray tube and extending to the outer limits of the detector array.

Typically, images are produced for each 360° rotation, permitting a high number of measurement data to be acquired and sufficient dose to be applied. While the scan is being performed, attenuation profiles, also referred to as samples or projections, are obtained. Attenuation profiles are really nothing other than a collection of the signals obtained from all the detector channels at a given angular position of the tube-detector unit. Modern CT systems (Fig. 6.4) acquire approximately 1400 projections over 360°, or about four projections per degree. Each attenuation profile comprises the data obtained from about 1500 detector channels, about 30 channels per degree in case of a 50° fan beam. While the patient table is moving continuously through the gantry, a digital radiograph (“scanogramm” or “localizer”, Fig. 6.2) is produced on which the desired sections can be planned. For a CT examination of the spine or the head, the gantry is angled to the optimal orientation (Fig. 6.3).

Fig. 6.1

Fig. 6.2

Fig. 6.3

Fig. 6.4

Multiple-Row Detector Spiral CT

Multiple-row detector CT (MDCT) is the latest scanner development. Rather than one detector row, multiple detector rows are placed opposite the x-ray tube. This shortens the examination time and improves the temporal resolution, allowing, for instance, the determination of the rate of vascular enhancement.

The detector rows along the z-axis opposite the x-ray tube are unequal in width, with the outer rows wider than the inner rows to provide better conditions for image reconstruction after data acquisition (see pages 9-11 and 206).

Dual Source CT

This newest technique features two detector units and two X-ray tubes in one gantry and is described in more detail on pages 198-201.

Comparison of Conventional CT with Spiral CT

In conventional CT, a series of equally spaced images is acquired sequentially through a specific region, e.g. the abdomen or the head (Fig. 7.1). There is a short pause after each section in order to advance the patient table to the next preset position. The section thickness and overlap/intersection gap are selected at the outset. The raw data for each image level is stored separately. The short pause between sections allows the conscious patient to breathe without causing major respiratory artifacts.

Fig. 7.1

However, the examination may take several minutes, depending on the body region and the size of the patient. Proper timing of image acquisition after i.v. contrast media is particularly important for assessing perfusion effects. CT is the technique of choice for acquiring complete 2D axial images of the body without the disadvantages of superimposed bone and / or air as seen in conventional x-ray images.

Both single-row detector CT (SDCT) and multiple-row detector CT (MDCT) continuously acquire data of the patient while the examination table moves through the gantry. The x-ray tube describes an apparent helical path around the patient (Fig. 7.2). If table advance is coordinated with the time required for a 360° rotation (pitch factor), data acquisition is complete and uninterrupted. This modern technique has greatly improved CT because respiratory artifacts and inconsistencies do not affect the single dataset as markedly as in conventional CT. The single dataset can be used to reconstruct slices of differing thickness or at differing intervals. Even overlapping slices can be reconstructed.

Fig. 7.2

In modern MDCT units with 16 up to 64 detector rows, data acquisition time for e.g. the thorax no longer exceeds the duration most patients can hold their breath: Even with narrow collimations, the entire thorax can be scanned within 7-10 seconds [47]. Even a CTA of the carotid arteries and circle of Willis with a pitch of 1.5 and rotation time of 0.37 seconds/rotation requires only 5 seconds for a scan range of 350 mm (64 x 0.6 mm collimation). Due to increased speed, most modern CT units would pass the renal excretion of CM, so that a longer delay time or a short break are required to document the patient`s renal excretion function. Therefore, the „neck of the bottle“ in the workflow is no longer the data acquisition time, but sometimes the size of the corresponding data files in cases of complex MIP- / MPR-recontructions at the local workstation.

One of the advantages of the helical technique is that lesions smaller than the conventional thickness of a slice can be detected. Small liver metastases (7) will be missed if inconsistent depth of respiration results in them not being included in the section (Fig. 7.3a). The metastases would appear in overlapping reconstructions from the dataset of the helical technique (Fig. 7.3b).

Fig. 7.3a Conventional CT

Fig. 7.3b Spiral-CT

Spatial Resolution

Fig. 8.1a

Fig. 8.1b

The image quality should improve with smaller voxels, but this only applies to the spatial resolution since a thinner section lowers the signal-to-noise ratio. Another disadvantage of thinner sections is the inevitable increase in the radiation dose to the patient (see page 175). Nonetheless, smaller voxels with identical measurements in all three dimensions (isotropic voxels) offer a crucial advantage: The multiplanar reconstruction (MPR) in coronal, sagittal or other planes displays the reconstructed images free of any step-like contour (Fig. 8.2). Using voxels of unequal dimension (anisotropic voxels) for MPR is burdened by a serrated appearance of the reconstructed images (Fig. 8.3), which, for instance, can make it difficult to exclude a fracture (Fig. 148.5b).

Fig. 8.2 MPR from isotropic voxels

Fig. 8.3 MPR from anisotropic voxels

Pitch

By now, several definitions exist for the pitch, which describes the rate of table increment per rotation in millimeter and section thickness.

A slowly moving table per rotation generates a tight acquisition spiral (Fig. 8.4a). Increasing the table increment per rotation without changing section thickness or rotation speed creates interscan spaces of the acquisition spiral (Fig. 8.4b).

The mostly used definition of the pitch describes the table travel (feed) per gantry rotation, expressed in millimeters, and selected collimation, also expressed in millimeters.

Fig. 8.4

The new scanners give the examiner the option to select the craniocaudal extension (z-axis) of the region to be examined on the topogram as well as the rotation time, section collimation (thin or thick sections?) and examination time (breath-holding intervals?). The software, e.g., “SureView®,” calculates the suitable pitch, usually providing values between 0.5 and 2.0.

Section Collimation: Resolution Along the Z-Axis

The resolution (along the body axis or z-axis) of the images can also be adapted to the particular clinical question by the choice of the collimation. Sections between 5 and 8 mm generally are total-ly adequate for standard examinations of the abdomen. However, the exact localization of small fracture fragments or the evaluation of subtle pulmonary changes require thin slices between 0.5 and 2 mm. What determines the section thickness?

The term collimation describes how thin or thick the acquired slices can be preselected along the longitudinal axis of the patient (= z-axis). The examiner can limit the fan-like x-ray beam emitted from the x-ray tube by a collimator, whereby the collimator’s aperture determines whether the fan passing through the collimator and collected by the detector units behind the patient is either wide (Fig. 9.1) or narrow (Fig. 9.2), with the narrow beam allowing a better spatial resolution along the z-axis of the patient. The collimator cannot only be placed next to the x-ray tube, but also in front of the detectors, i.e., “behind” the patient as seen from the x-ray source.

Depending on the width of collimator’s aperture, the units with only one detector row behind the patient (single section) can generate sections with a width of 10 mm, 8 mm, 5 mm or even 1 mm. A CT examination obtained with very thin sections is also called a high resolution CT (HRCT) and, if the sections are at the sub-millimeter level, ultra high resolution CT (UHRCT). The UHRCT is used for the petrous bone with about 0.5 mm sections to detect delicate fracture lines through the cranial base or auditory ossicles in the tympanic cavity (see pages 46–49). For the liver, however, the examination is dominated by the contrast resolution since the question here is the detectability of hepatic metastases (here somewhat thicker sections).

Fig. 9.1 Wide section collimation

Fig. 9.2 Narrow section collimation

Adaptive Array Design

A further development of the single-slice spiral technology is the introduction of the multislice technique, which has not one detector rows but several detector rows stacked perpendicular to the z-axis opposite the x-ray source. This enables the simultaneous acquisition of several sections.

The detector rows are not inevitably equal in width. The adaptive array design consists of detectors that increase in width from the center to the edge of the detector ring and consequently allows various combinations of thickness and numbers of acquired sections.

Fig. 10.1 Detector design of a 4-row unit, as found in the Siemens Sensation 4

Fig. 10.2 Detector design of a 6-row unit, as found in the Siemens Emotion 6

For instance, a 16-slice examination can be performed with 16 thin sections of a higher resolution (for the Siemens Sensation 16, this means 16 x 0.75 mm) or with 16 sections of twice the thickness. For an iliofemoral CTA (see page 188), it is preferable to acquire a long volume along the z-axis in a single run, of course with a selected wide collimation of 16 x 1.5 mm.

Fig. 11.1 Detector design of a 16-row unit, as found in the Siemens Sensation 16

The development of the CT hardware did not end with 16 slices and faster data acquisition can already be achieved with 32- and 64-row scanners. The trend to thinner slices is associated with higher patient exposure to radiation, requiring additional and already introduced measures for exposure reduction (see pages 174-177).

When both liver and pancreas are included, many users prefer a reduced slice thickness from 10 mm to 3 mm to improve image sharpness. This increases, however, the noise level by approxi-mately 80%. Therefore it would be necessary to employ 80% more mA or to lengthen the scan time (this increases the mAs product) to maintain image quality.

Reconstruction Algorithm

Spiral users have an additional advantage: In the spiral image reconstruction process, most of the data points were not actually measured in the particular slice being reconstructed (Fig. 11.2). Instead, data are acquired outside this slice (•) and interpolated with more importance, or “contribution”, being attached to the data located closest to the slice (X). In other words: The data point closest to the slice receives more weight, or counts more, in the reconstruction of an image at the desired table position.

This results in an interesting phenomenon. The patient dose (actually given in mGy) is determined by the mAs per rotation divided by the pitch, and the image dose is equal to the mAs per rotation without considering the pitch. If for instance 150 mAs per rotation with a pitch of 1.5 are employed, the patient dose in mGy is linear related to 100 mAs, and the image dose is related to 150 mAs. Therefore spiral users can improve contrast detectability by selecting high mA values, increase the spatial resolution (image sharpness) by reducing slice thickness, and employ pitch to adjust the length of the spiral range as desired, all while reducing the patient`s dose! More slices can be acquired without increasing the dose or stressing the x-ray tube.

This technique is especially helpful when data are reformatted to create other 2D views, like sagittal, oblique, coronal, or 3D views (MIP, surface shaded imaging, see pp. 8 and 13).

Fig. 11.2 Wide (360°) spiral reconstruction algorithm

The data obtained at the detector channel are passed on, profile for profile, to the detector electronics as electric signals corresponding to the actual x-ray attenuation. These electric signals are digitized and then transmitted to the image processor. At this stage, the images are reconstructed by means of the “pipeline principle”, consisting of preprocessing, convolution, and back projection (Fig. 12.1).

Preprocessing includes all the corrections taken to prepare the measured scan data for reconstruction, e.g., correction for dark current, dose output, calibration, channel correction, beam hardening, and spacing errors. These corrections are performed to further minimize the slight variations inherently found in the tube and detector components of the imaging chain.

Convolution is basically the use of negative values to correct for smearing inherent to simple back projection. If, for instance, a cylindric water phantom is scanned and reconstructed without convolution, the edges of this phantom will be extremely blurry (Fig. 12.2a): What happens when just eight attenuation profiles of a small, highly absorbent cylindrical object are superimposed to create an image? Since the same part of the cylinder is measured by two overlapping projections, a star-shaped image is produced instead of what is in reality a cylinder. By introducing negative values just beyond the positive portion of the attenuation profiles, the edges of this cylinder can be sharply depicted (Fig. 12.2b).

Back projection involves the reassigning of the convolved scan data to a 2D image matrix representing the section of the patient that is scanned. This is performed profile for profile for the entire image reconstruction process. The image matrix can be thought of as analogous to a chessboard, consisting of typically 512 x 512 or 1024 x 1024 picture elements, usually called “pixels”. Back projection results in an exact density being assigned to each of these pixels, which are then displayed as a lighter or darker shade of gray. The lighter the shade of gray, the higher the density of the tissue within the pixel (e.g., bone).

The Influence of kV

When examining anatomic regions with higher absorption (e.g., CT of the head, shoulders, thoracic or lumbar spine, pelvis, and larger patients), it is often advisable to use higher kV levels in addition to, or instead of, higher mA values: when you choose higher kV, you are hardening the x-ray beam. Thus x-rays can penetrate anatomic regions with higher absorption more easily. As a positive side effect, the lower energy components of the radiation are reduced, which is desirable since low energy x-rays are absorbed by the patient and do not contribute to the image. For imaging of infants or bolus tracking, it may be advisable to utilize kV lower than the standard setting.

Tube Current [mAs]

The tube current, stated in milliampere-seconds [mAs], also has a significant effect on the radiation dose delivered to the patient. A patient with more body width requires an increase in the tube current to achieve an adequate image quality. Thus, more corpulent patients receive a larger radiation dose than, for instance, children with a markedly smaller body width.

Body regions with skeletal structures that absorb or scatter radiation, such as shoulder and pelvis, require a higher tube current than, for instance, the neck, a slender abdominal torso or the legs. This relationship has been actively applied to radiation protection for some time now (compare with page 177).

Scan Time

It is advantageous to select a scan time as short as possible, particularly in abdominal or chest studies where heart movement and peristalsis may degrade image quality. Other CT investigations can also benefit from fast scan times due to decreased probability of involuntary patient motion. On the other hand, it may be necessary to select a longer scan time to provide sufficient dose or to enable more samples for maximal spatial resolution. Some users may also consciously choose longer scan times to lower the mA setting and thus increase the likelihood of longer x-ray tube life.

Fig. 12.1 The pipeline principle of image reconstruction

Fig. 12.2a Back projection without convolution

Fig. 12.2b Back projection with convolution

3D Reconstructions

Because the helical or spiral technique acquires a continuous, single volume dataset for an entire body region, imaging of fractures and blood vessels has improved markedly. Several different methods of 3D reconstruction have become established:

Maximal Intensity Projection

MIP is a mathematical method that extracts hyperintense voxels from 2D or 3D datasets [6, 7]. These voxels are selected from several different angles through the dataset and then projected as a 2D image (Fig. 13.1). A 3D impression is acquired by altering the projection angle in small steps and then viewing the reconstructed images in quick succession (i.e., in cine mode). This procedure is also used for examining contrast-enhanced blood vessels.

Fig. 13.1

Multiplanar Reconstruction

This technique makes it possible to reconstruct coronal and sagittal as well as oblique planes. MPR has become a valuable tool in the diagnosis of fractures and other orthopedic indications. For example, conventional axial sections do not always provide enough information about fractures. A good example is the undisplaced hairline fracture (*) without cortical discontinuity that can be more effectively demonstrated by MPR (Fig. 13.2a).

Fig. 13.2a

3D Surface Shaded Display

This method shows the surface of an organ or a bone that has been defined in Hounsfield units above a particular threshold value. The angle of view, as well as the location of a hypothetical source of light (from which the computer calculates shadowing) are crucial for obtaining optimal reconstructions. The fracture of the distal radius shown in the MPR in Figure 13.2a is seen clearly in the bone surface in Figure 13.2b.

(Figs. 13.2a and 13.2b supplied with the kind permission of J. Brackins Romero, M. D., Recklinghausen, Germany)

Fig. 13.2b

3D surface shaded displays are also valuable in planning surgery as in the case of the traumatic injury to the spinal column seen in Figures 13.3 a, b, and c. Since the angle of view can be freely determined, the thoracic compression fracture (*) and the state of the intervertebral foramina can be examined from several different angles (anterior in Fig. 13.3a and lateral in Fig. 13.3b). The sagittal MPR in Figure 13.3c determines whether any bone fragments have become dislocated into the spinal canal (compare with myelography CT on page 147).

Fig. 13.3a

Fig. 13.3b

Fig. 13.3c

Basic Rules of Reading CT Examinations

Anatomic Orientation

An image on the display is not only a 2D representation of anatomy, it contains information about the mean attenuation of tissue in a matrix consisting of about 1024 x 1024 elements (pixels). A section (Fig. 14.1) has a defined thickness (dS) and is composed of a matrix of cubic or cuboid units (voxels) of identical size. This technical aspect is the reason for the partial volume effects explained below. An image is usually displayed as if the body were viewed from caudal. Thus the right side of the patient is on the left side of the image and vice versa (Fig. 14.1). For example, the liver (122) is located in the right half of the body, but appears in the left half of the image. Organs of the left side such as the stomach (129) and the spleen (133) appear on the right half of an image. Anterior aspects of the body, for example the abdominal wall, are represented in the upper parts of an image, posterior aspects such as the spine (50) are lower. With this system CT images are more easily compared with conventional x-ray-images.

Fig. 14.1

Partial Volume Effects

The radiologist determines the thickness of the image (dS). 4–6 mm are usually chosen for thoracic or abdominal examinations, and 2–5 mm for the skull, spine, orbits, or petrosal bones. A structure may therefore be included in the entire thickness of a slice (Fig. 14.2a) or in only a part of it (Fig. 14.3a). The gray scale value of a voxel depends on the mean attenuation of all structures within it. If a structure has a regular shape within a section, it will appear well defined. This is the case for the abdominal aorta (89) and the inferior vena cava (80) shown in Figures 14.2a, b.

Partial volume effects occur when structures do not occupy the entire thickness of a slice, for example when a section includes part of a vertebral body (50) and part of a disk (50e) the anatomy will be poorly defined (Figs. 14.3a, b). This is also true if an organ tapers within a section as seen in Figures 14.4a, b. This is the reason for the poor definition of the renal poles or the borders of the gallbladder (126) or urinary bladder. Artifacts caused by breathing during image acquisition are discussed on page 19.

Fig. 14.2a

Fig. 14.2b

Fig. 14.3a

Fig. 14.3b

Fig. 14.4a

Fig. 14.4b

Distinguishing Between Nodular and Tubular Structures

It is essential to differentiate between possibly enlarged or affected LNs and vessels or muscles which have been cut in transverse section. This may be extremely difficult in a single image because these structures have similar density values (gray tones). One should therefore always analyze adjacent cranial and caudal images and compare the structures in question to determine whether they are nodular swellings or continue as more or less tubular structures (Fig. 15.1): A lymph node (6) will appear in only one or two slices and cannot be traced in adjacent images (compare Figs. 15.1a, b, and c). The aorta (89) or the inferior cava (80), or a muscle, for example the iliopsoas (31), can be traced through a cranio-caudal series of images.

If there is a suspicious nodular swelling in one image, it should become an automatic reaction to compare adjacent levels to clarify whether it is simply a vessel or muscle in cross-section. This procedure will also enable quick identification of the partial volume effects described on the previous page.

Fig. 15.1

Densitometry (Measurement of Density)

If it is uncertain, for example, whether fluid found in the pleural cavity is a pleural effusion or a hemothorax, a measurement of the liquid’s density will clarify the differential diagnosis. The same applies to focal lesions in the parenchyma of the liver or the kidney. However, it is not advisable to carry out measurements of single voxels (=volume element, see Fig. 14.1) since such data are liable to statistical fluctuations which can make the attenuation unreliable. It is more accurate to position a larger “region of interest” (ROI) consisting of several voxels in a focal lesion, a structure, or an amount of fluid. The computer calculates the mean density levels of all voxels and also provides the standard deviation (SD).

One must be particularly careful not to overlook beam-hardening artifacts (Fig. 19.2) or partial volume effects. If a mass does not extend through the entire thickness of a slice, measurements of density will include the tissue next to it (Figs. 121.2 and 133.1–133.3). The density of a mass will be measured correctly only if it fills the entire thickness of the slice (dS) (Fig. 15.2). It is then more likely that measurements will include only the mass (hatched area in Fig. 15.2a). If dS is greater than the mass’s diameter, for example a small lesion in an unfavorable position, it can only appear in partial volume at any scan level (Fig. 15.2b).

Fig. 15.2

Density Levels of Different Types of Tissues

Modern equipment has a capacity of 4096 gray tones, which represent different density levels in HUs. The density of water was arbitrarily set at 0 HU and that of air at –1000 HU (Table 16.1a). The monitor can display a maximum of 256 gray tones. However, the human eye is able to discriminate only approximately 20. Since the densities of human tissues extend over a fairly narrow range (a window) of the total spectrum (Table 16.1b), it is possible to select a window setting to represent the density of the tissue of interest. The mean density level of the window should be set as close as possible to the density level of the tissue to be examined. The lung, with its high air content, is best examined at a low HU window setting (Fig. 17.1c), whereas bones require an adjustment to high levels (Fig. 17.2c). The width of the window influences the contrast of the images: the narrower the window, the greater the contrast since the 20 gray tones cover only a small scale of densities.

It is noteworthy that the density levels of almost all soft-tissue organs lie within a narrow range between 10 and 90 HUs (Table 16.1b). The only exception is the lung and, as mentioned above, this requires a special window setting (Figs. 17.1a–c). With respect to hemorrhages, it should be taken into account that the density level of recently coagulated blood lies about 30 HU above that of fresh blood. This density drops again in older hemorrhages or liquefied thromboses. An exudate with a protein content above 30 g/l cannot be readily distinguished from a transudate (protein content below 30 g/l) at conventional window settings. In addition, the high degree of overlap between the densities of, for example, lymphomas, spleen, muscles, and pancreas makes it clear that it is not possible to deduce, from density levels alone, what substance or tissue is present.

Finally, standard density values also fluctuate between individuals, depending as well on the amount of CM in the circulating blood and in the organs. The latter aspect is of particular importance for the examination of the urogenital system, since i.v. CM is rapidly excreted by the kidney, resulting in rising density levels in the parenchyma during the scanning procedure. This effect can be put to use when judging kidney function (see Fig. 135.1).

Documentation of Different Windows

When the images have been acquired, a hard copy is printed for documentation. For example: in order to examine the mediastinum and the soft tissues of the thoracic wall, the window is set such that muscles (13, 14, 20–26), vessels (89, 90, 92…), and fat are clearly represented in shades of gray. The soft-tissue window (Fig. 17.1a) is centered at 50HU with a width of about 350HU. The result is a representation of density values from –125HU (50–350/2) up to +225HU (50+350/2). All tissues with a density lower than –125HU, such as the lung, are represented in black. Those with density levels above +225 appear white and their internal structural features cannot be differentiated.

If lung parenchyma is to be examined, for example when scanning for nodules, the window center will be lower at about –200HU, and the window wider (2000HU). Low-density pulmonary structures (96) can be much more clearly differentiated in this so-called lung window (Fig. 17.1c).

Fig. 17.1a Soft tissue window

Fig. 17.1b

Fig. 17.1c Lung window

In order to achieve maximal contrast between gray and white matter in the brain, it is necessary to select a special brain window because the density values of gray and white matter differ only slightly. The brain window must be very narrow (80 to 100HU => high contrast) and the center must lie close to the mean density of cerebral tissue (35HU) to demonstrate these slight differences (Fig. 17.2a