JP4089387B2

JP4089387B2 - X-ray CT system

Info

Publication number: JP4089387B2
Application number: JP2002317844A
Authority: JP
Inventors: 達今西
Original assignee: Shimadzu Corp
Current assignee: Shimadzu Corp
Priority date: 2002-10-31
Filing date: 2002-10-31
Publication date: 2008-05-28
Anticipated expiration: 2022-10-31
Also published as: JP2004147945A

Description

【０００１】
【発明の属する技術分野】
この発明は、Ｘ線ＣＴ撮影実行中は予め算出保持された設定管電流にしたがってＸ線ファンビームを被検体の体軸の方向にスキャン（走査）させるＸ線ＣＴ装置に係り、特に被検体が体軸の方向のズレを起こして生じるＸ線管の設定管電流の狂いを確実かつ容易に修正できるようにするための技術に関する。
【０００２】
【従来の技術】
図１２は従来のＸ線ＣＴ装置のＸ線撮影台の概略構成を示す図である。図１２のＸ線ＣＴ装置の場合、Ｘ線ＣＴ撮影に先立ってＸ線ＣＴ撮影計画用の２次元Ｘ線透過画像を撮影する。被検体（患者）Ｍを挟んでＸ線管５１とＸ線検出器５２とが対向して静止した状態のままで、天板５３を被検体Ｍの体軸Ｚの方向へ移動させてＸ線管５１とＸ線検出器５２の間に被検体Ｍを送り込みながら、Ｘ線管５１からＸ線ファンビームＦＢを被検体Ｍへ一定の管電流で連続照射すると同時に、Ｘ線検出器５２から出力されるＸ線透過信号を収集する。収集されたＸ線透過信号に対して必要な画像処理が行われ、図１３に示すように、Ｘ線ＣＴ撮影対象の関心部位を含む２次元Ｘ線透過画像Ｐａが作成されてＸ線ＣＴ撮影計画用の画像として表示モニタ５４に映し出される。オペレータは、表示された２次元Ｘ線透過画像Ｐａを見ながら、撮影計画に必要な操作をおこなう。
【０００３】
また、従来のＸ線ＣＴ装置の場合、Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像Ｐａの撮影実行に伴ってＸ線検出器５２から収集されるＸ線透過信号に基づき被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に追随して変化しながら透過Ｘ線の平均線量を略一定の適切量に保てるＸ線管５１の設定電流が予め算出されて被検体Ｍの体軸Ｚ上の位置と対応付けられたかたちで保持される。そして、Ｘ線ＣＴ撮影の実行中は、予め算出保持された設定管電流に従ってＸ線ファンビームＦＢが被検体Ｍを体軸Ｚの方向にスキャンしてゆく。
【０００４】
被検体ＭのＸ線透過率の場合、臓器存在の有無や存在臓器の種類など被検体Ｍの体内状況に応じて被検体Ｍの体軸Ｚの方向に沿ってＸ線透過率の平均レベルが変動するので、Ｘ線ＣＴ撮影の実行中、Ｘ線管５１の管電流が一定のままだと、Ｘ線ＣＴ画像の間で、被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に応じた画像の全体的明るさの相違が生じる等の不都合があり、この不都合を解消する為にＸ線管５１の管電流は被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に追随して変化させる必要がある。またＸ線検出器５２に入射する透過Ｘ線は、Ｘ線ＣＴ画像の画質を保持できるだけの線量があって、同時に被検体ＭのＸ線被曝量を極力抑えられる線量である必要がある。これらの必要性のためにＸ線管５１の設定管電流を透過Ｘ線の平均線量を略一定の適切量に保てるという条件の下に算出する。
【０００５】
【発明が解決しようとする課題】
しかしながら、従来のＸ線ＣＴ装置の場合、予め算出保持したＸ線管５１の設定管電流が時として狂うという問題がある。Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像の撮影以降に被検体Ｍが体軸Ｚの方向にズレを起こすと、Ｘ線管５１の設定電流と被検体Ｍの体軸Ｚ上の位置との対応関係にもズレが起こり、Ｘ線管５１の設定管電流が狂う。例えば、撮影中の静止の必要性が理解できない幼児や痴呆性老人などの場合はズレを起こし易い。最悪、設定管電流の狂いを修正するにはＸ線ＣＴ撮影計画用の画像を撮り直さなければならない。オペレータが被検体Ｍのズレを見過ごしたり、被検体ＭのズレがＸ線ＣＴ撮影開始後に起こったりした時は、設定管電流が狂ったままで撮影をすることになる。
【０００６】
この発明は、上記の事情に鑑み、被検体に体軸の方向のズレが起きることで生じるＸ線管の設定管電流の狂いを確実かつ容易に修正することができるＸ線ＣＴ装置を提供することを目的とする。
【０００７】
【課題を解決するための手段】
この発明は、このような目的を達成するために、次のような構成をとる。即ち、請求項１に記載のＸ線ＣＴ装置は、被検体の体軸の方向に沿って生じるＸ線透過率の平均レベルの変動に追随して変化しながら透過Ｘ線の平均線量を略一定の適切量に保てるようにＸ線管の設定管電流を算出し被検体の体軸上の位置と対応付けしたかたちで保持すると共に、Ｘ線ＣＴ撮影の実行中は予め算出保持された設定管電流にしたがってＸ線ファンビームを被検体の体軸の方向にスキャン（走査）させるように構成されたＸ線ＣＴ装置において、（Ａ）Ｘ線ＣＴ撮影実行時のＸ線ファンビームのスキャンに伴ってＸ線検出器から収集されるＸ線透過信号に基づき被検体の体軸の方向に沿って透過Ｘ線の平均線量を逐次求出する平均線量逐次求出手段と、（Ｂ）逐次求出される各平均線量が略一定の適切量であるか否かを判定して否が途切れず続くスキャン区間（線量非一定スキャン区間）を求出する線量非一定区間求出手段と、（Ｃ）平均線量が略一定の適切量でなくなる前後で設定管電流の変化が有るか無いかを判定する設定管電流変化判定手段と、（Ｄ）線量非一定区間求出手段で線量非一定スキャン区間が求出されると線量非一定スキャン区間の長さと設定管電流変化判定手段による設定管電流の変化の有無とにしたがって設定管電流と被検体の体軸上の位置との対応関係を即時シフトさせる対応関係シフト手段とを備えていることを特徴とするものである。
【０００８】
（作用・効果）請求項１に記載の発明では、Ｘ線ＣＴ撮影の実行中、予め算出保持された設定管電流にしたがってＸ線ファンビームを被検体の体軸の方向にスキャン（走査）させてゆくのと同時並行して、平均線量逐次求出手段が、Ｘ線ファンビームのスキャンに伴ってＸ線検出器から収集されるＸ線透過信号に基づき被検体の体軸の方向に沿って透過Ｘ線の平均線量を逐次求出すると共に、線量非一定区間求出手段が、逐次求出される平均線量が略一定の適切量であるか否かを判定して線量非一定スキャン区間を求出するのに加え、設定管電流変化判定手段が、平均線量が略一定の適切量でなくなる前後で設定管電流の変化の有りか無しを判定する。そして、線量非一定スキャン区間が求出されると、対応関係シフト手段が線量非一定スキャン区間の長さと設定管電流の変化の有無とにしたがって設定管電流と被検体の体軸上の位置との間の対応関係を即時シフトさせる。
【０００９】
請求項１に記載の発明の場合、Ｘ線管の設定管電流は被検体の体軸上の位置と対応付けしたかたちで保持されているので、被検体に体軸の方向のズレが起こると、設定管電流と被検体の体軸上の位置との間の対応関係にもズレが生じてＸ線管の設定管電流が狂う。一方、被検体の体軸の方向のＸ線透過率の平均レベルの変動は、平均レベルが略同一で相当長のスキャン区間にわたって連続して続く状態が、略同一の平均レベルが別のレベルに変わる他は同様にして繰り返されるというＸ線透過率の平均レベルがステップ型変化をするタイプの変動であるので、設定管電流も同様にステップ型変化となる。そのため、被検体に体軸の方向のズレが起きても、全スキャン区間にわたって全ての設定管電流が狂うのではなく、設定管電流の変わり目の前または後の被検体の体軸の方向のズレの距離に応じたスキャン区間でのみ設定管電流が狂うだけとなる。
他方、Ｘ線管の設定管電流は透過Ｘ線の平均線量が常に略一定の適切量に保たれるように算出されているので、Ｘ線管の設定管電流が狂っているスキャン区間は逐次求出される平均線量が略一定の適切量ではない状態が途切れずに続く線量非一定スキャン区間に対応する。そして、Ｘ線管の設定管電流が狂っているスキャン区間は、被検体に体軸の方向のズレの距離に対応しているので、線量非一定スキャン区間は、被検体に起こった体軸の方向のズレの距離と対応する。
【００１０】
また、被検体に起こる体軸の方向のズレの向きがＸ線ファンビームのスキャン方向と同一の場合は、設定管電流の狂いが設定管電流の変わり目から始まるので、平均線量が略一定の適切量でなくなる前後で設定管電流が変化する。逆に被検体に起こる体軸の方向のズレの向きがＸ線ファンビームのスキャンの方向と反対の場合は、設定管電流の狂いが設定管電流の変わり目の手前から始まるので、平均線量が略一定の適切量でなくなる前後で設定管電流が変化しない。したがって、設定管電流変化判定手段の判定による設定管電流の変化の有無（以下、適宜「設定管電流変化の有無」と略記）は被検体に起こった体軸の方向のズレの向きと対応する。
したがって、線量非一定スキャン区間が求出された場合、設定管電流と被検体の体軸上の位置との間の対応関係を線量非一定スキャン区間の長さと設定管電流変化の有無とにしたがって即時シフトすれば、被検体の体軸の方向のズレで生じた設定管電流と被検体の体軸上の位置との間の対応関係のズレが速やかに解消され、設定管電流は直ちに正常状態に復帰する。つまり、設定管電流を狂わせる被検体のズレが自動的に検出され、設定管電流の狂いが自動的に修正されるのである。
【００１１】
さらに、この発明は、上記目的を達成するために、次のような構成をとる。即ち、請求項２に記載のＸ線ＣＴ装置は、被検体の体軸の方向に沿って生じるＸ線透過率の平均レベルの変動に追随して変化しながら透過Ｘ線の平均線量を略一定の適切量に保てるようにＸ線管の設定管電流を算出し被検体の体軸上の位置と対応付けしたかたちで保持すると共に、Ｘ線ＣＴ撮影の実行中は予め算出保持された設定管電流にしたがってＸ線ファンビームを被検体の体軸の方向にスキャン（走査）させるように構成されたＸ線ＣＴ装置において、（ａ）Ｘ線ＣＴ撮影実行時のＸ線ファンビームのスキャンに伴ってＸ線検出器から収集されるＸ線透過信号に基づき被検体の体軸の方向に沿って透過Ｘ線の平均線量を逐次求出する平均線量逐次求出手段と、（ｂ）逐次求出される各平均線量が略一定の適切量であるか否かを判定して否が途切れず続くスキャン区間（線量非一定スキャン区間）を求出する線量非一定区間求出手段と、（ｃ）平均線量が略一定の適切量でなくなる前後で設定管電流の変化が有るか無いかを判定する設定管電流変化判定手段と、（ｄ）線量非一定区間求出手段で線量非一定スキャン区間が求出されると線量非一定スキャン区間の長さと設定管電流変化判定手段による設定管電流の変化の有無とにしたがって被検体の体軸の方向に対する被検体とＸ線管・Ｘ線検出器との位置関係を即時シフトさせる位置関係シフト手段を備えていることを特徴とするものである。
【００１２】
（作用・効果）請求項２に記載の発明の場合、線量非一定区間求出手段が線量非一定スキャン区間を求出すると、位置関係シフト手段が線量非一定スキャン区間の長さと設定管電流変化判定手段による設定管電流変化の有無とにしたがって被検体の体軸の方向に対する被検体ないしＸ線管およびＸ線検出器の位置関係を即時シフトさせる他は請求項１に記載の発明と全く同じである。そして、上述したように、線量非一定スキャン区間は、被検体に起こった体軸の方向のズレの距離と対応し、設定管電流変化の有無が被検体に起こった体軸の方向のズレの向きと対応しているので、Ｘ線管およびＸ線検出器に対する被検体の相対的な位置関係を被検体の体軸の方向に沿って線量非一定スキャン区間の長さと設定管電流の変化の有無とにしたがって即時シフトすれば、被検体の体軸の方向のズレで生じた被検体の体軸の方向に対する被検体ないしＸ線管およびＸ線検出器の位置関係のズレが速やかに解消され、設定管電流は直ちに正常状態に復帰する。つまり、設定管電流を狂わせる被検体のズレが自動的に検出され、設定管電流の狂いが自動的に修正されるのである。被検体をシフトさせる場合は被検体が元の正常な位置に戻ることで被検体をズレが直接解消され、Ｘ線管およびＸ線検出器をシフトさせる場合はＸ線管およびＸ線検出器が被検体のズレに合わせて移動することで被検体のズレが間接的に解消される。
【００１３】
また、請求項３の発明は、請求項１または２に記載のＸ線ＣＴ装置において、Ｘ線管の設定管電流が、Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像の撮影実行に伴ってＸ線検出器から収集されるＸ線透過信号に基づき算出されるように構成されているものである。
（作用・効果）請求項３に記載の発明によれば、Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像の撮影実行に伴ってＸ線検出器から収集されるＸ線透過信号に基づきＸ線管の設定管電流が算出されるので、Ｘ線管の設定管電流の算出にあたって被検体のＸ線透過率に関するデータを改めて収集する必要がない。
【００１４】
【発明の実施の形態】
続いて、この発明のＸ線ＣＴ装置の実施例を、図面を参照しながら詳しく説明する。
〔第１実施例〕
図１は第１実施例に係るＸ線ＣＴ装置の全体構成を示すブロック図である。
第１実施例のＸ線ＣＴ装置は、図１に示すように、Ｘ線ファンビームＦＢを被検体Ｍに照射するＸ線管１と、Ｘ線ファンビームＦＢの広がりに沿ってライン状に並んでいる多数のＸ線検出素子（図示省略）の列が被検体Ｍの体軸Ｚ方向に１列ないし数列配置されていると共にＸ線ファンビームＦＢの照射に伴ってＸ線透過信号（Ｘ線検出データ）を出力するＸ線検出器２とが被検体Ｍを挟んで対向するようにして配設されたガントリ３を備えると共に、被検体Ｍを載置したままでＸ線管１とＸ線検出器２との間を被検体Ｍの体軸Ｚの方向に往復移動させられてガントリ３を出入りする天板４とを備え、Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像の撮影と、２次元Ｘ線透過画像を用いて立てられたＸ線ＣＴ撮影計画に沿ってＸ線ＣＴ撮影とがおこなえるように構成されている。
【００１５】
２次元Ｘ線透過画像の撮影の場合、被検体Ｍを挟んでＸ線管１とＸ線検出器２が対向して静止した状態のままで、天板４を被検体Ｍの体軸Ｚの方向に移動させてＸ線管１とＸ線検出器２の間を被検体Ｍを移動させながら、Ｘ線管１からＸ線ファンビームＦＢを一定の管電流で連続照射すると同時に、Ｘ線検出器２から出力されるＸ線透過信号をデータ収集部（ＤＡＳ）５が収集する。
Ｘ線ＣＴ画像の撮影の場合、Ｘ線管１とＸ線検出器２を対向配置状態に維持したまま被検体Ｍの周りを回転させた状態で、天板４を被検体Ｍの体軸Ｚの方向に移動させてＸ線管１とＸ線検出器２の間へ被検体Ｍを送り込みながら、Ｘ線管１からＸ線ファンビームＦＢを連続照射すると同時に、Ｘ線検出器２から出力されるＸ線透過信号をデータ収集部５が収集する。したがって、第１実施例装置のＸ線ＣＴ画像の撮影は螺旋スキャン方式ということになる。
なお、Ｘ線管１およびＸ線検出器２の回転や天板４の移動はメカニカル制御部６によりコントロールされ、Ｘ線管１によるＸ線ファンビームＦＢの照射はＸ線曝射制御部７によりコントロールされる。
【００１６】
また、データ収集部５の後段には、２次元Ｘ線透過画像撮影でデータ収集部５が収集したＸ線透過信号を処理して２次元Ｘ線透過画像を作成したり、Ｘ線ＣＴ撮影でデータ収集部５が収集したＸ線透過信号を処理してＸ線ＣＴ画像を作成したりする画像再構成部８と、画像再構成部８で作成された２次元Ｘ線透過画像やＸ線ＣＴ画像を記憶する画像メモリ部９と、画像メモリ部９に記憶されている画像や入力操作用画面などを表示する表示モニタ１０と、撮影に必要な指令やデータなどの入力操作をおこなう為の操作卓１１とが設けられているのに加え、メカニカル制御部６、Ｘ線曝射制御部７、画像再構成部８、画像メモリ部９や表示モニタ１０が適切に作動するように操作卓１１による入力操作や撮影の進行状況に応じて指令を送出するホストコンピュータ１２が設けられている。
【００１７】
さらに、第１実施例のＸ線ＣＴ撮影は、２次元Ｘ線透過画像の撮影の実行に伴ってＸ線検出器２から収集されるＸ線透過信号に基づき被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に追随して変化しながら透過Ｘ線の平均線量を略一定の適切量に保てるようにＸ線管１の設定管電流を算出する設定管電流算出部１３と、設定管電流算出部１３で算出した設定管電流を被検体Ｍの体軸Ｚ上の位置と対応付けたかたちで保持する設定管電流保持部１４を備え、Ｘ線ＣＴ撮影の実行中は設定管電流算出部１３と設定管電流保持部１４により予め算出保持された設定管電流にしたがってＸ線ファンビームＦＢが被検体Ｍを体軸Ｚの方向に沿ってスキャン（走査）してゆくように構成されている。
【００１８】
被検体ＭのＸ線透過率の場合、臓器存在の有無や存在臓器の種類など被検体Ｍの体内状況に応じて被検体Ｍの体軸Ｚの方向に沿ってＸ線透過率の平均レベルが変動するので、Ｘ線ＣＴ撮影の実行中、Ｘ線管１の管電流が一定のままだと、Ｘ線ＣＴ画像の間で、被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に応じた画像の全体的明るさの相違が生じる等の不都合があり、この不都合を解消する為にＸ線管１の管電流を被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に追随して変化させる。また、Ｘ線検出器２に入射する透過Ｘ線は、Ｘ線ＣＴ画像の画質保持に必要な線量があって、かつ被検体ＭのＸ線被曝量を極力抑えられる線量である必要がある。これらの必要性を満たす為に設定管電流算出部１３が設定管電流を透過Ｘ線の平均線量を略一定の適切量に保てるという条件の下に算出する。
【００１９】
２次元Ｘ線透過画像の撮影によって、図２に示すように、Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像ＰＡが得られる場合、設定管電流算出部１３は、被検体Ｍの体軸Ｚに沿って被検体Ｍの体軸Ｚに垂直に次々に設定される被検体Ｍの各スライス断面ＱＡ〜ＱＺについて、それぞれに属するＸ線透過信号を加算・平均化して透過Ｘ線の平均線量ＴＡ〜ＴＺを求出する。具体的には、例えばＸ線検出器２においてＸ線ファンビームＦＢの広がりに沿ってライン状に並んでいる１列分のＸ線検出素子の全Ｘ線透過信号を被検体Ｍの各スライス断面ＱＡ〜ＱＺ毎に加算・平均化することで平均線量ＴＡ〜ＴＺが求まる。これらの各スライス断面ＱＡ〜ＱＺの間で起こる透過Ｘ線の平均線量Ｔの変化が、被検体Ｍの体軸Ｚの方向に沿って生じるＸ線透過率の平均レベルの変動に対応する。
【００２０】
さらに設定管電流算出部１３は、各平均線量ＴＡ〜ＴＺを同じ線量となる水の厚み（水換算体厚みｔ）に換算して各スライス断面ＱＡ〜ＱＺを想定した体軸Ｚ上の位置Ｚａ〜Ｚｚにおける被検体Ｍの水換算体厚みｔａ〜ｔｚを求出する。被検体Ｍの体軸Ｚを横軸にとり、被検体Ｍの水換算体厚みｔを縦軸にとって図示すると、図３に破線で示す如くになる。被検体Ｍの水換算体厚みｔは平均線量Ｔに逆比例の関係（水換算体厚みｔが薄いほど平均線量Ｔが増加する関係）にあるので、平均線量Ｔだけでなく水換算体厚みｔも、被検体Ｍの体軸Ｚ上の位置Ｚａ〜Ｚｚにおける被検体ＭのＸ線透過率の平均レベルのデータ（Ｘ線透過率の平均レベル情報）となる。
【００２１】
一方、Ｘ線管１の管電流と透過Ｘ線の線量は良好な正比例関係にあるので、一定値の透過Ｘ線の基準平均線量Ｔ0 、さらには基準水換算体厚みｔ0 の時の基準管電流Ｉ0 を予め定めておく。この基準平均線量Ｔ0 はＸ線ＣＴ画像の画質保持に必要な線量であって、かつ被検体ＭのＸ線被曝量を極力抑えられる適切量である。そして、設定管電流算出部１３は、水換算体厚みｔａ〜ｔｚ（または平均線量ＴＡ〜ＴＺ）の各スライス断面ＱＡ〜ＱＺについて基準平均線量Ｔ0 が得られる管電流Ｉを例えば設定管電流Ｉ＝Ｉ0 ×ｔ÷ｔ0 （又はこれと等価なＩ＝Ｉ0 ×Ｔ0 ÷Ｔ）等の式による演算で算出すると共に、求出した設定管電流Ｉａ〜Ｉｚを設定管電流保持部１４が被検体Ｍの体軸Ｚ上の位置Ｚａ〜Ｚｚと対応付けて保存する。なお、管電流Ｉの演算はＩ0 ×ｔ÷ｔ0 又はＩ0 ×Ｔ0 ÷Ｔの式によらずに適当な他の式を用いて行ってもよい。
【００２２】
この結果、図３に実線で示すように、設定管電流Ｉａ〜Ｉｚは、被検体Ｍの体軸Ｚの方向に沿って生じる被検体ＭのＸ線透過率の平均レベルの変動に追随して変化するものになる。同時にＸ線管１が各設定管電流Ｉａ〜Ｉｚで被検体ＭにＸ線ファンビームＦＢを照射した時の透過Ｘ線の各平均線量Ｕは、常に略基準平均線量Ｔ0 となるので、Ｘ線ＣＴ画像の画質保持に必要な線量であって、しかも被検体ＭのＸ線被曝量を極力抑えられる線量である。
【００２３】
さらに第１実施例のＸ線ＣＴ装置は、Ｘ線ＣＴ撮影実行時のＸ線ファンビームＦＢのスキャンに伴ってＸ線検出器２から収集されるＸ線透過信号に基づき被検体Ｍの体軸Ｚの方向に沿って透過Ｘ線の平均線量Ｕを逐次求出する平均線量逐次求出部１５と、平均線量逐次求出部１５で逐次求出される各平均線量Ｕが略基準平均線量Ｔ0 であるか否かを逐次判定して否が途切れず続くスキャン区間（線量非一定スキャン区間）を求出する線量非一定区間求出部１６と、平均線量Ｕが略一定の適切量でなくなる前後で設定管電流の変化が有るか無いかを判定する設定管電流変化判定部１７と、線量非一定区間求出部１６で線量非一定スキャン区間が求出されると線量非一定スキャン区間の長さと設定管電流変化の有無とにしたがって設定管電流Ｉと被検体Ｍの体軸Ｚ上の位置との対応関係を即時シフトさせる対応関係シフト部１８とを備えている。
【００２４】
平均線量逐次求出部１５は、例えばＸ線検出器２においてＸ線ファンビームＦＢの広がりに沿ってライン状に並んでいる１列分のＸ線検出素子が各スライス断面ＱＡ〜ＱＺのところで３６０°または１８０°回転する間に収集される全Ｘ線透過信号を逐次加算・平均化することで透過Ｘ線の平均線量Ｕを求出する。平均線量Ｕは、例えば各スライス断面ＱＡ〜ＱＺのところでＸ線検出器２がＸ線ＣＴ撮影計画用の２次元Ｘ線透過画像ＰＡの撮影を行った時の位置に来た瞬間に収集される全Ｘ線透過信号を逐次加算・平均化することで算出することもできる。
【００２５】
そして、被検体Ｍに体軸Ｚの方向のズレが無くて設定管電流Ｉが正常な間は、図４に示すように、平均線量Ｕは略基準平均線量Ｔ0 である。しかし、被検体Ｍに体軸Ｚの方向のズレが起こると、図５に示すように、被検体ＭのＸ線透過率の平均レベルの変動にもズレが起こり、設定管電流Ｉと被検体Ｍの体軸Ｚ上の位置との間の対応関係が崩れ、設定管電流Ｉの狂うところが出てくる。
【００２６】
一方、図３に示すように、被検体Ｍの体軸Ｚの方向のＸ線透過率の平均レベルの変動は、平均レベルが略同一で相当長のスキャン区間にわたって連続して続く状態が、略同一の平均レベルが別のレベルに変わる他は同様にして繰り返されるというＸ線透過率の平均レベルがステップ型変化をするタイプの変動であるので、同様に設定管電流Ｉもステップ型変化になる。そのため、被検体Ｍに体軸Ｚの方向のズレが起こっても、全スキャン区間にわたって全ての設定管電流が狂うわけではなく、被検体Ｍのズレの向きに応じて各設定管電流の変わり目の前または後の被検体Ｍのズレの距離に相当するスキャン区間のみの設定管電流が狂うだけである。被検体Ｍのズレの向きがＸ線ファンビームＦＢのスキャン方向と同じならば、図６に示すように、各設定管電流の変わり目から始まって被検体Ｍのズレの距離迄の間のスキャン区間ＳＴで設定管電流が狂うだけである。
【００２７】
他方、設定管電流が狂っているスキャン区間ＳＴは、図６に示すように、平均線量Ｕが略基準平均線量Ｔ0 でない状態が途切れずに続く線量非一定スキャン区間ＴＭに等しい。また設定管電流が狂っているスキャン区間ＳＴは、前述のように被検体Ｍに起こった体軸Ｚの方向のズレの距離ΔＺに対応しているので、線量非一定区間求出部１６で求出する線量非一定スキャン区間ＴＭは被検体Ｍに起こった体軸Ｚの方向のズレの距離ΔＺに対応している。
【００２８】
また、被検体Ｍに起こる体軸Ｚの方向のズレの向きが、Ｘ線ファンビームＦＢのスキャン方向と同じ場合は、図７に示すように、設定管電流Ｉの狂いが設定管電流の変わり目から始まるので、平均線量Ｕが略一定の適切量でなくなる前後で設定管電流が変化する。逆に被検体Ｍに起こる体軸Ｚの方向のズレの向きが、Ｘ線ファンビームＦＢのスキャン方向と反対の場合は、図８に示すように、設定管電流Ｉの狂いが設定管電流の変わり目の手前から始まるので、平均線量Ｕが略一定の適切量でなくなる前後で設定管電流は変化しない。このように、設定管電流変化判定部１７による設定管電流変化の有無は被検体Ｍに起こる体軸Ｚの方向のズレの向きに対応する。
【００２９】
したがって、対応関係シフト部１８が、設定管電流Ｉと被検体Ｍの体軸Ｚ上の位置との間の対応関係を、線量非一定スキャン区間ＴＭに見合う距離だけ、設定管電流の変化有無に見合う向きに即時シフトさせると、被検体Ｍの体軸Ｚの方向のズレで生じた設定管電流Ｉと被検体Ｍの体軸Ｚ上の位置との対応関係のズレは速やかに解消され、設定管電流Ｉの狂いは直ちに修正される。なお、いったん修正が完了した後、被検体Ｍが体軸Ｚの方向のズレをまた起こして設定管電流が再び狂った場合も、上記と同じプロセスで修正処理が繰り返される。
【００３０】
また、第１実施例の装置の場合、Ｘ線管１の設定管電流ＩはＸ線ＣＴ撮影計画用の２次元Ｘ線透過画像の撮影実行に伴ってＸ線検出器２から収集されるＸ線透過信号に基づき算出されるので、Ｘ線管１の設定管電流Ｉの算出に際して被検体ＭのＸ線透過率に関するデータを改めて収集しなくて済む利点がある。
【００３１】
次に、第１実施例のＸ線ＣＴ装置におけるＸ線管１の設定管電流Ｉの狂い修正の状況をより具体的に説明する。図９は設定管電流保持部１４による設定管電流の保持状況を示す模式図、図１０は第１実施例における設定管電流の狂い修正プロセスを示すフローチャートである。以下では、２次元Ｘ線透過画像の撮影が済んで撮影計画が立てられた段階から説明する。なお、２次元Ｘ線透過画像の撮影が済んでからＸ線ＣＴ撮影が始まる迄の間に被検体ＭがＸ線ファンビームＦＢのスキャン方向と同じ向きに距離ΔＺのズレを起こしたものとする。
【００３２】
〔ステップＳ１〕被検体Ｍの体軸Ｚに沿って被検体Ｍの体軸Ｚに垂直に次々に設定される被検体Ｍの各スライス断面ＱＡ〜ＱＺについて、設定管電流算出部１３が設定管電流Ｉａ〜Ｉｚを算出する。
【００３３】
〔ステップＳ２〕設定管電流保持部１４が、図９（ａ）に示すように、設定管電流Ｉａ〜Ｉｚを各スライス断面ＱＡ〜ＱＺの被検体Ｍの体軸Ｚ上の位置Ｚａ〜Ｚｚと対応付けて保持する。
【００３４】
〔ステップＳ３〕Ｘ線ＣＴ撮影の開始に伴って平均線量逐次求出部１５が透過Ｘ線の平均線量Ｕの逐次求出処理をスタートさせる。
【００３５】
〔ステップＳ４〕線量非一定区間求出部１６が線量非一定スキャン区間の求出処理をスタートさせ、設定管電流変化判定部１７が設定管電流変化の有無の判定処理をスタートさせる。
【００３６】
〔ステップＳ５〕線量非一定スキャン区間求出の有無をチェックし、線量非一定スキャン区間ＴＭが求出されなければ、ステップＳ７に飛ぶ。線量非一定スキャン区間ＴＭが求出されたら、次のステップＳ６へ進む。なお、被検体Ｍのズレの向きがＸ線ファンビームＦＢのスキャン方向であるので、設定管電流変化判定部１７で設定管電流変化有りと判定される。
【００３７】
〔ステップＳ６〕対応関係シフト部１８が、設定管電流Ｉと被検体Ｍの体軸Ｚ上の位置との対応関係を、線量非一定スキャン区間ＴＭに見合う距離だけ、Ｘ線ファンビームＦＢのスキャン方向の向きに即時シフトさせる。具体的には、図９（ｂ）に示すように、設定管電流保持部１４により被検体Ｍの体軸Ｚ上の各位置Ｚａ〜Ｚｚを被検体Ｍのズレの距離ΔＺだけ増加する処理が行われる。被検体Ｍの体軸Ｚの方向のズレで生じた設定管電流Ｉの狂いは自動的に修正される。
【００３８】
〔ステップＳ７〕Ｘ線ＣＴ撮影が継続中であれば、ステップＳ５へ戻り、以下のプロセスを続行する。Ｘ線ＣＴ撮影が完了ならば、Ｘ線管１の設定管電流の狂い修正処理も終了となる。
【００３９】
〔第２実施例〕
図１１は第２実施例に係るＸ線ＣＴ装置の全体構成を示すブロック図である。第２実施例のＸ線ＣＴ装置は、対応関係シフト部１８の代わりに、線量非一定区間求出部１６で線量非一定スキャン区間が求出されると線量非一定スキャン区間ＴＭの長さと設定管電流変化判定部１７による設定管電流の変化の有無とにしたがって被検体Ｍの体軸Ｚの方向に対する被検体Ｍの位置を即時シフトさせる位置関係シフト部１９を備えている他は、第１実施例の装置と実質的に同一であるので、相違する部分のみを説明し、共通する部分の説明は省略する。
【００４０】
即ち、第２実施例の装置では、第１実施例の場合と同様、Ｘ線ＣＴ撮影実行時に平均線量逐次求出部１５が透過Ｘ線の平均線量Ｕを逐次求出するのに伴い、線量非一定区間求出部１６が線量非一定スキャン区間ＴＭを求出すると共に、設定管電流変化判定部１７が設定管電流変化の有無を判定する。そして、第２実施例の装置の場合、線量非一定スキャン区間ＴＭが求出されると、位置関係シフト部１９が線量非一定スキャン区間ＴＭの長さと設定管電流変化の有無とにしたがって被検体Ｍの体軸Ｚの方向に対する被検体Ｍの位置を即時シフトさせる。
【００４１】
前述したように、線量非一定スキャン区間ＴＭは、被検体Ｍに起こった体軸Ｚの方向のズレの距離と対応し、設定管電流変化の有無が被検体Ｍに起こった体軸Ｚの方向のズレの向きと対応しているので、位置関係シフト部１９による即時シフトによって被検体Ｍが元の正しい位置へ速やかに戻り、被検体Ｍに起こった体軸Ｚの方向のズレによる設定管電流の狂いは直ちに修正される。
【００４２】
第２実施例の場合、具体的には、位置関係シフト部１９は、被検体Ｍに起こった体軸Ｚの方向へのズレとちょうど逆のズレが天板４に起きるように、線量非一定スキャン区間ＴＭと設定管電流変化の有無にしたがって天板４を移動させる指令をメカニカル制御部６に送出し、被検体Ｍのズレを天板４のズレで相殺するかたちで被検体Ｍを元の正しい位置へ戻す構成となっている。
なお、第２実施例の場合、設定管電流の狂い修正後、もう一度、Ｘ線ファンビームＦＢを最初からスキャンさせるように構成されている。
【００４３】
この発明は、上記実施の形態に限られることはなく、下記のように変形実施することができる。
（１）第１実施例の装置では、各スライス断面ＱＡ〜ＱＺを被検体Ｍの体軸Ｚに垂直に設定したが、各スライス断面ＱＡ〜ＱＺは被検体Ｍの体軸Ｚに対し斜めに設定してもよい。例えばガントリ３が傾斜（ティルティング）している場合、ガントリ３の傾斜角度に対応した分だけスライス断面ＱＡ〜ＱＺは被検体Ｍの体軸Ｚに対し斜めに設定されることになる。
【００４４】
（２）第２実施例の装置では、被検体Ｍのズレを天板４のズレで相殺するように天板４を移動させて設定管電流の狂いを修正する構成であったが、Ｘ線管１およびＸ線検出器２が被検体Ｍの体軸Ｚに沿って移動できる場合は被検体ＭのズレをＸ線管１およびＸ線検出器２のズレで相殺するようにＸ線管１およびＸ線検出器２を移動させて設定管電流の狂いを修正する構成としてもよい。
【００４５】
（３）上記の実施例の装置は、Ｘ線ＣＴ撮影の場合、被検体Ｍを連続移送しながらＸ線ファンビームＦＢを連続照射する螺旋スキャン方式であったが、この発明のＸ線ＣＴ装置は、Ｘ線ＣＴ撮影の場合、被検体Ｍを間歇移送しながらＸ線ファンビームＦＢを間歇照射する間歇スキャン方式であってもよい。
【００４６】
（４）本発明のＸ線ＣＴ装置は、医療用装置に限らず、工業用装置にも適用することができる。
【００４７】
【発明の効果】
以上に詳述したように、請求項１に記載のＸ線ＣＴ装置によれば、Ｘ線ＣＴ撮影実行中、被検体に起こったズレの距離を、線量非一定スキャン区間というかたちで検出すると同時に、被検体に起こったズレの向きを、設定管電流変化の有無というかたちで検出し、検出された線量非一定スキャン区間の長さと設定管電流変化の有無とにしたがって設定管電流と被検体の体軸上の位置との間の対応関係を即時シフトすることにより、設定管電流の狂いを引き起こした設定管電流と被検体の体軸上の位置との対応関係のズレを速やかに解消し、設定管電流を直ちに正常状態に復帰させる構成を備えていて、設定管電流を狂わせる被検体のズレが自動的に検出され、設定管電流の狂いが自動的に修正されるので、設定管電流の狂いを確実かつ容易に修正することができる。
【００４８】
さらに、請求項２に記載のＸ線ＣＴ装置によれば、Ｘ線ＣＴ撮影実行中、被検体に起こったズレの距離を、線量非一定スキャン区間というかたちで検出すると同時に、被検体に起こったズレの向きを、設定管電流変化の有無というかたちで検出し、検出された線量非一定スキャン区間の長さと設定管電流の変化の有無とにしたがって被検体の体軸の方向に対する被検体ないしＸ線管およびＸ線検出器の位置関係を即時シフトすることにより、設定管電流の狂いを引き起こした被検体の体軸の方向に対する被検体ないしＸ線管およびＸ線検出器の位置関係のズレを速やかに解消し、設定管電流を直ちに正常状態に復帰させる構成を備えていて、設定管電流を狂わせる被検体のズレが自動的に検出され、設定管電流の狂いが自動的に修正されるので、設定管電流の狂いを確実かつ容易に修正することができる。
【図面の簡単な説明】
【図１】第１実施例のＸ線ＣＴ装置の全体構成を示すブロック図である。
【図２】Ｘ線ＣＴ撮影計画用の２次元Ｘ線透過画像の一例を示す模式図である。
【図３】Ｘ線管の設定管電流の変化とＸ線透過率の平均レベルの変動を対比して示すグラフである。
【図４】設定管電流に狂いのない場合の設定管電流と透過Ｘ線の平均線量を対比して示すグラフである。
【図５】被検体に体軸の方向のズレが起こった場合のＸ線管の設定管電流の変化とＸ線透過率の平均レベルの変動を対比して示すグラフである。
【図６】設定管電流に狂いがある場合の設定管電流と透過Ｘ線の平均線量を対比して示すグラフである。
【図７】被検体のズレの向きがＸ線ファンビームのスキャン方向と同一の場合の線量非一定スキャン区間の出現状況を示すグラフである。
【図８】被検体のズレの向きがＸ線ファンビームのスキャン方向と反対の場合の線量非一定スキャン区間の出現状況を示すグラフである。
【図９】設定管電流保持部による設定管電流の保持状況を示す模式図である。
【図１０】第１実施例の設定管電流の狂い修正プロセスを示すフローチャートである。
【図１１】第２実施例のＸ線ＣＴ装置の全体構成を示すブロック図である。
【図１２】従来のＸ線ＣＴ装置のＸ線撮像台の概略構成図である。
【図１３】従来装置におけるＸ線ＣＴ撮影計画用の画像例を示す模式図である。
【符号の説明】
１ … Ｘ線管
２ … Ｘ線検出器
１５ … 平均線量逐次求出部（平均線量逐次求出手段）
１６ … 線量非一定区間求出部（線量非一定区間求出手段）
１７ … 設定管電流変化判定部（設定管電流変化判定手段）
１８ … 対応関係シフト部（対応関係シフト手段）
１９ … 位置関係シフト部（位置関係シフト手段）
Ｉ … 設定管電流
Ｉａ〜Ｉｚ … 設定管電流
Ｍ … 被検体
ＰＡ … ２次元Ｘ線透過画像
ＴＭ … 線量非一定スキャン区間
Ｚ … 体軸
Ｚａ〜Ｚｚ … 体軸上の位置
Ｕ … 透過Ｘ線の平均線量
Ｕ … 透過Ｘ線の平均線量Ｚａ〜Ｚｚ … 体軸上の位置？[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT apparatus that scans an X-ray fan beam in the direction of the body axis of a subject according to a set tube current calculated and held in advance during the execution of X-ray CT imaging. The present invention relates to a technique for reliably and easily correcting a deviation in a set tube current of an X-ray tube caused by a deviation in the direction of a body axis.
[0002]
[Prior art]
FIG. 12 is a diagram showing a schematic configuration of an X-ray imaging table of a conventional X-ray CT apparatus. In the case of the X-ray CT apparatus of FIG. 12, a two-dimensional X-ray transmission image for X-ray CT imaging plan is acquired prior to X-ray CT imaging. The X-ray tube 51 and the X-ray detector 52 face each other across the subject (patient) M and remain stationary, and the top 53 is moved in the direction of the body axis Z of the subject M to obtain X-rays. While the subject M is being sent between the tube 51 and the X-ray detector 52, the X-ray fan beam FB is continuously irradiated from the X-ray tube 51 to the subject M with a constant tube current, and at the same time, output from the X-ray detector 52. Collect the transmitted X-ray transmission signal. Necessary image processing is performed on the collected X-ray transmission signal, and as shown in FIG. 13, a two-dimensional X-ray transmission image Pa including a region of interest as an X-ray CT imaging target is created and X-ray CT imaging is performed. It is displayed on the display monitor 54 as an image for planning. The operator performs operations necessary for the imaging plan while viewing the displayed two-dimensional X-ray transmission image Pa.
[0003]
Further, in the case of the conventional X-ray CT apparatus, the subject M is detected based on the X-ray transmission signal collected from the X-ray detector 52 when the two-dimensional X-ray transmission image Pa for X-ray CT imaging plan is acquired. The set current of the X-ray tube 51 that can keep the average dose of transmitted X-rays at a substantially constant appropriate amount while following and changing the average level of X-ray transmittance generated along the direction of the body axis Z is calculated in advance. And held in a form associated with the position of the subject M on the body axis Z. During execution of the X-ray CT imaging, the X-ray fan beam FB scans the subject M in the direction of the body axis Z according to the preset tube current calculated and held.
[0004]
In the case of the X-ray transmittance of the subject M, the average level of the X-ray transmittance along the direction of the body axis Z of the subject M depends on the internal state of the subject M, such as the presence or absence of an organ and the type of the existing organ. Therefore, if the tube current of the X-ray tube 51 remains constant during execution of X-ray CT imaging, X-ray transmission that occurs along the direction of the body axis Z of the subject M between X-ray CT images. In order to eliminate this inconvenience, such as a difference in the overall brightness of the image according to the fluctuation of the average level of the rate, the tube current of the X-ray tube 51 is directed in the direction of the body axis Z of the subject M. It is necessary to change following the fluctuation of the average level of the X-ray transmittance generated along the line. Further, the transmitted X-rays incident on the X-ray detector 52 must have a dose that can maintain the image quality of the X-ray CT image, and at the same time, a dose that can suppress the X-ray exposure dose of the subject M as much as possible. For these needs, the setting tube current of the X-ray tube 51 is calculated under the condition that the average dose of transmitted X-rays can be maintained at a substantially constant appropriate amount.
[0005]
[Problems to be solved by the invention]
However, in the case of the conventional X-ray CT apparatus, there is a problem that the preset tube current of the X-ray tube 51 calculated and held in advance sometimes goes wrong. If the subject M shifts in the direction of the body axis Z after the acquisition of the two-dimensional X-ray transmission image for the X-ray CT imaging plan, the set current of the X-ray tube 51 and the position of the subject M on the body axis Z And the set tube current of the X-ray tube 51 goes wrong. For example, in the case of an infant or a demented elderly person who cannot understand the necessity of stillness during photographing, deviation is likely to occur. In the worst case, the X-ray CT imaging plan image must be taken again to correct the setting tube current error. When the operator overlooks the deviation of the subject M or the deviation of the subject M occurs after the start of X-ray CT imaging, the imaging is performed with the set tube current being out of order.
[0006]
In view of the above circumstances, the present invention provides an X-ray CT apparatus capable of reliably and easily correcting a deviation in a set tube current of an X-ray tube caused by a deviation in the direction of the body axis in a subject. For the purpose.
[0007]
[Means for Solving the Problems]
In order to achieve such an object, the present invention has the following configuration. That is, the X-ray CT apparatus according to claim 1 is configured such that the average dose of transmitted X-rays is substantially constant while changing following the change in the average level of X-ray transmittance generated along the direction of the body axis of the subject. The set tube current of the X-ray tube is calculated so as to be maintained at an appropriate amount, and is held in association with the position on the body axis of the subject, and the set tube previously calculated and held during execution of X-ray CT imaging In the X-ray CT apparatus configured to scan the X-ray fan beam in the direction of the body axis of the subject according to the current, (A) Accompanying the scanning of the X-ray fan beam when performing X-ray CT imaging Average dose sequential acquisition means for sequentially determining the average dose of transmitted X-rays along the direction of the body axis of the subject based on the X-ray transmission signal collected from the X-ray detector, and (B) sequential acquisition To determine whether each average dose to be applied is a substantially constant appropriate amount Dose non-constant interval obtaining means for obtaining a scan interval (dose non-constant scan interval) that continues without interruption, and (C) there is no change in the setting tube current before and after the average dose is not a substantially constant appropriate amount A setting tube current change determining means for determining whether or not (D) a non-constant dose scanning section is obtained by a non-constant dose finding means, and the length of the non-constant scan section and the setting by the setting tube current change judging means Corresponding relationship shift means for immediately shifting the corresponding relationship between the set tube current and the position of the subject on the body axis according to the presence or absence of a change in the tube current is provided.
[0008]
(Operation / Effect) According to the first aspect of the present invention, during execution of X-ray CT imaging, the X-ray fan beam is scanned in the direction of the body axis of the subject according to the preset tube current calculated and held. At the same time, the average dose sequential obtaining means is arranged along the direction of the body axis of the subject based on the X-ray transmission signal collected from the X-ray detector as the X-ray fan beam is scanned. The average dose of transmitted X-rays is sequentially obtained, and the dose non-constant interval obtaining means determines whether or not the sequentially obtained average dose is a substantially constant appropriate amount and determines the dose non-constant scan interval. In addition to the determination, the setting tube current change determining means determines whether or not there is a change in the setting tube current before and after the average dose is not a substantially constant appropriate amount. Then, when the dose non-constant scan section is obtained, the correspondence shift means determines the set tube current and the position of the subject on the body axis according to the length of the dose non-constant scan section and whether or not the set tube current has changed. Immediately shift the correspondence between.
[0009]
In the case of the invention described in claim 1, since the set tube current of the X-ray tube is held in a form associated with the position on the body axis of the subject, if the subject is displaced in the direction of the body axis, Also, the correspondence between the set tube current and the position on the body axis of the subject is shifted, and the set tube current of the X-ray tube goes wrong. On the other hand, the fluctuation of the average level of the X-ray transmittance in the direction of the body axis of the subject is a state in which the average level is substantially the same and continues continuously over a correspondingly long scan section. Since the average level of the X-ray transmittance, which is repeated in the same manner except that it changes, is a type of change that changes stepwise, the set tube current also changes stepwise. Therefore, even if a deviation in the direction of the body axis occurs in the subject, not all the set tube currents are distorted over the entire scan section, but a deviation in the direction of the body axis of the subject before or after the change of the set tube current. The set tube current only goes wrong in the scan interval according to the distance.
On the other hand, the set tube current of the X-ray tube is calculated so that the average dose of transmitted X-rays is always maintained at a substantially constant appropriate amount. This corresponds to a dose non-constant scan section in which a state in which the calculated average dose is not a substantially constant appropriate amount continues without interruption. Since the scan section in which the set tube current of the X-ray tube is distorted corresponds to the distance of deviation in the direction of the body axis relative to the subject, the dose non-constant scan section corresponds to the body axis that occurred in the subject. Corresponds to the distance of the direction deviation.
[0010]
Also, if the direction of the deviation of the body axis direction that occurs in the subject is the same as the scan direction of the X-ray fan beam, the deviation of the setting tube current starts from the transition of the setting tube current, so the average dose is appropriately constant. The set tube current changes before and after the quantity disappears. On the other hand, if the direction of the deviation of the body axis direction that occurs in the subject is opposite to the X-ray fan beam scan direction, the deviation of the set tube current starts just before the change of the set tube current, so the average dose is approximately The set tube current does not change before and after the fixed amount is not reached. Therefore, the presence or absence of a change in the setting tube current (hereinafter, abbreviated as “presence or absence of a change in the setting tube current” as appropriate) determined by the setting tube current change determination means corresponds to the direction of deviation of the body axis direction that has occurred in the subject. .
Therefore, when a non-constant dose scan interval is obtained, the correspondence between the set tube current and the position of the subject on the body axis is determined according to the length of the non-constant scan interval and whether there is a change in the set tube current. If the shift is done immediately, the deviation of the correspondence between the set tube current and the position of the subject on the body axis that is caused by the deviation in the direction of the body axis of the subject is quickly eliminated, and the set tube current is immediately in a normal state. Return to. In other words, the deviation of the subject that causes the set tube current to change is automatically detected, and the set tube current change is automatically corrected.
[0011]
Furthermore, in order to achieve the above object, the present invention has the following configuration. That is, the X-ray CT apparatus according to claim 2 keeps the average dose of transmitted X-rays substantially constant while changing following the change in the average level of the X-ray transmittance generated along the direction of the body axis of the subject. The set tube current of the X-ray tube is calculated so as to be maintained at an appropriate amount, and is held in association with the position on the body axis of the subject, and the set tube previously calculated and held during execution of X-ray CT imaging In the X-ray CT apparatus configured to scan the X-ray fan beam in the direction of the body axis of the subject according to the current, (a) accompanying the scan of the X-ray fan beam when performing X-ray CT imaging Average dose sequential acquisition means for sequentially determining the average dose of transmitted X-rays along the direction of the body axis of the subject based on the X-ray transmission signal collected from the X-ray detector, and (b) sequential acquisition To determine whether each average dose to be applied is a substantially constant appropriate amount Non-constant dose section finding means for finding a scan section (dose non-constant scan section) that continues without interruption, and (c) there is no change in the setting tube current before and after the average dose is not a substantially constant appropriate amount A setting tube current change determining means for determining whether or not (d) when a dose non-constant scan section is obtained by the dose non-constant section finding means, the length of the dose non-constant scan section and the setting by the setting tube current change determining means A positional relationship shift means for immediately shifting the positional relationship between the subject and the X-ray tube / X-ray detector with respect to the direction of the body axis of the subject according to whether or not the tube current has changed is provided. It is.
[0012]
(Operation / Effect) In the case of the invention according to claim 2, when the dose non-constant interval finding means finds the dose non-constant scan interval, the positional relationship shift means changes the length of the dose non-constant scan interval and the set tube current change. The same as the invention according to claim 1, except that the positional relationship between the subject or the X-ray tube and the X-ray detector with respect to the direction of the body axis of the subject is immediately shifted according to whether or not the setting tube current is changed by the judging means. It is. As described above, the dose non-constant scan section corresponds to the distance of the deviation of the body axis direction that has occurred in the subject, and the presence or absence of the change in the setting tube current has occurred in the direction of the body axis direction of the subject. Therefore, the relative positional relationship of the subject with respect to the X-ray tube and the X-ray detector is changed along the direction of the body axis of the subject with the length of the non-constant scan interval and the change in the set tube current. If an immediate shift is made according to the presence / absence of the presence / absence, the positional deviation of the subject or the X-ray tube and the X-ray detector with respect to the direction of the body axis of the subject caused by the deviation of the direction of the body axis of the subject is quickly eliminated. The set tube current immediately returns to the normal state. In other words, the deviation of the subject that causes the set tube current to change is automatically detected, and the set tube current change is automatically corrected. When shifting the subject, the subject is returned to the original normal position, and the deviation of the subject is directly eliminated. When shifting the X-ray tube and the X-ray detector, the X-ray tube and the X-ray detector are The displacement of the subject is indirectly eliminated by moving according to the displacement of the subject.
[0013]
According to a third aspect of the present invention, in the X-ray CT apparatus according to the first or second aspect, the set tube current of the X-ray tube is accompanied with execution of imaging of a two-dimensional X-ray transmission image for X-ray CT imaging planning. The calculation is based on the X-ray transmission signal collected from the X-ray detector.
(Operation / Effect) According to the third aspect of the present invention, X based on the X-ray transmission signal collected from the X-ray detector when the two-dimensional X-ray transmission image for the X-ray CT imaging plan is acquired. Since the setting tube current of the X-ray tube is calculated, it is not necessary to newly collect data relating to the X-ray transmittance of the subject when calculating the setting tube current of the X-ray tube.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Subsequently, an embodiment of the X-ray CT apparatus of the present invention will be described in detail with reference to the drawings.
[First embodiment]
FIG. 1 is a block diagram showing the overall configuration of the X-ray CT apparatus according to the first embodiment.
As shown in FIG. 1, the X-ray CT apparatus of the first embodiment is arranged in a line along the X-ray tube 1 that irradiates the subject M with the X-ray fan beam FB and the spread of the X-ray fan beam FB. A number of X-ray detection elements (not shown) are arranged in one or several rows in the body axis Z direction of the subject M, and an X-ray transmission signal (X-ray) is generated with irradiation of the X-ray fan beam FB. The gantry 3 is disposed so that the X-ray detector 2 that outputs (detection data) faces the subject M, and the X-ray tube 1 and the X-ray are placed while the subject M is placed. A top plate 4 that moves back and forth between the detector 2 in the direction of the body axis Z of the subject M and enters and exits the gantry 3, and captures a two-dimensional X-ray transmission image for X-ray CT imaging planning; X-ray CT imaging is based on an X-ray CT imaging plan established using a two-dimensional X-ray transmission image. It is configured so as playable.
[0015]
In the case of taking a two-dimensional X-ray transmission image, the top plate 4 is placed on the body axis Z of the subject M while the X-ray tube 1 and the X-ray detector 2 face each other with the subject M sandwiched therebetween and remain stationary. X-ray detection while simultaneously irradiating the X-ray fan beam FB from the X-ray tube 1 with a constant tube current while moving the subject M between the X-ray tube 1 and the X-ray detector 2. The data collection unit (DAS) 5 collects the X-ray transmission signal output from the device 2.
In the case of taking an X-ray CT image, the top plate 4 is rotated around the subject M while the X-ray tube 1 and the X-ray detector 2 are kept facing each other. The X-ray fan beam FB is continuously irradiated from the X-ray tube 1 while the subject M is sent between the X-ray tube 1 and the X-ray detector 2 and is output from the X-ray detector 2 at the same time. The data collection unit 5 collects X-ray transmission signals. Therefore, imaging of the X-ray CT image of the first embodiment apparatus is a spiral scan method.
The rotation of the X-ray tube 1 and the X-ray detector 2 and the movement of the top plate 4 are controlled by the mechanical control unit 6, and the X-ray fan beam FB is irradiated by the X-ray tube 1 by the X-ray exposure control unit 7. Controlled.
[0016]
Further, in the subsequent stage of the data collection unit 5, a X-ray transmission signal collected by the data collection unit 5 by processing the two-dimensional X-ray transmission image is processed to create a two-dimensional X-ray transmission image, or by X-ray CT imaging. An image reconstruction unit 8 that processes an X-ray transmission signal collected by the data collection unit 5 to create an X-ray CT image, and a two-dimensional X-ray transmission image or X-ray CT created by the image reconstruction unit 8 An image memory unit 9 for storing images, a display monitor 10 for displaying images stored in the image memory unit 9, screens for input operations, and the like, and operations for performing input operations such as commands and data necessary for shooting In addition to the console 11, the console 11 controls the mechanical control unit 6, the X-ray exposure control unit 7, the image reconstruction unit 8, the image memory unit 9, and the display monitor 10 to operate appropriately. Sends commands according to input operations and shooting progress Strike computer 12 is provided.
[0017]
Furthermore, the X-ray CT imaging of the first embodiment is performed in the direction of the body axis Z of the subject M based on the X-ray transmission signal collected from the X-ray detector 2 in accordance with the execution of the imaging of the two-dimensional X-ray transmission image. The setting tube current for calculating the setting tube current of the X-ray tube 1 so that the average dose of the transmitted X-rays can be maintained at a substantially constant appropriate amount while following the change in the average level of the X-ray transmittance generated along A calculation unit 13 and a setting tube current holding unit 14 for holding the setting tube current calculated by the setting tube current calculation unit 13 in association with the position on the body axis Z of the subject M are provided, and X-ray CT imaging is performed. During execution, the X-ray fan beam FB scans the subject M along the direction of the body axis Z according to the set tube current calculated and held in advance by the set tube current calculating unit 13 and the set tube current holding unit 14. It is configured to go.
[0018]
In the case of the X-ray transmittance of the subject M, the average level of the X-ray transmittance along the direction of the body axis Z of the subject M depends on the internal state of the subject M, such as the presence or absence of an organ and the type of the existing organ. X-ray transmission that occurs along the direction of the body axis Z of the subject M between X-ray CT images if the tube current of the X-ray tube 1 remains constant during execution of X-ray CT imaging. There is a disadvantage that the overall brightness of the image varies depending on the fluctuation of the average level of the rate, and in order to eliminate this inconvenience, the tube current of the X-ray tube 1 is moved in the direction of the body axis Z of the subject M. The average level of the X-ray transmittance generated along the line is changed following the fluctuation. Further, the transmitted X-ray incident on the X-ray detector 2 needs to have a dose necessary for maintaining the image quality of the X-ray CT image and a dose that can suppress the X-ray exposure dose of the subject M as much as possible. In order to satisfy these needs, the setting tube current calculation unit 13 calculates the setting tube current under the condition that the average dose of transmitted X-rays can be maintained at a substantially constant appropriate amount.
[0019]
When a two-dimensional X-ray transmission image PA is obtained by capturing a two-dimensional X-ray transmission image as shown in FIG. 2, the setting tube current calculation unit 13 determines the body axis of the subject M. For each slice cross section QA to QZ of the subject M set one after another perpendicularly to the body axis Z of the subject M along the Z, the X-ray transmission signals belonging to each are added and averaged to average the transmitted X-ray dose Find TA-TZ. Specifically, for example, in the X-ray detector 2, all the X-ray transmission signals of one row of X-ray detection elements arranged in a line along the spread of the X-ray fan beam FB are cross-sectional slices of the subject M. The average doses TA to TZ are obtained by adding and averaging for each of QA to QZ. The change in the average X-ray dose T of the transmitted X-ray that occurs between these slice sections QA to QZ corresponds to the change in the average level of the X-ray transmittance that occurs along the direction of the body axis Z of the subject M.
[0020]
Further, the setting tube current calculation unit 13 converts the average doses TA to TZ into the water thickness (water equivalent body thickness t) having the same dose, and the position Za on the body axis Z assuming the slice slices QA to QZ. The water equivalent body thicknesses ta to tz of the subject M in ~ Zz are obtained. When the body axis Z of the subject M is taken along the horizontal axis and the water equivalent body thickness t of the subject M is taken along the vertical axis, the result is as shown by a broken line in FIG. Since the water equivalent body thickness t of the subject M is inversely proportional to the average dose T (the relation that the average dose T increases as the water equivalent body thickness t decreases), not only the average dose T but also the water equivalent body thickness t. Is also the data of the average level of the X-ray transmittance of the subject M at the positions Za to Zz on the body axis Z of the subject M (average level information of the X-ray transmittance).
[0021]
On the other hand, since the tube current of the X-ray tube 1 and the transmitted X-ray dose are in a good direct proportional relationship, the reference average current T0 of the transmitted X-ray with a constant value, and further the reference tube current at the reference water equivalent body thickness t0. I0 is predetermined. The reference average dose T0 is a dose necessary for maintaining the image quality of the X-ray CT image, and is an appropriate amount that can suppress the X-ray exposure dose of the subject M as much as possible. Then, the setting tube current calculation unit 13 determines the tube current I from which the reference average dose T0 is obtained for each slice section QA to QZ of the water equivalent body thickness ta to tz (or the average dose TA to TZ), for example, the setting tube current I = The calculated set tube currents Ia to Iz are calculated by the set tube current holding unit 14 of the subject M and calculated by calculation using an equation such as I 0 × t ÷ t 0 (or equivalent I = I 0 × T 0 ÷ T). Stored in association with the positions Za to Zz on the body axis Z. Note that the calculation of the tube current I may be performed using any other appropriate expression instead of the expression of I0 * t / t0 or I0 * T0 / T.
[0022]
As a result, as shown by the solid line in FIG. 3, the set tube currents Ia to Iz follow the fluctuation of the average level of the X-ray transmittance of the subject M that occurs along the direction of the body axis Z of the subject M. It will change. At the same time, the average dose U of transmitted X-rays when the X-ray tube 1 irradiates the subject M with the X-ray fan beam FB at the set tube currents Ia to Iz is always substantially the reference average dose T0. This is a dose necessary for maintaining the image quality of the CT image, and is a dose that can suppress the X-ray exposure dose of the subject M as much as possible.
[0023]
Furthermore, the X-ray CT apparatus of the first embodiment is based on the X-ray transmission signal collected from the X-ray detector 2 along with the scan of the X-ray fan beam FB when X-ray CT imaging is executed. An average dose sequential calculation unit 15 that sequentially calculates an average dose U of transmitted X-rays along the direction Z, and each average dose U that is sequentially calculated by the average dose sequential calculation unit 15 is a substantially reference average dose T0. Before and after the average dose U is no longer a substantially constant appropriate amount. When the setting tube current change determination unit 17 for determining whether or not there is a change in the setting tube current and the dose non-constant interval obtaining unit 16 find the dose non-constant scan interval, the length of the dose non-constant scan interval And the setting tube current I according to the presence or absence of the setting tube current change And a correspondence relationship shift unit 18 to immediately shift the correspondence between the position on the body axis Z of the sample M.
[0024]
For example, in the X-ray detector 2, the average dose sequential obtaining unit 15 includes 360 rows of X-ray detection elements arranged in a line along the spread of the X-ray fan beam FB at the slice cross sections QA to QZ. The average dose U of the transmitted X-ray is obtained by sequentially adding and averaging all the X-ray transmitted signals collected during the rotation of 180 ° or 180 °. The average dose U is collected, for example, at the moment when the X-ray detector 2 comes to the position when the X-ray CT imaging plan two-dimensional X-ray transmission image PA is taken at each slice cross section QA to QZ. It can also be calculated by sequentially adding and averaging all X-ray transmission signals.
[0025]
While the subject M is not displaced in the direction of the body axis Z and the set tube current I is normal, the average dose U is substantially the reference average dose T0 as shown in FIG. However, when a deviation in the direction of the body axis Z occurs in the subject M, as shown in FIG. 5, a deviation also occurs in the fluctuation of the average level of the X-ray transmittance of the subject M, and the set tube current I and the subject The correspondence relationship between the position of M on the body axis Z collapses, and the setting tube current I goes out of place.
[0026]
On the other hand, as shown in FIG. 3, the variation in the average level of the X-ray transmittance in the direction of the body axis Z of the subject M is substantially the same as the state in which the average level is substantially the same and continues continuously over a considerably long scan section. Since the average level of the X-ray transmittance, which is repeated in the same manner except that the same average level is changed to another level, is a type of variation that changes stepwise, similarly, the setting tube current I also changes stepwise. . For this reason, even if a deviation in the direction of the body axis Z occurs in the subject M, not all the set tube currents are distorted over the entire scan section, and the change of each set tube current depends on the direction of the deviation of the subject M. Only the set tube current in the scan section corresponding to the distance of the displacement of the subject M before or after is incorrect. If the direction of misalignment of the subject M is the same as the scan direction of the X-ray fan beam FB, as shown in FIG. 6, the scan section starts from the change of each set tube current and extends to the misalignment distance of the subject M. Only the set tube current goes wrong at ST.
[0027]
On the other hand, as shown in FIG. 6, the scan section ST in which the set tube current is out of order is equal to a dose non-constant scan section TM in which the state in which the average dose U is not substantially the reference average dose T0 continues without interruption. In addition, the scan section ST in which the set tube current is distorted corresponds to the deviation ΔZ in the direction of the body axis Z that has occurred in the subject M as described above. The emitted dose non-constant scan section TM corresponds to a deviation distance ΔZ in the direction of the body axis Z occurring in the subject M.
[0028]
Further, when the direction of deviation in the direction of the body axis Z occurring in the subject M is the same as the scanning direction of the X-ray fan beam FB, as shown in FIG. 7, the deviation of the set tube current I is the change of the set tube current. Therefore, the setting tube current changes before and after the average dose U is not a substantially constant appropriate amount. Conversely, when the direction of deviation in the direction of the body axis Z occurring in the subject M is opposite to the scanning direction of the X-ray fan beam FB, as shown in FIG. Since it starts before the turning point, the set tube current does not change before and after the average dose U is not a substantially constant appropriate amount. Thus, the presence or absence of the setting tube current change by the setting tube current change determination unit 17 corresponds to the direction of deviation of the direction of the body axis Z occurring in the subject M.
[0029]
Accordingly, the correspondence shift unit 18 sets the correspondence between the setting tube current I and the position of the subject M on the body axis Z to the presence or absence of the change in the setting tube current by a distance corresponding to the dose non-constant scan section TM. When the shift is made immediately in the appropriate direction, the deviation of the correspondence between the setting tube current I and the position of the subject M on the body axis Z caused by the deviation in the direction of the body axis Z of the subject M is quickly eliminated. The deviation of the tube current I is corrected immediately. In addition, after the correction is once completed, the correction process is repeated in the same process as described above even when the subject M is displaced in the direction of the body axis Z and the set tube current is changed again.
[0030]
In the case of the apparatus of the first embodiment, the set tube current I of the X-ray tube 1 is collected from the X-ray detector 2 as the two-dimensional X-ray transmission image for X-ray CT imaging plan is acquired. Since the calculation is based on the ray transmission signal, there is an advantage that data relating to the X-ray transmittance of the subject M need not be collected again when calculating the set tube current I of the X-ray tube 1.
[0031]
Next, the situation of correcting the deviation of the set tube current I of the X-ray tube 1 in the X-ray CT apparatus of the first embodiment will be described more specifically. FIG. 9 is a schematic diagram showing how the setting tube current holding unit 14 holds the setting tube current, and FIG. 10 is a flowchart showing a setting tube current error correction process in the first embodiment. In the following, description will be made from the stage when the imaging plan is made after the imaging of the two-dimensional X-ray transmission image is completed. Note that it is assumed that the subject M has shifted by the distance ΔZ in the same direction as the scanning direction of the X-ray fan beam FB after the two-dimensional X-ray transmission image has been captured and before the X-ray CT imaging is started. .
[0032]
[Step S1] The setting tube current calculation unit 13 sets the setting tube for each slice cross section QA to QZ of the subject M set one after another along the body axis Z of the subject M and perpendicularly to the body axis Z of the subject M. Currents Ia to Iz are calculated.
[0033]
[Step S2] As shown in FIG. 9A, the setting tube current holding unit 14 converts the setting tube currents Ia to Iz to positions Za to Zz on the body axis Z of the subject M in each slice section QA to QZ. Hold in association.
[0034]
[Step S3] Along with the start of X-ray CT imaging, the average dose sequential acquisition unit 15 starts a process of sequentially determining the average dose U of transmitted X-rays.
[0035]
[Step S4] The dose non-constant interval finding unit 16 starts the finding process of the dose non-constant scan interval, and the setting tube current change determining unit 17 starts the determining process of whether or not the setting tube current has changed.
[0036]
[Step S5] The presence / absence of non-constant dose scanning section is checked. If the non-constant scanning section TM is not determined, the process jumps to Step S7. When the dose non-constant scan section TM is obtained, the process proceeds to the next step S6. Since the direction of deviation of the subject M is the scanning direction of the X-ray fan beam FB, the setting tube current change determination unit 17 determines that there is a setting tube current change.
[0037]
[Step S6] The correspondence shift unit 18 scans the X-ray fan beam FB for the correspondence between the set tube current I and the position of the subject M on the body axis Z by a distance corresponding to the dose non-constant scan section TM. Immediate shift in direction. Specifically, as shown in FIG. 9B, a process of increasing each position Za to Zz on the body axis Z of the subject M by the distance ΔZ of the subject M by the setting tube current holding unit 14. Done. The deviation of the set tube current I caused by the deviation in the direction of the body axis Z of the subject M is automatically corrected.
[0038]
[Step S7] If X-ray CT imaging is continuing, the process returns to step S5 and the following process is continued. When the X-ray CT imaging is completed, the process for correcting the set tube current of the X-ray tube 1 is also completed.
[0039]
[Second Embodiment]
FIG. 11 is a block diagram showing the overall configuration of the X-ray CT apparatus according to the second embodiment. In the X-ray CT apparatus of the second embodiment, instead of the correspondence shift unit 18, when the non-constant dose section finding unit 16 obtains the non-constant dose scan section, the length and setting of the non-constant dose scan section TM are set. Except for having a positional relationship shift unit 19 that immediately shifts the position of the subject M with respect to the direction of the body axis Z of the subject M according to the presence or absence of a change in the set tube current by the tube current change determination unit 17. Since it is substantially the same as the apparatus of the embodiment, only different parts will be described, and description of common parts will be omitted.
[0040]
That is, in the apparatus of the second embodiment, as in the case of the first embodiment, when the average dose sequential calculation unit 15 sequentially calculates the average dose U of transmitted X-rays when performing X-ray CT imaging, the dose The non-constant interval obtaining unit 16 obtains the dose non-constant scan interval TM, and the setting tube current change determining unit 17 determines the presence or absence of the setting tube current change. In the case of the apparatus of the second embodiment, when the dose non-constant scan section TM is obtained, the positional relationship shift unit 19 determines the subject according to the length of the dose non-constant scan section TM and whether there is a change in the set tube current. The position of the subject M with respect to the direction of the body axis Z of M is immediately shifted.
[0041]
As described above, the dose non-constant scan section TM corresponds to the distance of the displacement in the direction of the body axis Z that occurred in the subject M, and the direction of the body axis Z in which the presence or absence of the set tube current change occurred in the subject M. Therefore, the subject M quickly returns to the original correct position by the immediate shift by the positional relationship shift unit 19, and the set tube current due to the deviation of the direction of the body axis Z that has occurred in the subject M. Will be corrected immediately.
[0042]
In the case of the second embodiment, specifically, the positional relationship shift unit 19 makes the dose non-constant so that a deviation exactly opposite to the deviation in the direction of the body axis Z that has occurred in the subject M occurs in the top 4. A command to move the top 4 according to the scan section TM and whether there is a change in the set tube current is sent to the mechanical control unit 6, and the subject M is returned to the original by offsetting the offset of the subject M with the offset of the top 4. It is configured to return to the correct position.
In the case of the second embodiment, the X-ray fan beam FB is again scanned from the beginning after correcting the deviation of the setting tube current.
[0043]
The present invention is not limited to the above-described embodiment, and can be modified as follows.
(1) In the apparatus of the first embodiment, each slice section QA to QZ is set perpendicular to the body axis Z of the subject M, but each slice section QA to QZ is oblique to the body axis Z of the subject M. It may be set. For example, when the gantry 3 is tilted, the slice cross sections QA to QZ are set obliquely with respect to the body axis Z of the subject M by an amount corresponding to the tilt angle of the gantry 3.
[0044]
(2) In the apparatus of the second embodiment, the top plate 4 is moved so that the displacement of the subject M is offset by the displacement of the top plate 4 to correct the setting tube current deviation. When the tube 1 and the X-ray detector 2 can move along the body axis Z of the subject M, the X-ray tube 1 so that the deviation of the subject M is offset by the deviation of the X-ray tube 1 and the X-ray detector 2. Alternatively, the X-ray detector 2 may be moved to correct the setting tube current deviation.
[0045]
(3) In the case of X-ray CT imaging, the apparatus of the above-mentioned embodiment is a spiral scan system that continuously irradiates the X-ray fan beam FB while continuously transferring the subject M. The X-ray CT apparatus of the present invention In the case of X-ray CT imaging, an intermittent scan method in which the X-ray fan beam FB is intermittently irradiated while the subject M is intermittently transferred may be used.
[0046]
(4) The X-ray CT apparatus of the present invention can be applied not only to medical devices but also to industrial devices.
[0047]
【The invention's effect】
As described above in detail, according to the X-ray CT apparatus of the first aspect, during the execution of X-ray CT imaging, the distance of the deviation occurring in the subject is detected in the form of a non-constant dose scanning section. The direction of the deviation that occurred in the subject is detected in the form of the presence or absence of a change in the setting tube current, and the setting tube current and the subject By immediately shifting the correspondence between the position on the body axis, the deviation of the correspondence between the setting tube current and the position on the body axis of the subject that caused the deviation of the setting tube current is quickly resolved. It is equipped with a configuration that immediately returns the setting tube current to the normal state, and the deviation of the subject that deviates the setting tube current is automatically detected, and the error in the setting tube current is automatically corrected. Make crazy sure and easy It can be positive to.
[0048]
Furthermore, according to the X-ray CT apparatus of claim 2, during the X-ray CT imaging, the distance of the deviation that occurred in the subject is detected in the form of a non-constant dose scanning section, and at the same time, The direction of deviation is detected in the form of whether there is a change in the setting tube current, and the subject or X with respect to the body axis direction of the subject according to the detected length of the non-constant scan interval and the presence or absence of the change in the setting tube current By immediately shifting the positional relationship between the X-ray tube and the X-ray detector, the positional relationship between the subject or the X-ray tube and the X-ray detector with respect to the direction of the body axis of the subject causing the deviation of the setting tube current is shifted. It has a configuration that quickly eliminates the setting tube current and immediately returns it to the normal state, and the deviation of the subject that causes the setting tube current to go wrong is automatically detected, and the setting tube current error is automatically corrected. , It can be reliably and easily correct the deviation of the set tube current.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an overall configuration of an X-ray CT apparatus according to a first embodiment.
FIG. 2 is a schematic diagram showing an example of a two-dimensional X-ray transmission image for an X-ray CT imaging plan.
FIG. 3 is a graph showing the change in the set tube current of the X-ray tube and the change in the average level of the X-ray transmittance.
FIG. 4 is a graph showing a comparison between the set tube current and the average dose of transmitted X-rays when there is no deviation in the set tube current.
FIG. 5 is a graph showing a comparison between a change in the set tube current of the X-ray tube and a change in the average level of the X-ray transmittance when a deviation in the direction of the body axis occurs in the subject.
FIG. 6 is a graph showing a comparison between a set tube current and an average dose of transmitted X-rays when there is a deviation in the set tube current.
FIG. 7 is a graph showing the appearance of a non-constant dose scan section when the direction of deviation of the subject is the same as the scan direction of the X-ray fan beam.
FIG. 8 is a graph showing the appearance of a dose non-constant scan section when the direction of deviation of the subject is opposite to the scan direction of the X-ray fan beam.
FIG. 9 is a schematic diagram showing a holding state of a setting tube current by a setting tube current holding unit.
FIG. 10 is a flowchart showing a setting tube current error correction process according to the first embodiment;
FIG. 11 is a block diagram showing an overall configuration of an X-ray CT apparatus according to a second embodiment.
FIG. 12 is a schematic configuration diagram of an X-ray imaging stand of a conventional X-ray CT apparatus.
FIG. 13 is a schematic diagram illustrating an image example for an X-ray CT imaging plan in a conventional apparatus.
[Explanation of symbols]
1 ... X-ray tube
2 ... X-ray detector
15 ... Average dose sequential calculation section (average dose sequential calculation means)
16 ... Dose non-constant interval finding section (Dose non-constant interval finding means)
17 ... Setting tube current change determining unit (setting tube current change determining means)
18: Corresponding relationship shift unit (corresponding relationship shifting means)
19: Positional relation shift unit (positional relation shift means)
I… Setting tube current
Ia to Iz ... Setting tube current
M… Subject
PA ... 2D X-ray transmission image
TM… Dose non-constant scan section
Z ... body axis
Za to Zz ... Positions on the body axis
U: Average dose of transmitted X-ray
U ... Average dose of transmitted X-rays Za to Zz ... Position on the body axis?

Claims

Setting tube current of the X-ray tube so that the average dose of transmitted X-rays can be maintained at a substantially constant appropriate amount while following and changing the average level of X-ray transmittance generated along the direction of the body axis of the subject. Is calculated and held in association with the position on the body axis of the subject, and during execution of X-ray CT imaging, the X-ray fan beam is applied to the body axis of the subject according to the preset tube current calculated and held. In an X-ray CT imaging apparatus configured to scan in a direction (scanning), (A) an X-ray transmission signal collected from an X-ray detector along with scanning of an X-ray fan beam at the time of X-ray CT imaging execution And (B) an average dose sequentially obtaining means for sequentially obtaining an average dose of transmitted X-rays along the direction of the body axis of the subject, and (B) each average dose obtained sequentially is a substantially constant appropriate amount. The scan interval (dose (D) non-constant interval finding means for obtaining a constant scan interval), and (C) setting tube current change determination for determining whether or not there is a change in the set tube current before and after the average dose is not a substantially constant appropriate amount. And (D) when the non-constant dose scanning section is obtained by the non-constant dose finding means, the length of the non-constant scan section and the presence / absence of a change in the setting tube current by the setting tube current change determining means An X-ray CT imaging apparatus comprising: correspondence shift means for immediately shifting the correspondence between the set tube current and the position of the subject on the body axis.

Setting tube current of the X-ray tube so that the average dose of transmitted X-rays can be maintained at a substantially constant appropriate amount while following and changing the average level of X-ray transmittance generated along the direction of the body axis of the subject. Is calculated and held in association with the position on the body axis of the subject, and during execution of X-ray CT imaging, the X-ray fan beam is applied to the body axis of the subject according to the preset tube current calculated and held. In an X-ray CT apparatus configured to scan in a direction (scanning), (a) an X-ray transmission signal collected from an X-ray detector along with scanning of an X-ray fan beam at the time of X-ray CT imaging execution An average dose sequential calculation means for sequentially calculating an average dose of transmitted X-rays along the direction of the body axis of the subject, and (b) whether each average dose calculated sequentially is a substantially constant appropriate amount The scan interval (Dose is not unique) A dose non-constant interval obtaining means for obtaining a scan interval), and (c) a setting tube current change judging means for judging whether or not there is a change in the set tube current before and after the average dose is not a substantially constant appropriate amount. And (d) when the non-constant dose scan section is obtained by the non-constant dose section finding means, the subject is determined according to the length of the non-constant scan section and the presence or absence of the change in the set tube current by the set tube current change judging means An X-ray CT apparatus comprising: a positional relationship shift means for immediately shifting the positional relationship between the subject and the X-ray tube / X-ray detector with respect to the direction of the body axis of the sample.

3. The X-ray CT apparatus according to claim 1, wherein the set tube current of the X-ray tube is collected from the X-ray detector as the two-dimensional X-ray transmission image for the X-ray CT imaging plan is acquired. An X-ray CT apparatus configured to be calculated based on an X-ray transmission signal.