Arkadiusz

Polacin,

Evaluation and Image

PhD

Wihi

#{149}

A. Kalender,

PhD

Guy

Marchal,

#{149}

MD

of Section

Noise

Spiral computed tomography (CT) offers continuous volume scanning of complete organs or body sections within a single breath hold. Almost all image quality characteristics of spiral CT are identical to those of conventional section-by-section CT; however, there is a change in pixel noise values and degradation in the shape of the section sensitivity profiles (SSPs). Computer simulations, phantom measurements, and clinical studies were used in evaluating the SSP and noise characteristics of two new section-interpolation algorithms. The results were compared with standard CT and spiral CT data processed with the commonly employed linear section-interpolation algorithm. Degradation of SSP quality was insignificant for a table feed distance per 360#{176} revolution equal to the section thickness when the new algorithms were applied; noise values, however, increased. SSP width increased for table feed distances greater than the section width, the effect being less pronounced with the new algorithms. The value of these algorithms is primanly seen in the improved quality of multiplanar reformations and cine and three-dimensional displays.

Index terms: Computed tomography (CT), image processing #{149} Computed tomography (CT), image quality #{149} Computed tomography (CT), physics ‘ Computed tomography (CT), spiral technology

Sensitivity in Spiral CT’

S

PIRAL

was lice

computed introduced

in 1989

(1-5)

Profiles

tomography into clinical and

has

since

section

may

cause

omission

of struc-

tures or lesions and degradation the quality of multiplanar and dimensional It has been

displays. demonstrated

section

CT with

respect

185:29-35

in three-

that

image quality of spiral scanning equivalent to that of standard to most

the

is singlepa-

rameters; only image noise and the shape of the section sensitivity profiles (SSPs) are changed (5,23-25). The magnitude and direction of these effects depend on the scanning parameters employed and the reconstruction algorithms used. The shape of the resulting SSPs is a particular point of concern. We investigated different sectioninterpolation algorithms by means of computer simulations, phantom ex-

periments, termine and the

and

clinical

studies

their effects on SSPs, quality of multiplanar

termine whether pitch of greater 1992;

been

investigated and applied in a large number of studies (6-22). Spiral or continuous-volume scanning involves moving the patient through the gantry while CT data are simultaneously acquired with multiple 360#{176} acquisilions (5). This scanning mode is of particular clinical interest in contrast media studies, in which complete organs or a large volume need to be measured during the short time of maximum enhancement, or in examinations of organs subject to motion, in which misregistration from section to

mations and three-dimensional plays. In particular, we sought

Radiology

(CT) prac-

acceptable

image

to denoise, refordis-

to de-

spiral CT with a than 1 can produce

quality.

(Pitch

is de-

1 From Siemens Medical Systems, Henkestrasse 127, 8520 Erlangen, Germany (A.P., W.A.K.); and the Department of Radiology, University Hospitals, Louvain, Belgium (G.M.). From the 1991 RSNA scientific assembly. Received February 27, 1992; revision requested April 22; revision received May 18; accepted May 28. Address reprint requests to W.A.K.

C RSNA,

1992

fined

as the

table

360#{176} rotation section

feed

divided

distance

per

by the

nominal

thickness.)

AND

MATERIALS

METHODS

Section-Interpolation

Algorithms

Direct reconstruction of images from data obtained over any 360#{176} segment of a spiral CT acquisition will result in motion artifacts due to patient transport. To avoid such been must

artifacts, the data that would have obtained in the planar geometry be synthesized from the spiral data

set. This has to be done point by point for a complete data set (ie, for all projections over the 360#{176} angular range of rotation and for all data within each projection) (5). The new data set, which can be calculated for any arbitrary table position z within

the scanned volume, is subsequently subjected to the same image reconstruction process as any data set obtained with conventional single-section scanning. There are a number of alternatives for synthesizing “planar” data sets from spiral data. In the simplest case, this is done by means of linear interpolation (LI) between the two neighboring data points obtained at the same angle of rotation, z’ and z’ + d, with

d denoting

the table

feed

distance

per 360#{176} rotation (Fig la). Thereby, data from a range corresponding to 2 x 360#{176} are used, with z’ varied from z d to z. -

We refer LI.

to this

type

of processing

as 360#{176}

To limit the SSP broadening that results from using this large range, we exploit data for interpolation that were obtained with 180#{176} opposite views. This is equivalent to calculating a second spiral from the original that is offset by a distance d/2, as depicted in Figure lb. Thereby, data are available for interpolation that were obtained a total

closer range

being

used.

to the desired z axis position, of only 2 x (180#{176}+ fan angle)

We use the term

180#{176} LI when

Abbreviations: FWHM = full width at half maximum, FWTA = full width at tenth area, FWTM = full width at tenth maximum, HI =

higher-order lation,

SSP

interpolation, =

section

sensitivity

LI

=

linear

interpo-

proffle.

29

a.

b.

Figure

1.

would views

have been obtained

Data processing one to limit

allows

points

U between

in spiral in planar

CT. (a) Linear

geometry. (b) Interpolation the scan range used per image.

z” and z’

+ d

is em-

Rel. 1.2

ployed. A better

estimate

be obtained terpolation two

data

of the true

In our

z’ and z’ + d is most commonly data points and data points

between

points

between

measured

used to estimate data that derived from 180#{176} opposite

value I

I

may

value

by means of higher-order in(HI) with use of more than points.

interpolation

1.1

implementation,

we employed a modified cubic spline interpolation between points z z”, z’ + d, and z” + d (Fig lb). We use the term 180#{176} HI for this procedure.

1.0

-

0.9

-

0.8

-

0.7

Noise

Analysis

0.6

It has been shown that noise is reduced by a factor of V’2/3 in spiral CT with 360#{176} U processing when compared with the in standard single-section CT with the same nominal section thickness and radiation dose (5,24,25). In 180#{176} LI, noise

to increase

the central

noise

by a factor

ray in comparison

in 360#{176} LI, as is derived

is

of V2 for with

the

in the Ap-

pendix. Compared with the noise in standard CT at the same section thickness and

dose,

this means

or 1.15.

a noise

The increase

increase

of \/4/3

is less for off-axis region of interest 4 cm in

rays; for a central diameter, the average noise increase is expected to be 13%. For 180#{176} HI, no general prediction can be made, since the algorithm will adapt to the object. Simulation

of SSPs

It has been shown that SSPs in spiral CT with 360#{176} LI result as a convolution of the original SSP with the table motion function, which is given by a triangle of 2d base width and unity area; results of simulations and measurements were found to be in excellent agreement (25). The same agreement was found for 180#{176} LI with this simulation method; the triangular table motion function at the center of rotation has a base width of only d and unity area. For 180#{176} HI, a different approach had to be created.

We synthesized

CT projection

data of an infinitely

thin disk scanned

spiral

reconstructed

geometry

and

as a function of table position, directly yielding the SSP. This method was also applied to the other section-interpolation 30

Radiology

#{149}

in

images

SSP

area.

0.4

noise

expected

of 0.5

0.3

FWTA

0.2

I

0.1

FWTM I

0.0 -123

-10.0

-5.0

-7.5

-2.5

I

II

0.0

2.5

5.0

7.5

Figure

2.

algorithms, with those method. Figures

Width

parameters

[mm]

for SSPs. (See text for explanation.)

providing results consistent of the earlier convolution

of Merit

for

SSPs

The full width at half maximum (FWHM) is the most widely used descriptor of SSPs; this value is typically used to describe the nominal section width. The FWHM does not give any indication of whether the profile approaches the ideal form of a rectangle or deviates from this form substantially. In standard CT, section proffles approximate the ideal rectangular form well, in particular when postpatient collimators are used. In spiral CT, the scan is obtained

with

tion width; owing tive section width, broadened proffle, proffle is degraded

a selected

12.5

10.0

Z-positlon

nominal

sec-

to the motion, an effecthe FWHM of the will result. Even if the substantially, this will

not necessarily result in a large increase in the FWHM of the profile (5,25). We therefore employed additional descriptors, as illustrated in Figure 2. We determined the full width at one-tenth of the maximum (FWTM) and the full width at the points of the profiles where 90% of the area

is covered

and

10% of the area

is ex-

duded (full width at tenth area [FWTAJ). We consider FWTA the most adequate measure

of SSP quality.

Measurements All measurements Somatom Medical

Plus-S Systems,

were CT scanner Erlangen,

obtained

with

a

(Siemens Germany),

which acquires a full 360#{176} data set in 1 second and offers a spiral CT option with 32 seconds of continuous scanning. Section widths of 1-10 mm can be chosen; table feed speeds of 1-10 mm/sec are available. October

1992

Rd.

Rd.

value

I.

I

-

Processing LO

,.‘I

algorithm

none

-

value

I

(d-0)

-

360L1

\#{149}\

:/

:..:.:

0.8

Table 0.6

0.4

feed

L

d=5rnm/’

I

I

-

-

0.2

0.c

.12.5

.10.0

.7#{149}5 -5.0

.2.5

0.0

2.5

5.0

7.5 10.0 1 -5 Z-positlon [mm]

b.

a. rd.

rd.

FWHM

FWTM

5.

....G... -#{149}-

-.e.-

.

360

LX

180 180

LI HI

0.

1.

2.

0.

3.

1.

Pitch

3.

2. Pitch

d. rd.

FWTA

I Figure

5.

3.

SSP5

in spiral

CT for

different

section-interpolation

algo-

rithms at 5-mm nominal section thickness. (a, b) Profile plots for table feed speeds of 5 mm/sec (a) and 10 mm/sec (b). (c-c) Plots of FWHM FWTM (d), and FWTA (e), relative to the value for the original profile (d = 0), as a function of pitch.

4.

(c),

3.

Table 1 Image Noise

2.

in Spiral

Algorithm

Predicted

360#{176} LI

180#{176}LI 180#{176} HI

CT

0.82 1.13 Not available

Measured 0.83 1.12 1.29

0. 0.

1.

2.

3.

e.

We employed an experimental software package that allowed table feed speeds up to 20 mm/sec

and

and 180#{176} HI algorithms. Noise was measured

included

the

in a water

of

180#{176} LI

phan-

torn 20 cm in diameter. To evaluate spatial resolution in the longitudinal direction or

Volume

185

Number

#{149}

1

Note-Values given are relative standard CT at the same nominal thickness and dose. in

Pitch

z axis, we scanned spheres in a water phantom. We employed acrylic spheres

and 10 mm in diameter

and a wooden

sphere 10 mm in diameter. SSPs were measured with a phantom containing a 0.5-mm-thick disk oriented exactly perpendicular to the z axis. Phantom mea-

4

to the

noise

section

surements were obtained with both standard CT and spiral scanning for direct comparison.

Raw data from spiral CT acquisitions were available for a large number of cmical studies, obtained mA over 32 seconds

at 137 kVp and 145 with varying section

Radiology

#{149} 31

and table feed values.

thickness

We arbi-

trarily selected 20 studies of the abdomen and processed them with the three different algorithms for direct comparison.

RESULTS Noise

was

measured

for

standard

CT and spiral CT with 120 kVp, 165 mA, 5-mm nominal section thickness, and 5-mm/sec table feed speed. Re-

sults

are given

in Table

1; they

were

in excellent agreement with the predictions in the “Noise Analysis” seclion. Simulated SSPs for the different data processing algorithms are plotted in Figure 3a and 3b. It is apparent that SSPs are broadened in spiral CT for all algorithms. However, this effect is much

reduced

with

the

180#{176} algo-

rithms. In particular, even the profiles for 180#{176} HI at a pitch of 2 (ie, a table feed distance per 360#{176} revolution equal to twice thickness) are

those

the still

nominal narrower

confirmed parameters the width Figure

lower

of 1. This is

3c-3e.

These

results

apply

water confirmed

given

above

(Fig

the

re-

(Table

noise

FWHM FWTA FWHM FWfM FWTA

spheres

are

more

While

the

and

shape

pronounced

dard section-by-section however.

addi-

of the in stan-

CT scanning, in noise

levels

is apparent in the transaxial CT images, the effects of the SSPs on image quality were relatively hard to assess in clinical studies (Fig 5a-5c). Since SSP5

determine

spatial

resolution

The 180#{176} algorithms

proved resolution substantially these displays. These findings consistently Results

ues 32

obtained obtained

greater Radiology

#{149}

than

the

for Standard

CT

()* (.)*

(mm)t (pj)t

(j)t

case

were

quality.

is highest

for

180#{176} HI pro-

CT and Spiral

CT at 5-mm

CT

=

5 mm/sec.

=

10 mm/sec.

360#{176} LI

180#{176} LI

180#{176} HI

5.0

6.3

5.0

5.0

6.1

11.1

8.0

7.4

5.0

8.3

5.9

5.4

5.0

10.8

6.5

6.4

6.1

19.8

11.3

10.3

5.0

14.1

8.4

7.7

for

imin were

for all studies. at table feed val-

nominal

section

limited is particularly

scans

5-mm

lion and

in imthe

slightly

This

obtained

tion thicknesses. in Figure 6 were

nominal

with

large

occurs particularly in lung nodule screening (4,6,9,11,15) or in examination of patients who are not able to cooperate. Some studies show deci-

sec-

The images shown obtained with a

section

table

feed

acquisition;

thickness speed

over

effective

sive trast

and a

180#{176} HI,

respectively

These differences can ated in the multiplanar (Fig 6d-6i).

(Table

advantages medium

for spiral applications.

CT in conDupuy et

al (13) and Costello et al (18) reported improved image quality for spiral CT

sec-

thicknesses (FWHM) of 10.8, 6.5, 6.4 mm resulted for 360#{176} LI, 180#{176} 2).

best be apprecireformations

even

when

using

amounts

been

demonstrated

Simulations

and

Spiral CT is a promising CT technique. Its particular advantages result

ments yield consistent results. In transaxial CT reconstructions,

from terest

the fact that are scanned

the volumes continuously,

of inpro-

SSP degradation ticed in most

viding

a seamless

or overlapping

set

of images in every case. of particular importance anatomic

or involuntary

regions

subject

patient

This feature in scanning to voluntary

motion.

This

feed

is

of

CT on SSPs is well understood

(25) and has detail above.

DISCUSSION

reduced

a contrast medium bolus. With respect to image quality, the only concerns brought forward so far refer to the shape of SSPs (5,23-25). The influence of patient transport in

spiral

in

the longitudinal direction, the effect of data processing can best be demonstrated in multiplanar reformations

(Fig 5d-5i).

age

LI, and

difference

and

1). Multipla-

elongation

in the

Descriptors)

(SSP

Table feed speed t Table feed speed

10-mm/sec 32-second

this

1800 algorithms

Thickness

*

in spiral

CT;

Results Section

Descriptor

thickness

distortion

by the

Spiral

nar displays were generated for standard CT images obtained at 5-mm increments and spiral CT images calculated in 1-mm increments (Fig 4b). It is evident that the spheres are displayed elongated to different degrees tional

HI).

Standard

to

phantom

right = 180#{176} is increased

comparison of standard and spiral CT images. left = standard CT, upper right = 360#{176} LI, Noise is lowest for 360#{176} LI processing resolution

2

Simulation Nominal

thicknesses provided original profile is the same. There was good agreement between simulations and measurements; on average, differences between measured and calculated width parameters were about 0.1 mm. Spiral scans of the spheres in the

20-cm-diameter

(b) coronal

= 180#{176} LI, lower longitudinal direction

section of the

4a) visually

CT images of spheres; reformations (upper

Multiplanar

left

in the cessing.

quantitatively by the width given in Table 2. Plots of parameters are given in

arbitrary the shape

sults

4.

(a) CT images,

Table

section than

for 360#{176} LI at a pitch

Figure

the

distance

nominal

ticular details

structures

cases

can hardly be nowhen the table

is less

section

in experi-

than

or equal

thickness,

to

in par-

for thin sections. Attention to like small calcifications or bone

reveals

this

influence, October

how1992

t

r.

:.

--

3-

:

I

‘

t

,

,I

-

k

I

.

JiL d.

-

JL

g

‘

#{216}r, e.

h.___

.?t

V

,.

, I.

Figure 5. Spiral CT studies (32-second acquisition, 5-mm section thickness, 5-mm/sec table feed speed) of upper abdomen. (a-c) CT images at level of upper pole of the left kidney. (d-f) Coronal reformations through the spine. (g-i) Sagittal reformations through the left kidney. (a, d, g = 360#{176} LI; b, e, h = 180#{176}LI; c, f, i = 180#{176}HI.) The continuous acquisition in spiral CT provides excellent-quality multiplanar reformations. Note the absence of contour irregularities, which are commonly seen when reformations are obtained from consecutive breath-hold CT scans acquired at 5-mm section thickness. Anatomic detail is best displayed in reformatted images obtained with 180#{176} HI (note clear delineation of the diaphragm).

ever. The effect may become pronounced when thick sections are used in combination with high table feed values. Particular advantages of spiral scanfling and of the different data processing algorithms become apparent in multiplanar reformations. It is understood

that

spiral

scanning

elongate structures nal direction. This c

#{149} Number

tends

to

in the longitudieffect can be re1

duced substantially with the new algorithms, especially with 180#{176} HI. However, it is more important to realize that the same detrimental effect occurs in standard CT (Fig 4b). In standard CT, the choice of thickness and spacing of sections is decisive. Resolution in the longitudinal direction is much inferior to resolution in the x, y, or image plane in any case. Spiral

CT offers

considerable

im-

provement

in this

us to reconstruct table positions

ing. This provides tours

in multiplanar

Steps

commonly

respect,

images at arbitrarily

as it allows for

arbitrary fine spac-

smoothness

of con-

reformations.

done

in contiguous

section-by-section scanning, in particular for thick sections, are strongly reduced. Discontinuities due to incon-

sistent

breathing

are practically

levels eliminated.

between

scans

In addition, Radiology

#{149} 33

-.-..v

-.

L.

\

\

‘

‘

-

#{149}t:

.

1_ a.

,

b.

Figure

6.

quisition,

Spiral 5-mm

CT studies section

(32-second

thickness,

10-mm/sec

table feed speed) of patient with carcinoma of the right kidney. (a-c) Bolus-enhanced images show the hypervascular and partially

necrotic

carcinoma

C-

ac-

in the

CT

posterior

I

as-

pect of the upper pole of the right kidney. Note the differences in blurring between the lower edge of the liver, the anterior surface of the right kidney, the posterior surface of the left kidney, and the aorta. Also note the improved image contrast for the 180#{176} algonthms (arrows in c). (d-f) Coronal reformations. (g-i) Sagittal reformations. (a, d, g = 360#{176} LI; b, e, h

spiral lively fully

=

c, f, i

180#{176}LI;

=

180#{176}HI.)

scanning allows us to retrospecselect the table position that yields the anatomic or patho-

e.

h.

logic detail of interest in the section. This is particularly effective in interaclive cine displays (12,16,19). Efforts to scan the largest possible volume with spiral scanning in a given lime (ie, by choosing table feed values greater than the nominal seclion thickness) have to be seen and weighed

in the

context

of 5SF

degra-

dation (Fig 3). The selection of parameters will depend on the clinical demand and the diagnostic task. However, even for a pitch of greater than 1, scanning is continuous and in no case do gaps result or will lesions be overlooked. For lesions smaller than the section thickness, a reduclion in contrast may be observed, owing to the partial volume effect. Lung scanning-with the inherent high contrast of most lesions-will hardly be limited by this effect and therefore may become a primary field of application. In addition to producing increased scan volumes, pitch values of greater

than

1 bring

about

in mean organ dose, posure is distributed ume. Scanning

than 1 offers but it requires

with

a reduction

as the given over a larger

exvol-

1.

choices of section thickness, table feed value, and section interpolation algorithm still need to be determined for the various clinical applications.

P2(O)

=

O/’rrP+i(O

of greater

potential advantages, 180#{176} data processing to keep

adequate

This demands additional tional efforts and causes creased noise. The most

34

#{149} Radiology

-

.P1(0)for0

a pitch

SSPs.

computaslightly inappropriate

of image

ir)

+ (1

-

P(0)

+

(2

-

-

TI)

0/u)P1#{247}1(0)]dO

+

uT

[(2

+

(0/ui

-

-

0/ur)P+i(0

1)P1+1(0

We can rearrange

-

rr)

-

2ui)]dO.

the above

(A3)

by chang-

in the second

inte-

C [0,u-r)

0/ui)P1+1(0

P1+i(O

(1

-

2r

‘180LI =

(A2)

in

ing integration limits gral, which leads to

0/u)

P(0)d0,

J0 [(O/’rr)P+1(O

I180’LI

pixel

or

.

algorithms

derivation

noise for the 180#{176} LI algorithm, let us concentrate on the center of rotation and, correspondingly, on data measured with the central detector. For each section position z and angular position 0, the synthesized projection value P2(0) is calculated as

f2i

‘180’LI

resulting

APPENDIX To simplify

value (5,25). To get the image pixel value, we have to integrate over all projections:

-

-

ri)

+ (0/u

-

1)

2rr) for 0 C [rr, 2u),

where j is the number of the last scan reaching angle 0 before reaching position z and P(0) is a corresponding projection

f +

2[(O/’rr)P3+1(O

(1

-

-

ur)

0/ur)P1+1(0)]dO.

(A4)

(Al) Denoting variance of projection data as o.2(0) and taking into account statistical independence of the corresponding cornponents, we can express the variance of

October

1992

image

pixel noise

f

80’LI +

We can &(0)

3.

by

(I

-

4.

0/u)2o(8)]d0.

assume

further

(A5)

that a(0

ur)

-

(ie, the noise level is the same for all projections). This allows us to write cTso.u

4

=

J2

f

[(0/rr)2

+ (I

(A6)

Evaluation

of the integral

(A7) With

use of Equation

(A2)

Thus,

(ie,

8.

=

2w

4

(A8)

(A9

‘

that for image

References Kalender WA, Seissler W, Vock P. Singlebreath-hold spiral volumetric CT by continuous patient translation and scanner rotalion (abstr). Radiology 1989; 173(P):414. Vock

P,Jung

breathhold iary system

11.

H, Kalender

WA.

Single-

12.

13.

14.

15.

185

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M, Kalender volumetric CT with sin-

technique.

Radiology

16.

1990;

Kalender WA, Seissler W, Klotz E, Vock Spiral volumetric CT with single-breath-

P. 17.

hold technique, continuous transport, and continuous scanner rotation. Radiology 1990; 176:181-183. Costello P, Anderson W, Blume D. Spiral volumetric CT in the search for a pulmonary nodule (abstr). Radiology 1990; 177(P):295. Zimmerman PA, Gusnard DA. Craniocervical spiral CT (abstr). Radiology 1990; 177(P):12l. Bautz W, Skalej M, Kalender WA. Diagnosis of pancreatic disease: clinical impact of spiral CT (abstr). Radiology 1990; 177(P):

18.

19.

20.

1

Midorikawa Suzuki K

S, Hashimoto N, Katakura T, New approach to lung cancer with helical volume CT (abstr). Radiology 1990; 177(P):174. Kaneko M, Eguchi K, Ono R, et al Threedisplay

of bronchial

images

21.

by

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2.

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for 180#{176} LI is

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1.

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Thus, the standard deviation of the pixel noise for 180#{176} LI interpolation in the

with

7.

results

u=lcrcr.

pared

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154.

f21 a(O)d0

the variance

6.

for recon-

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5.

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CT of hilar

176:864-867.

0/ir)2]dO.

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=

cr

=

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#{149} 35